Sunday, December 30, 2012

The first plants

Who were the first plants?   At the risk of a scolding from some of my fellow botanists, I’m going to define plants broadly as organisms that use sunlight to make carbohydrates, and release oxygen gas as a byproduct -   in other words, organisms that photosynthesize.   There is in fact a diverse array of organisms that photosynthesize, including some bacteria and various kinds of algae that are only remotely related to one another; not to mention photosynthetic sea slugs!  So used in that way, "plant" is more of an ecological term for organisms that provide the base of the global food chain, not a precise taxonomic category. (see my post of October 5, 2011, "Plants and animals and kleptoplasts - oh my!")

My goal today is in fact to explore where photosynthesis came from, and we won't be distracted by taxonomic issues.  The first organisms to practice photosynthesis were bacteria, and the cyanobacteria were the first to employ modern photosynthesis, in which oxygen is released as a byproduct.  Bacteria are prokaryotic organisms that have a simple cell structure without nucleus or internal organelles.  They don’t have chloroplasts like true plants or algae, but their entire cell is adapted to conduct photosynthesis.  In my October 5, 2011  post, I described how cyanobacteria in fact were captured and domesticated to become the first chloroplasts.   I also argued as devil’s advocate that cyanobacteria are the only true plants – the multicellular organisms that we call plants are merely the luxury condominiums in which those captured cyanobacteria live!
Cyanobacteria come in many different forms and resemble true eukaryotic algae.  Mat-forming cyanobacteria
 that build stromatolites are similar to to A and B, in which cells are bound by a mucilaginous matrix. on the
surface of a rock.  Drawing from Haupt, Plant Morphology, McGraw-Hill, 1953.

Photosynthesis was invented only once, and passed on to various eukaryotic organisms as chloroplasts, which have been captured, stolen, and recaptured many times.  But when did it all start?  We travel to Australia, where we find some important clues.

In a few shallow, highly saline lagoons along the west coast of Australia, peculiar knobby pillars of rock called stromatolites stand like the disarrayed remnants of a terracotta army, eroded and distorted beyond recognition.  These monoliths were not carved by some ancient civilization, however, but are built up very slowly by microscopic living organisms.   On the tops of the knobs, you can find mats of living microorganisms, held together by a mucilaginous glue.  The glue is secreted primarily by cyanobacteria.  Many bacteria form mats like this.  It’s a good way to anchor yourself to a suitable location.  
Stromatolites at Shark Bay, Western Australia.  Photo by Paul
Harrison via Wikipedia.
Cyanobacteria are massively abundant organisms.  Many live as free-floating plankton or tangles of filaments, and account for as much as 50% of the photosynthesis occurring in open waters (Fig. 2). Others are attached to rocks as filaments or mats.   They are easily confused with true algae, which are eukaryotic organisms with nuclei, chloroplasts, and other organelles.  Before electron microscopy showed us the difference, cyanobacteria were called “blue-green algae.”

Microbial mats on the tops of stromatolites form deposits that extend the knobby pillars slowly upwards.  Particles of sediment and lime precipitated from the water get trapped in the sticky matrix, and periodically bury the living microbes.   The resourceful cyanobacteria in those instances migrate to the top of the sediment and begin a new mat.  This results in a fine structure of alternating light and dark bands.  Stromatolites are thus, like coral reefs, built by the living organisms that inhabit them.   There is a website devoted to Shark Bay in Western Australia ( that includes facts, photos and a video swim through a grove of stromatolites.
Stromatolites turn out to be one of our most important clues as to the origin of plant life.  They have been around for about 3.5 million years.  Ancient fossilized stromatolites, which can also be found in parts of Australia, are in fact among the earliest signs of any kind of life on this planet.  They are abundant throughout much of the geological record, but became rather scarce around 500 million years ago.  This was the time of the  “Cambrian Explosion”, when many new kinds of animals appeared.  Stromatolites came under attack by voracious grazing animals equipped with hard, scraping mouth parts.  After that, they survived only in restricted sites too salty for such animals. 

Though there is still debate about whether the earliest stromatolites were formed by cyanobacteria or some earlier form of life, or even by some physical process, there is no doubt that they were building stromatolites  by 2.7 billion years ago – still a heck of a long ago!   Fossilized cells identifiable as cyanobacteria have also been found in slightly different types of rocks, the Warrawoona and Apex cherts of Australia, and these appear also to date back to 3.5 billion years ago.   Cyanobacteria were the plants, the photosynthetic organisms, that supported the Earth’s early ecosystem.  They did so virtually alone form almost 2 billion years, after which the first signs of eukaryotic algae began to appear.

The early photosynthesizers must have gradually built up enormous populations, for the oxygen they produced eventually transformed the vast oceans of our planet, and then the very atmosphere itself.  The scarcity of oxidized (“rusted”) minerals in the Earth’s oldest rocks (older than 3 billion years), indicates that there was very little free oxygen in the atmosphere at that time, so the oxidizing of crustal rocks is also evidence of plant life.  Iron is particularly abundant on Earth, and quite prone to rusting.  In the ancient seas there was a steady supply of iron bubbling up from underwater volcanic fissures, and from eroding surface rocks.  In its unoxidized state, iron is soluble in water, but when it oxidizes it forms insoluble molecules of hematite or magnetite, which sink to the bottom of the sea.  When oxygen became available in comparably huge quantities there were spectacular depositions of iron.  This resulted in distinctive and extensive rock layers known as the Banded Iron Formations.  The “rusting of the earth,” as it was called by Schopf (2006), is the source of most of the iron ore that is being ravenously consumed by modern civilization.

There is some evidence of limited iron formations about 3.5 billion years ago, but they did not become truly massive until the mid-2-billions.   This suggests that oxygen buildup may have occurred sporadically and slowly at first, but became overwhelming between 2.7 and 2.5 billion years ago.  Oxygen makes up 21% of our current atmosphere, but 20 times as much plant-generated oxygen may be tied up in banded iron formations.  The formation of iron deposits declined rapidly after about 2 billion years ago, as the supply of dissolved iron was depleted, and oxygen then began to build up in the atmosphere.   The transformed atmosphere made it possible for more complex organisms to evolve, leading to eukaryotic cells and the modern world of algae, plants, fungi and animals.

So cyanobacteria invented photosynthesis as we know it today, and were the first functional plants.  The story does not end there.  In the two billion years of their unchallenged domination of the Earth, cyanobacteria also invented the rudiments of aerobic respiration and nitrogen-fixation, two other essential metabolic processes - cyanobacteria had to do virtually everything themselves!  Aerobic respiration is the process by  which organisms “burn” carbohydrates to fuel their metabolic processes.  It is obviously essential  for animals, but also for plants.  How else could they  utilize the carbohydrate reserves that they produced for themselves?

In terms of starting ingredients, photosynthesis is easy.  All you  need are carbon dioxide and water, and both are abundant on the planet.  But to make certain other things, like protein, DNA, ATP and other vital organic molecules, you also need nitrogen, which isn’t so easy to come by.  Wait-a-minute, isn’t the Earth’s atmosphere about 70% nitrogen?  Yes, it is, but it’s nitrogen that is hard to use.  Nitrogen gas consists of two nitrogen atoms bound tightly together by a triple bond.  Most organisms can’t break those bonds to make use of the abundant nitrogen supply.  Enter the cyanobacteria.  Somewhere along the way they acquired the genes to split nitrogen, either through mutation and natural selection, or from some other bacterium that invented it first.   Bacteria freely share genes through the process of horizontal gene transfer, a process we exploit in genetic engineering.

Once the nitrogen molecule has been split, cyanobacteria are able to attach hydrogen atoms to the nitrogen, making ammonia, and ammonia then can become the amino group required to make the building blocks of protein – amino acids.  So cyanobacteria, arguably the most successful and certainly the most long-lived group of organisms, make carbohydrates, metabolize carbohydrates for energy, and make their own protein.  The rest of us have only stolen from them!


Schopf, J. W. 2006.  Fossil evidence of Archaean life.  Phil. Trans. R. Soc. B 361: 869–885.

Website for images and more information about stromatolites in Australia:  (under "Nature of Shark Bay")

Tuesday, November 6, 2012

Mosses of Central Florida 2. Octoblepharum albidum

Leaves of Octoblepharum are crowded in upward-facing rosettes,
resembling tiny bromeliads. 
 [For other mosses in this series, see the Table of Contents]

Octoblepharum albidum Hedw. is one of the more common and distinctive mosses in our area.  Its long, thick, strap-like leaves, typically pale green in color, and arranged in tight rosettes make it easily recognizable.  In local flatwoods and sandhill associations, it occurs in relatively dry banks, on roots or the bases of trees.  I have recently found it also growing on the trunks of date palms planted on the University of South Florida campus.

Octoblepharum is one of the few genera of mosses with leaves more than one cell thick.  Large, water-storage cells lie above and below 1-2 layers of thin photosynthetic cells in the center. Ovoid sporangia are erect and symmetrical (compare with those in Leucobryum, which are bent to the side).

The pale-colored, strap-shaped leaves of Octoblepharum are readily recognized.
 According to Reese (1984) the species is found throughout the tropics, but outside of Florida has been found only in a few places around New Orleans, Louisiana, and long ago near Matagorda Bay in Texas.  In the herbarium at the University of South Florida, the species is represented by collections as far north as Levy and Marion Counties, and is abundant in the southern and west-central part of the state.

Reference: Reese, W. D. 1984.  Mosses of the Gulf South.  Louisiana State University Press.  Baton Rouge and London.

Thursday, October 18, 2012

Making the Ancestor Problem Go Away

At the end of my recent post entitled “The Great Botanical Butter Battle Book” (which you would be well-advised to review before plunging into the even murkier aspects of the debate that follow!), I ended with the implication that, in the traditional taxonomic view, a classification system consisting entirely of monophyletic taxa  is not only undesirable, but also impossible.  Every genus, it is argued,  logically must have originated from a species in an earlier genus, rendering the earlier genus paraphyletic. All efforts to correct this by eliminating paraphyletic genera cause the system to collapse into a single genus. This is sometimes referred to as the “ancestor problem.”  It is a problem that arises with a steadily improving fossil record and actual knowledge of ancestor-descendant relationships.

Recall that monophyletic taxa are complete branches, or clades, of a phylogenetic tree, beginning with the founding ancestral species and including all the descendant species of the common ancestor (all twigs of the branch).   According to the prevailing practice of phylogenetic taxonomy, a clade-taxon at one taxonomic rank (e.g. family) can be subdivided into smaller clade-taxa (e.g. genera) using the same criteria, but none of the subclades can be given the same rank as the main clade (e.g. there cannot be a family within a family).   That is why birds and reptiles cannot  both be ranked as formal classes, as was done traditionally.

A similar example involves dogs and cats, which represent two of the several modern families in the mammal order Carnivora.    The common ancestor, and most likely a group of related species that preceded the split into dog and cat clades (along with the others that don't need to be mentioned), are in a taxonomic no-man’s land.  If we place that ancestral cluster of species, or "stem group," into a family, that family will by definition be paraphyletic, because both the dog and cat clades that we now consider formal families developed as subclades from within that ancient family.

Phylogenetic taxonomy has the logical goal of accurately incorporating the actual branching pattern of evolutionary history into a formalized classification system.  Attempting to put traditional taxonomic boxes (i.e. genera, families, etc.) around clades, however, does create some awkward situations, particularly as we know more and more about ancestral groups of organisms.

As we envision the process of evolution, clades
branch to beget new clades.  Successful
clades further branch into  bushy clusters of
species that  we recognize as genera.   
The hypothetical diagram at the right  represents a common pattern of evolution, with new clades arising from older clades, each in turn developing distinctive adaptations, blossoming out into a diverse group of species, and then declining into extinction as still newer groups come to dominate.  Assuming a fairly complete fossil record of this group, traditional (also called "evolutionary") and phylogenetic taxonomists would have rather different ideas about how to classify them.

Traditional taxonomists would view the four groups as a succession of distinct genera, one on top of the other.  They would see nothing wrong with new genera arising from older genera in sequence over time – it is necessary in fact, if we are to simultaneously believe in evolution and have a system of classification.

Phylogenetic taxonomists view the diagram differently.  A clade is a clade, from the founding species through all of its descendants.  Subclades are viewed as nested within the main clade, rather than emerging on top of them. If all four were considered genera, both A and C would be paraphyletic, because some parts have been removed. "A" can be considered a monophyletic genus, only if B, C, and D are included within it.  If so, the latter 3 genera must be given lower rank than A (e.g. subgenera).   Traditional taxonomists charge that this "lumping" solution would result in a collapse of the entire taxonomic system, for surely genus A itself evolved from some earlier genus, and so on back to the first genus of organisms.

Alternatively, B and D could be recognized as genera, but C and A would have to have higher rank (e.g. C as a subfamily that included genus D, and A as a family that included subfamily C and genus B).  In that scenario, the remaining contents of A and C would have to be split into comparable subgenera and genera.  However, "splitting" like that would just result in even more small, unclassifiable stem groups, and not really solve the problem.  

Many taxonomists, particularly botanists, contend that the ancestor problem is not worth worrying about, because we are highly unlikely to ever have a specimen of an actual ancestor to classify and name, and if we do we won’t know it and it won’t matter.  It’s true that we really don’t have a lot of fossil plants with which to “connect the dots,” and may never directly confront the ancestor problem. Zoologists, however, with a better fossil record, don’t get off so easy.

For example, wee now know quite a lot about our own family of primates, the hominids.  Before the genus Homo existed, there was another genus of hominids, traditionally known as Australopithecus.  Unless you believe in special creation of humans, there has to be a connection between our genus and earlier genera.  The very first species of Homo most certainly descended from a species of Australopithecus, which therefore would be paraphyletic and illegal in phylogenetic classification.  Some anthropologists have dutifully tried to weed out the paraphyly, but no matter how you lump or split the various parts of the hominid tree, there is some part of it that is still directly ancestral to the first Homo.  Is that ancestor unnamable?  Can it be placed in a genus at all?

In the end, the best that hominid taxonomists can do is to “minimize paraphyly” by making the ancestral stem genus as small as possible.  This was the approach of Cela-Conde and Ayala (2003), who recognized the genus Praeanthropus for the stubbornly paraphyletic residue (5 species) of Australopithecines that directly preceded the first humans and the more narrowly defined Australopithecus.  So it seems that in this perspective of evolutionary history, paraphyletic taxa are logically unavoidable.

Yet monophyletic taxa are the foundation of phylogenetic taxonomy.  Is there no way out of this dilemma?

Proponents of the Phylocode movement, advocate simply naming all the branches of a phylogenetic tree, without ranking the parts as families, genera, etc.  This might effectively avoid the conflict of taxonomic philosophies.  Clades can and do arise from one another (e.g. birds from dinosaurs).  Each clade and subclade at every level can have a name, as long as we don't try to rank them as genera, families, classes, etc.  Paraphyly, it can be argued, is an artifact of trying to fit clades into the boxes of traditional classification.

The phylocode makes a lot of sense, and in practice, we don’t worry so much about ranking the bigger clades of life any more.  We talk of Magnolids, Eudicots, Monocots, Amborellids, etc.  in discussions of the evolution of flowering plants, but it seems that no one is seriously trying to squeeze them into classes, subclasses, etc. any more.  There seems to be no point to it. 

There is a snag, however, in going totally rank-less.   Our conventional binomial (“two-name”) system  for naming species requires the genus name plus a specific epithet (e.g. – Quercus is the name of the genus that includes Quercus rubra, the red oak).  So all species must be in a genus.   Attempts to find an alternate species naming system to accompany the Phylocode have failed to achieve any consensus.  So my further discussion and proposed resolution of the ancestor problem assumes that we need formally-defined genera in order to create names for all species, past and present.

It would appear that the prospects for monophyletic-only genera destabilize and collapse the more information we have about ancestral organisms.  But does it really?  Perhaps all this discussion of ancestors having names, and genera evolving from genera, is putting the cart before the horse. 

Going back to the “father” of modern phylogenetic taxonomy, Willi Hennig, we see that his system centered around the process of cladistics and the resulting cladograms.  Cladistics is an objective mathematical process for comparing taxa by coding each for a very specific set of characters (e.g. leaves simple vs compound, stamens 6 vs 5, etc.).  The cladogram is generated by comparing taxa pair by pair.  It superficially resembles a phylogenetic tree as it consists of lines connecting taxa in a branching pattern. Taxa most similar in terms of shared character states come out close together on the cladogram, while those sharing relatively few character states come out distant from each other on the cladogram. 

The difference between a cladogram and a traditional phylogenetic tree (though the two are often considered the same these days) is that the cladogram is purely a diagram of similarity among the taxa being compared.  The cladistics process itself is not prejudiced by whether or not any of the taxa in the comparison are ancestors of any other taxa.  Though we call the stem line that precedes each branch point the “hypothetical common ancestor” (probably a poor choice of words), it is only a mathematically generated line, and does not need to be classified or named.  In a traditional phylogenetic tree (as in the first diagram above, or the diagram published in the study of hominid evolution discussed above), there are built-in hypotheses about ancestors and descendants. 

Now suppose, by chance, that we actually had a common ancestor in a cladistic analysis.  It would be coded and compared like any of the other taxa in the analysis.  It could be classified by putting a box around its line on the cladogram, and named according to conventional rules.  Like the exasperated phylogenetic plant taxonomists mentioned above, we would not know for sure if any taxon were ancestral to any other, and it wouldn't matter.  The cladogram is "ancestor-neutral." 

After the cladogram is made and carved into genera, however, we can then make hypotheses about common ancestors.  This is a separate, secondary process.  At the right is a small portion of a cladogram, with two genera that are sister taxa.  The short horizontal lines on the branch leading to genus B, indicate new adaptations, (technically character state changes or apomorphies), that evolved in the common ancestor of B after its split from A.   There are no comparable character changes indicated on the line leading to genus A, indicating that A has not changed since the split, and is therefore not measurably different from the actual common ancestor.  A and B can be classified as genera based on the cladogram, but then as a separate process we can hypothesize that genus B evolved from a species in genus A.   Genus A is monophyletic by the rules of cladistic classification, but still can be considered ancestral to B.

There is one important assumption in this “solution” of the ancestor problem.  In the simplified diagram above, it is  genera that are the units of cladistic comparison.  The short horizontal lines on the line going to genus B in the diagram are generic level characters.  For  purposes of recognizing genera, we must assume that characters that distinguish genera from one another are of a greater magnitude than, or perhaps qualitatively different from, characters that distinguish species, otherwise we might logically be led to make genera out of smaller and smaller groups of species, even of individual species, following the branching pattern alone.  

The determination of generic-level characters is a subjective judgement that in this context inevitable.   Otherwise, how do we decide that our own genus, Homo, is in fact a genus, not just a section of Australopithecus?  Anthropologists (some at least) have emphasized the "minimization of paraphyly" by lowering the threshold for generic characters.  Other systematists might choose to minimize the proliferation of small, barely-distinguishable genera by raising that threshold.  I have argued that minimal ancestral genera, such as Praeanthropus, are not taxonomically paraphyletic, as they are not identified as ancestral in the cladistic process.  How broadly or narrowly such ancestral genera are defined becomes an issue of how much difference should be required to distinguish genera from one another. This issue is far less cataclismic than the black-and-white battle between pro- and anti-paraphyletic forces that has unnecessarily preoccupied systematists for so many decades.

I would not necessarily extend these arguments to higher levels of classification, to talk about “class-worthy” characters, or “phylum-worthy” characters, for example.  It is not necessary to go there.  It may be that un-ranked clade names are a better option at those levels.  Genera are uniquely important in the taxonomic hierarchy, however, as they are necessary for naming species by the binomial convention, so we must do what we can to  maintain their universal application for all appropriately characterized clusters of species, be they current or ancestral. 


Cela-Conde, C. J. and F. J. Ayala. 2003.  Genera of the human lineage PNAS 100: 7684-7689. 

Thursday, September 6, 2012

Mosses of Central Florida 1. Leucobryum albidum

[For other mosses in this series, see the Table of Contents]

To take a break from the more theoretical posts of late, I'd like to begin a series on the mosses of Central Florida, the subject of one of my current projects. This will ultimately be part of a pictorial key and guide to the mosses of this area. So far about 77 species have been recorded in our area, centering on Hillsborough County, and including the neighboring counties of Pasco, Hernando, Pinellas, Manatee, and Polk. More are sure to show up as we look more closely.

Looking closely is the key to finding and identifying the different kinds of mosses.  They are mostly small, and at first all look pretty much the same.  But there are huge differences in both vegetative and reproductive structures that make that closer look most rewarding.

I begin with one of the most abundant species in our area, Leucobryum albidum (Brid. ex P. Beauv.) Lindb. (Family Dicranaceae).  This species forms large, conspicuous mats on the dry ground of shady pine flatwoods and oak hammocks.  Like most mosses, Leucobryum soaks up water from rainstorms and dries out inbetween.  The leaves of this species are unusually thick.  There are several layers of cells, with only the single central layer green.  Above and below are essentially empty cells that fill up with water during a rainstorm.  In most mosses, leaves are just one cell thick, and dry up rather quickly during dry weather.  Mosses in general can tolerate desiccation for rather long periods of time, and spring back to life quickly when wet.

The distinctive succulent leaves of Leucobryum are short, stiff, and folded lengthwise, creating a distinct upper groove.  The leaves are crowded at the ends of upright stems, and the sporangia emerge from the tips of the stems after union of sperm and egg.  The sporangia are turned to the side and look distinctly like the heads of some kind of wading bird.

Another species, Leucobryum glaucum (Hedw.) Ångström has longer leaves, but is not as common, and grows in somewhat moister areas.

Thursday, August 30, 2012

The Great Botanical Butter Battle Book

In recent decades, there have been many often drastic changes in the taxonomic system of flowering plants (and indeed all organisms).  This is due to radical changes in taxonomic philosophy that began in the middle of the 20th Century, and which were in turn the result of vastly improved techniques for determining the pattern of evolutionary relationships among organisms.  Usually referred to as phylogenetic taxonomy, the primary philosophical goal of this approach is to make our classification system reflect the evolutionary history, or phylogeny, of organisms as exactly as possible.  Our traditional classification system deviated from this ideal in many of its details, and so the classification of organisms has changed considerably in the last 50 years. 

In a recent posting, I described the fate of the Snapdragon Family, which has been taxonomically “blown to smithereens” because it was polyphyletic.  A polyphyletic taxonomic group (taxon) is one that has members descended from very different ancestors, i.e. their resemblance to one another is superficial. Similar body forms in unrelated organisms with very different anatomy and chemistry, not to mention genetics, is due to independent adaptation to similar environmental conditions, or convergent evolution.  The former classification of whales and porpoises, along with tuna and salmon, in the category of “Fishes” is an extreme example.

No one really disagrees that such strange bedfellows need to be separated and returned to their true relatives, i.e. to be arranged in monophyletic categories - by definition a common ancestor and all its descendants.   Much of the taxonomic busywork of the past century has been to ferret out illicit polyphyletic associations and rearrange organisms into a classification based on true relationship.  That’s not where the butter battle comes in, however.

 The classic Butter Battle Book by Dr. Suess satirized the human propensity for escalating silly disagreements into cataclysmic space-time-continuum-threatening warfare.  The story began with an argument between two groups of whimsical creatures, one of which believed passionately that toast should be eaten butter-side up, and the other that it should be eaten butter-side down.  Something akin to this feud began in the botanical world during the latter part of the 20th century, and it’s not over yet.  It wasn’t about polyphyletic groups, but about a much subtler distinction that results in some traditional taxa being called “paraphyletic.”

Both traditional and phylogenetic systems of classification have the form of a hierarchy of categories.  The level of the hierarchy is referred to as ranks.  Domains are currently the highest rank of taxon, and they are subdivided into Kingdoms.  Progressively smaller subcategories are given lesser rank.  The  principle ranks are Domain, Kingdom, Phylum, Class, Order, Family, Genus,  and Species, and in-between ranks (subclass, superorder, etc.) can be inserted as needed.  

Phylogenetic taxonomy has two basic rules designed to make a classification system explicitly reflect evolutionary history:

1.  Only monophyletic groups - by definition a complete branch, or clade, of the phylogenetic tree, with the common ancestor and all of its descendents, can be formally recognized as taxa at each level of branching.  
 2.  Each branch point on a cladogram results in two sister taxa, which must be given equal rank (genus, family, class, etc.).  

   Applying these two rules, it was found that many traditional taxa were neither polyphyletic nor monophyletic, but fell into a fuzzy gray area called paraphyletic.  Paraphyletic taxa are incomplete clades (violating rule 1) - some part of the clade that should be just a subclade has been improperly promoted to the same rank as the main clade (violating rule 2 since they are not sister clades).
The traditional classification of higher vertebrates was
simple and intuitive, but misleading about
evolutionary relationships.  Clades A through F
retain certain ancestral characteristics, such as
scaly skin, that we associate with reptiles, while
the birds evolved highly distinctive wings, flight
feathers and other specialized features
associated with flight.
     This is probably as clear as mud at this point.  Let me use a familiar  example from the animal world to quickly explain this concept.  Reptiles and birds are two of the traditional classes of vertebrates, but modern studies have overwhelmingly confirmed that birds descended from reptiles, in particular from a group of dinosaurs that included velociraptors and Tyrannosaurus.   While birds are a monophyletic group, the reptiles are paraphyletic (also referred to as a grade to differentiate it from a clade).  

In the greatly simplified diagram to the right, clades A through F are different groups of reptiles.  Technically, the sister group of birds is clade F, which includes their closest dinosaur relatives.  Therefore, according to the rules, birds must be have the same rank as group F.  Birds are still a distinct group, but taxonomically must be considered just a highly specialized subgroup of reptiles.  Note also that the traditional classification obscures the fact that group F is much more closely related to birds than it is to group A (turtles and their relatives).  Remember that it is the goal of phylogenetic taxonomy to better reveal these relationships in the classification system.

Taxonomically, there are actually three choices in such a situation:  1) keep the status quo, recognizing the paraphyletic Reptilia and the monophyletic Aves as two equivalent classes.  This would be the preference of some traditional taxonomists; 2) combine the reptile and bird classes together into one class (“lumping”), in which birds become one of the subclasses of reptiles along with groups A through F; or 3) keep the birds as a class, but also promote each of  the main clades of reptiles to the rank of class, creating several additional classes (“splitting”). 
An acceptable solution in phylogenetic systematics
is to lump reptiles and birds into a single class, with
birds recognized as a subclass (along with clades A
 through F)

The alternate solution recognizes many classes of
reptiles along with the birds.
The second two options are equally acceptable to phylogenetic taxonomists, but this creates a dilemma with each paraphyletic situation encountered: Do we fragment the paraphyletic taxon (reptiles) into a number of smaller classes or submerge the distinctive group at the top (birds) into one big class with the reptiles?  Either way, the taxonomy reflects the phylogenetic history of the group, but one way ends up with a lot of smaller classes (harder to remember them all) or fewer big classes (obscuring the distinctive, sometimes revolutionary, adaptations of the top group).  This ranking decision is pretty much subjective.

This is all about the formal classification used primarily by scientists.  We can still talk about reptiles and birds informally.   No one is obligated to include birds in a field guide of common reptiles, for example. 

To finally get around to plants, I'll mention first in passing that the traditional classification of flowering plants into dicots and monocots created exactly the same situation as the reptiles and birds.  Dicots turned out to consist of a grade of separate clades, with monocots occupying the bird position at the top of the tree.  It still has not been fully agreed upon as to how to rank these clades.

Walter Judd, Roger Sanders and Michael Donoghue, in a truly landmark paper in 1994, identified a number of pairs of traditional angiosperm  families, of which one was paraphyletic and theo other monophyletic.  In most cases, the monophyletic family at the top of the tree was primarily temperate in distribution and consisted primarily of herbaceous plants, while the clades branching along the grade consist mainly of woody trees and shrubs.  The pairs include: Apocynaceae (oleander family)/Asclepiadaceae (milkweed family), Araliaceae (Aralia family)/Apiaceae (carrot family), Capparaceae/Brassicaceae (mustard family), among others.  To keep this posting under control, I’ll just talk briefly about the first case.

The Apocynaceae and Asclepiadaceae share a number of characters inherited from their common ancestor.  Each clade that branched off of the Apocynaceae grade had some distinctive characters, but were not deemed worthy of family status.  The upper subclade we call Asclepiadaceae, however, had some very innovative and distinctive floral characters that allowed this group to blossom into a “family” of many related genera.  The conclusion of Judd, Sanders, and Donoghue was however that the Asclepiadacease should be submerged into the Apocynaceae.
The traditional Apocynaceae, like this
Pachypodium, have relatively
conventional flowers with petals united
into a flaring tube that often twists like a

I’ve provided this lengthy explanation as a prelude to getting into the real butter battle, which may take one or two more posts to explore fully.  It is first of all a battle between two views of how we divide up an undisputed diagram of relationship into taxonomic entities that are convenient, practical, and informative.  Phylogenetic taxonomists, in forbidding  paraphyletic taxa, are emphasizing information on phylogenetic branching patterns.  Clades are real entities that represent evolutionary history and groups of related organisms.  They are predictive of the characteristics to expect within the group.  Recognizing birds and reptiles as equivalent groups obscures the fascinating relationship of birds to dinosaurs, and the fact that Tyrannosaurus rex is much more closely related to ostriches than it is to turtles. 
The Asclepiadaceae, represented by this milkweed
(Asclepias), has the same basic features as the Apocynaceae,
but adds some unique floral features related to specialized
modes of pollination.

 Traditional taxonomists, while probably not much concerned about the reptile/bird issue anymore, believe that allowing some paraphyletic groups to be recognized as taxa is not only harmless, but also desirable in order to simplify classifications and create user-friendly, easily recognizable taxa, and  to recognize highly innovative groups, like the birds or the Asclepiadaceae, as distinctive taxa of higher rank.  Birds have achieved the level of distinctiveness that is worthy of a new class of organisms, while the other clades of reptiles are still basically reptilian.  The differences between them are technical and not readily evident to the non-expert.   They point out that if one wants to see the actual pattern of relationship, one can look directly at the phylogenetic trees.  There is no need to create cumbersome, non-intuitive classifications just to duplicate the information that is easier to see in the diagrams themselves.  This sentiment is probably stronger among botanists that among zoologists.  

  Phylogenetic taxonomists appear to be winning the day, but there are many holdouts among traditionalists.  For the latter, the distinctions between reptiles and birds, dicots and monocots, Apocynaceae and Asclepiadaceae, are all simple, intuitive,informative, and useful.

There is, moreover, another dimension to the debate brought up by traditionalists that is not so easily dismissed, and which we might sum up as the “ancestor problem.”  This arises when one views, theoretically at least, the entire sweep of evolutionary history.  The first member (common ancestor) of every genus (or family, etc.) logically had ancestors that were in an older genus.  Whenever a new genus-worthy group of species became distinct, it would have rendered its ancestral genus paraphyletic (just as the evolution of birds as a distinct new class of organisms rendered the reptiles paraphyletic).  So the evolution of new genera (or families or classes) appears to be impossible according to the rules of phylogenetic taxonomy, or else to force the retroactive fragmentation of the older taxa. I will take up this tricky issue in a future posting (See "Making the Ancestor Problem Go Away,"  October 18, 2012).

Wednesday, July 11, 2012

The symmetry of plants

This Ficus tree has a broad canopy of leaves and
a broadly spreading root system connected by
a narrow trunk. Photo by William A. Essig.
I recently had the sad task of helping my family sort through the belongings of my late uncle Bill.  Bill had traveled a lot during his retirement, and one of our jobs was to sort through boxes of his travel photos.  One that particularly grabbed my attention, was a photo of a Ficus tree growing in Hawaii.  It reminded me not only of the magnificence of trees in general, and Shel Silverstein's book, "The Giving Tree,"  but also of the question of symmetry in plants.

Coral polyps have a wheel-like, or radial symmetry,
when viewed from the top.  You could slice it into
identical halves by cutting through the center at any
One of the things you learn in introductory biology or zoology, is that there are two basic symmetry patterns in animals: radial and bilateral.  Radial, as the name implies is a circular or wheel-like symmetry, such as found in sea anemones and corals.  Looking down on the top of such an organism, you can see that it could be cut in half at different angles and the two halves would always be essentially identical.  The only requirement is that the cut go through the center.  A sea star approximates radial symmetry,  but is off in the placement of some important organs.

A higher animal, like a lizard, can be split down
the center, from head to tail, to create two mirror-
image halves.  Cutting it any other way creates
very different looking halves.
The alternate symmetry, bilateral, means that the animal has two sides that are mirror images, but only if the cut is made a certain way.  Most familiar animals are built this way.  In humans, if you make the (imaginary!) cut, it has to follow the spine exactly, cutting the head and torso into equal left and right halves.  Any other cut, say from left to right, results in pieces that are quite different (e.g. front and back).  To make a long story short, bilateral symmetry is an important adaptation in animals for moving around, for pursuing food or mates, or for escaping enemies.  It results in paired legs and eyes, and puts the mouth at the forward moving end.  True radial symmetry works best for animals that sit in one spot gathering food from water currents.

What about plants - do they have symmetry?  Plants don't move, so symmetry would have a different meaning, if it exists at all.  In fact it does, but it's more complicated.  Plants exhibit both radial symmetry and bilateral symmetry, often at the same time. In flowers, it has a lot to do with pollination strategy.  A sunflower head has radial symmetry, allowing small insects to land on top, while a snapdragon flower has bilateral symmetry, inviting large bees to enter from the side.

The whole plant can have one or both symmetries also, as discussed by naturalist Francis Halle (2002).  Plants that are anchored to a single spot, like trees, exhibit an overall symmetry that is roughly radial. The weight is more-or-less evenly balanced around its central axis, which extends down the trunk into a woody taproot system.  A tree could hypothetically be spun around its central axis without affecting its environmental orientation. Like a coral polyp, a tree  gathers resources -sunlight and carbon dioxide - that are widely dispersed, and so must spread a wide net from its central trunk. Of course the symmetry is not exact because of the random nature of tree branching, but overall the crown of a typical tree is a rounded dome.  This symmetry is more obvious and exact in something like a tree fern or a single-stemmed palm tree, like a date palm, though these do not have woody taproot systems.

A woody tree or shrub has
an hour-glass symmetry, as
the massive crown is
balanced by an
equivalent woody root
Palms are not woody, but their
crown of massive leaves is balanced
at the other end of the trunk by a mass of a
adventitious roots.
The radial-symmetry of a tree has another dimension, a waist-like constriction, something like the shape of an old-fashioned hourglass. (Remember the Wicked Witch of the West showing Dorothy how much time she has left?)  A tree, like an hourglass, has has a broad top, a narrow waist, and a broad bottom.  the broad crown of the tree converges into the relatively narroy trunk, and the trunk then flares out into a root system that roughly mirrors the crown.  The roots of a tree spread widely in pursuit of thier own set of widely scattered resources: water and minerals.

Creeping ferns, gingers, or irises, on the other hand, have a roughly bilateral symmetry.  Each stem, or rhizome, "travels" horizontally through the soil over time, with the apical meristem creating new tissues as the older end of the rhizome ages and disintegrates.  As a rhizome grows forward,  it sprouts roots on its lower side and a series of leaves or leafy shoots on its upper side.  Such a plant does in fact move, albeit slowly, and through branching can come to cover a large area and live indefinitely.  One can make a longitudinal cut along the center of a rhizome, and the two halves will be more-or-less the same. Spinning such a plant around its stem axis would result in leaves and roots alternating in the wrong environment.

A liverwort body is a simple, bilaterally symmetrical ribbon
that grows at its tip. The specimen is Pallavacinia lyellii 
growing in central Florida.
The very first land plants, which probably resembled modern liverworts, had a ribbon-like structure that rested flat on the ground and grew horizontally from a primitive apical meristem.  Liverwort ribbons therefore  each have a bilateral symmetry, though they can branch to form  complex colonies.  Their distant cousins, the mosses, sometimes develop upright leafy shoots with radial symmetry.

Sphagnum moss forms miniature forests of upright,
tree-like shoots that emerge from underground stolons

Ancient vascular plants, like ferns and club mosses, grow from horizontal rhizomes, but sometimes have radially symmetrical upright shoots.  Creeping plants all produce new (adventitious) roots from their stem tissues as they go.  The evolution of real trees from such ancestors was a remarkable event.  Invention of secondary growth and wood was only part of the story.  The more radical change was the reorientation of the plant into a single, vertical, bipolar axis that included a woody branching root system extending down directly below the trunk, as pictured above.

The trunk of a banana plant is a pseudostem of soft
leaf sheaths rising from an underground rhizome.  The
leafy shoot is roughly radially symmetrical.  From Brown,
The Plant Kingdom, 1935,  Fig. 91.

Monocots reinvented creeping bilateral symmetry in their primary stems, but often have radially symmetrical upright shoots. New banana shoots, for example, sprout from short rhizomes  that have budded off of an older plant, and these upright shoots have leaves spreading out in a circular pattern that minimizes them shading each other.  Many palms, those referred to as "clumping," likewise spread by rhizomes, but send radially symmetrical shoots upwards.

In this historic photo from Singapore,
a traveler's palm, Ravenala madagascariensis,
has created a massive fan-shaped crown,
giving it a bilateral symmetry.
The upright shoots of a few plants, inexplicably have their leaves arranged in a different sort of bilateral symmetry, that of a fan.  The most spectacular example is the traveler's "palm" (not really a palm, but a member of the bird-of-paradise family).   The leaves of a bearded iris, and even those of common daylilies also present their leaves in a fan shape.   I'm still working on why plants would adopt such a configuration.

Reference: Halle, F.  2002.  In praise of plants.  Timber Press.  Portland, Oregon.

Tuesday, June 12, 2012

Plants that Generate Heat

A few weeks ago I took you on a field trip to the marshy meadows around Longmire, in Mt. Rainier National Park in Washington.  The most spectacular early spring wildflowers there are the western skunk cabbage. These exotic-looking members of the mostly tropical aroid family (Araceae) seem totally out of place here in the cool woods of the Pacific Northwest.  In May and June, it's still cold at night, the snow has just recently melted, and the water in the streams and marshes is frigid, yet these extravagant plants are in full bloom.

The eastern skunk cabbage (Symplocarpus foetidus)
generates enough heat to melt its way through snow
in early spring.   Photo by Sakaori, from
Wikimedia Commons
The eastern skunk cabbage blooms even earlier, even poking up through the snow.   The inflorescences of these plants actually generate heat, enough to melt the snow above them and continue their reproductive activities when most other plants are still hunkered down underground.  Presumably, this allows them to take advantage of the pollination services of some of the earliest insects to emerge in the spring.

But how and why do these plants do this?  Surprisingly, this ability evolved before the ancestors of these tropical plants moved northward.  Many tropical aroids living in 365/24/7 jungle steam baths also generate heat.  They do it also to attract pollinators, and that is the original reason.  The ability to melt snow is a bonus. Heat generation in plants is similar to that in animals, by the burning of carbohydrate or sometimes lipid reserves.

Philodendron bipinnatifidum (formerly
P. selloum) generates heat to attract
pollinating beetles.  Photo by
Tekwani, Wikimedia Commons
Tropical aroids that produce heat include species of  Philodendron and Amorphophallus, the dead horse arum (Helicodiceros muscivorus), and voodoo lily (Typhonium [Sauromatum] venosum).  The practice is not confined to aroids, but is also found in some waterlilies (famly Nymphaeceae), sacred lotus (family Nelumbaceae), and in a number of other plant families.  It is most often found associated with pollination by flies and beetles, dull-witted insects that find flowers primarily by scent.  The heat in some cases helps vaporize and disperse the scent of the flowers, and in the plants growing in cold places, it may provide a refuge for the insects, who sometimes spend the night where orgies of reproductive activity may take place. Temperatures up to 35 degrees C above ambient temperatures have been reported.  (See the essay by Roger Seymour at Plant Physiology online.)

Monday, May 28, 2012

Which organisms live the longest?

Pine Alpha, in the White Mountains of
California, was one of the first trees to
be dated at over 4000 years old.  The
oldest trees are no longer marked, in
order to avoid vandalism.  Note that
the tree is mostly dead.  A few living
twigs are sprouting from the backside.
Photo by Frederick C. Essig.
What are the oldest living organisms?  It all depends on how you set the rules.  Botany students may read in their texts that the bristlecone pines (Pinus longaeva) of California are the oldest, clocking in at nearly 5000 years.  Many species of tree can live 2000 years or more, as measured by counting their "rings" - the layers of wood laid down annually by the vascular cambium.  Trees are certainly the easiest individual organisms to date, but  are they really the oldest?

The question is not quite as simple as it seems.  I could make the argument that no bristlecone pine tree, or any tree, is more than 10 years old. The fact of the matter is that most of the 4000 rings in the tree are dead.  The living tissues of the bristlecone pine consist only of a thin living bark, the root tips, the leaves, and twig tips of the most recent growth.  No cell, tissue, or leaf is more than a few years old.  So, yes, a bristlecone pine tree has been sitting at this spot for more than 4000 years, and the current growth is the result of the continuous activity of meristems that originated in a single seedling that germinated  long ago, but none of the living parts are very old. The tissues of the tree constitute a genetic clone living on the edge of an accumulation of dead wood.
A tree is comparable in some ways to a coral colony.  The individual coral polyps live a short time, but build a small "house" for themselves on top of previous generations of houses.  The accumulation over many years results in a coral reef.  Black coral colonies have a distinct tree-like structure, and their age can be determined by techniques similar to counting tree rings  (  Some have been found to be about the same age as the oldest bristlecone pines.  The individual coral animals, or polyps, reproduce asexually, increasing the number if individuals and the number of branches, and all trace back to the original polyp that started the colony, so like the tree, there is a genetic continuity of the clone that is quite old, though no individual polyp is very old.  Clonal organisms, even those that leave remains like wood or a coral skeleton, represent longevity of genotypes, not of a functioning set of organs and tissues. 

A fairly well-documented article in Wikipedia, a "List of long-living organisms," explores different ways in which organisms can get old.  A number of other clonal plants and animals are mentioned.  Some clonal plants are estimated to be tens of thousands of years old, not because they leave layers of wood, but because they spread slowly outward, sprouting new roots and abandoning the original center of the colony to decay.  They form circular colonies similar to a fairy ring of mushrooms (fungal colonies likewise are potentially quite old).  A colony of quaking aspen (Populus tremuloides) - trees that propagate asexually from their root system, have an age estimated by some at over 80,000 years, but could be much older.  

Clonal plants that spread by horizontal
rhizomes, like this giant Gunnera, may
potentially be the oldest living "individuals,"
but  their age cannot really be measured.
Sea grass colonies have been estimated to be 200,000 years old, and it is theoretically possible that some ancient fern rhizomes have been creeping around for millions of years, leaving no traces of earlier rhizome segments by which to document their age.  In short, clonal plants are potentially immortal.  But does that really count? What is an "individual living organism?"  Do we define it genetically, or in terms of  how long tissues and organs keep going?

If we're talking about an integrated set of organs and tissues in a discrete individual, than a 120-year old human being is more impressive than any plant.  Some of our tissues turn over during our lifetime, but some do not.  I'm amazed that my own brain is still working at all!  Some kinds of tortoises can live more than 170 years, and koi fish have been reported to be over 200 years old.  Invertebrates like sea urchins and bivalve molluscs have been recorded at more than 200 years as well. (Sponges may live for 10,000 years or more, but are technically more like clonal colonies than individual animals.)  Animals are different from plants, beginning small, but complete, and getting bigger with age.  We have only one set of legs, eyes, etc., that have to last a lifetime.  So we are really older than any tree or clonal organism.

So you can make an argument for many different "oldest" organisms, depending on how you set the rules.  

Friday, May 4, 2012

The Invention and Reinvention of Trees

Most trees - plants with permanent, elevated,  leafy shoot systems - depend on wood for physical support and nutrient transport.  Wood consists of annual layers of secondary xylem, deposited by a cylindrical meristem called the vascular cambium.  The vascular cambium in most trees also lays down rings of secondary phloem, the necessary sugar transport tissue that carries food from the leaves back to the roots and other developing organs.  This is the standard model of trees and shrubs found throughout the gymnosperms and dicots (eudicots, magnolids and other ancient flowering plant lineages). 

Getting tall has its advantages in competing for light, dispersing seeds, etc., and on numerous occasions,  plants without a vascular cambium have found ways to do so.  Though perhaps not strictly-speaking trees, they are all interesting experiments that lasted for millions of years, or are still with us (e.g. palms, bamboos). The monocots, in particular, have a number of different forms of gigantism arising from rhizomatous ancestors all without any wood at all.

These tree ferns, growing in the temperate
rain forest of Australia, achieve considerable
height with their root-clad, upright rhizomes.
One very ancient form of non-woody gigantism, the tree fern, is still with us.  All forms of tree-like growth begin with low-growing herbaceous plants, usually with an underground rhizome system.  In the case of the tree fern, the rhizome has essentially "gone vertical."  This slender upright stem is strengthend by masses of fibers, but no wood.  It has no secondary growth and its "trunk" does not get thicker over time.  It is just about as thick at the top where the stem tissues are being laid down, as it is lower down. The thickness of the tree fern stem is enhanced by a thick mantle of fibrous  adventitious roots  (the tree fern fiber of horticultural commerce) that collectively serve as a water-absorbing sponge.  A massive terminal bud makes a single rosette of large compound leaves atop the thick stem apex and rarely branches.  Plants of this general form are sometimes referred to as pachycauls (“thick stems”), or rosette trees.  Palm trees and cycads are other common examples. 

Lepidodendron and Sigillaria were ancient relatives
of modern clubmosses.  Like the giant horsetails
featured earlier in The First "Bamboos," they had
meager layers of wood, but no secondary phloem.
From Smith, Cryptogamic Botany. 1955, Fig. 128

The first upright plants with a vascular cambium that produced layers of wood developed in parallel among club mosses and horsetails.  The problem was that they could not also produce layers of secondary phloem (food-conducting tissue) toward the outside, and their longevity was limited by that of the original phloem.   When a vascular cambium came along that could alternately produce xylem to the inside and phloem to the outside, truly large and long-lived trees became possible, and this led to the early explosion of seed plants (the first such trees were actually the seedless progymnosperms, which are believed to the the ancestors of the first seed plants).

As discussed earlier, the first monocots were seed plants that returned to the ancient underground lifestyle.  In the process, they lost all ability to make a vascular cambium.  So when various monocots found themselves in situations where getting taller would be advantageous, they had to reinvent the wheel, so-to-speak.  Bamboos spread underground via rhizomes like ordinary grasses, but their hollow, upright, leafy shoots have gotten taller and taller over time, adding thick bundles of fibers to their culm walls to support that upright growth.  In parts of Asia, they are aggressive enough to displace ordinary trees for many square miles.

The trunk of this Pigafetta palm growing
in Papua New Guinea, develops its full
thickness at the top, as the massive leaf
bases expand.

Palms like this Ptychosperma develop many
upright stems from a branching
underground rhizome system.

Palms appear also to have originated from underground plants.  Many still spread by rhizomes like the bamboos.  Their upright leafy shoots are not hollow, but filled with hard fibrous bundles, or sometimes with a softer, food-storing center (e.g. the true sago palms, genus Metroxylon).  Some, like the tropical mangrove palm, Nypa fruticans, retain a basically horizontal position, with only leaves and flowerstalks rising vertically.  The saw palmetto of Florida (Serenoa repens) has a similar habit with its stems mainly lying on the ground and occasionally turning upward.  Those palms that become solitary rosette trees, like the coconut, are actually exceptional in having given up their rhizomatous underground system.  They, like all monocots, lack secondary growth, but have enormous buds atop an expanded shoot apex, which is as thick as most of the rest of the trunk. 

Philodendron selloum achieves some modest
height by supporting its stem with prop roots.
Some would-be monocot trees don’t have quite such a thick trunk, but produce a series of adventitious prop roots, both for support and for additional water-absorption.  A simple example is Philodendron selloum, whose relatives usually climb up trees.  Though it can’t claim the tree-like dimensions of palms or bamboos, it is a giant within its family (Araceae).

The screw pine (Pandanus) is a monocot with
long strap-shaped leaves and a fibrous
trunk similar to that of palms.  It supports itself
with prop roots
This giant Pandanus in a New Guinea forest
has prop roots six inches thick.
The screw pines (genus Pandanus) also rely on prop roots to support their upward growth, but are able to achieve true tree size and compete with forest trees.  Unlike palms, screw pines sometimes produce a number of branches, but without secondary growth, the branches are progressively and permanently thinner as they spread their crown. 

Another approach to tree-ness is seen in bananas and some gingers.  What appears to be a trunk is actually mostly the concentric cylindrical bases of the leaves (the leaf sheathes).  Each new leaf that pushes up through the center of this false stem (pseudostem) has a longer cylindrical base than the previous, and so can achieve the proportions of a modest tree.   The true vertical stem rises through the center of the pseudostem only when it is time to flower and fruit.

The herbaceous pseudostem of a banana shoot
builds up as each tubular leaf sheath that
pokes up through the center is longer than
the previous one.
Banana "trees" are really giant herbs.  The soft shoots
bud off of an underground rhizome system and die
after fruiting

A cross-section of a banana trunk
reveals the nested series of leaf sheaths
from which it was built.  The solid circle
in the center is the stalk of the
inflorescence, which pushes the cluster
of flowers to the top of the plant. From Brown,
The Plant Kingdom, 1935. Fig. 92

A most interesting case of monocot gigantism is seen in the Egyptian papyrus (Cyperus papyrus in the Sedge Family, Cyperaceae).   This source of ancient paper and floating bassinets for infant prophets, is mostly a long smooth stem arising from an underground rhizome, with a crowded tuft of grass-like leaves and flowerstalks at its tip.  The smooth stem, from which the valuable fiber is obtained, can be 3 meters tall, and consists of a single elongate internode.  Other sedges have a similar stalk for elevating flowers above a grass-like clump, as do familiar plants like onions and amaryllis.  So papyrus “trees” are basically overgrown flowerstalks.

Papyrus shoots arise from underground rhizomes through the
elongation of a single internode at the base of the globe-shaped
cluster of leaves and flowers. From Kerner and Oliver, The
Natural History of Plants, 1904.

The globe-shaped cluster of leaves and
flowers of the Egyptian papyrus plant are
lifted to tree-like proportions by the
elongating flower stalk.

The most tree-like of all monocots are found in Dragon trees and their relatives (in the genera Dracaena and Cordyline) and in giant aloes.  Though they do not have a conventional vascular cambium, they have evolved a new way of expanding the older stems with a cambium-like layer that continuously produces whole new vascular bundles containing xylem and phloem.

The dragon tree (Dracaena) adds layers of whole
vascular bundles to continually thicken the stems.
From Kerner and Oliver, The Natural History of
Plants, 1904.
So the monocots have been successful in becoming tree-like in a variety of ways without a conventional vascular cambium, adding to their reputation as a varied and highly successful group of plants.