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.





The underground plant movement

Plants in general must bask in the sun day after day to soak up the energy they need for life activities.  In doing so, of course, they face certain hazards.  Being immobile, they are essentially "sitting ducks" for everything from plant-munching animals to fire, drought, and winter freezes.  As discussed earlier in "How the grass leaf got its stripes," plants in general employ one of two basic strategies for coping with such hazards.  Trees and shrubs tough it out with permanent woody shoot systems that resist damage with protective tissues or toxins or by dropping their leaves during the adverse seasons.  Their dormant buds are typically wrapped in tough scales.  They may simply rise above threats from grazing animals or ground fires, as do the Acacias of the tropical savannas.  Their competition with one another for light often results in forests. 

Grasses dominate in areas where moisture is too sparse for forests.  They have plenty of light, and escape most environmental threats by keeping their main stem system and buds below ground.  Like the periscopes of submarines, their leafy shoots and flowerstalks rise above ground to do their business, sending food reserves to storage organs below ground, or drawing upon those reserves to make seeds.  When things get too rough upstairs, these aerial shoots are abandoned, and replaced when better growing conditions return.

Grasses are not the only plants to "go underground."  A great many perennial herbs follow similar strategies, as do the unique class of plants called biennials.  They all achieve similar results, but what differs is the nature of the underground storage structures.  In different groups of plants, roots, stems, and even leaves, are recruited for this job.

The carrot, Daucus carota (Apiaceae) is mostly a thick taproot.
A short stem at the top produces the cluster of leaves.  Photo
by jonathunder, Wikimedia commons.

In most biennial herbs, a specialized taproot, or sometimes a hypocotyl  (to be explained shortly!) swells up with food reserves, resulting in a carrot, radish, beet, parsnip, turnip, or one of many other type of root vegetable.  These are radially symmetrical plants with an axial root system, similar to trees and shrubs.  The expansion of their storage organs in fact is due to the activity of a type of cambium similar to that which produces layers of wood in a tree.  They can be considered highly reduced trees in this respect. 

Biennials store up food reserves during their first season of growth, and use up those reserves making flowers and seeds during the second season.  Then they die.  Of course such swollen taproots are great sources of food for humans and other animals, and we cheat them out of reproducing by harvesting after the first season.

In the radish, Raphanus sativa (Brassicaceae), the
short section of the seedling stem below the cotyledons
(the hypocotyl) swells into a storage organ. From Transeau, et
al., 1940, Fig. 114.

All of the above-mentioned "root vegetables" appear to be actually roots, but some are actually a different, adjacent part of the plant.  When a seedling emerges from a seed, it consists of three sections: the root, the shoot, and something inbetween: the hypocotyl ("below the cotyledons").  The hypocotyl is the usually inconspicuous section of stem between the root proper and the seedling leaves (cotyledons) that mark the beginning of the leafy shoot.  In some biennials, such as radishes, it is this tiny section of stem that swells into the underground storage organ, not the root proper.  This is of utterly no importance to the master chef, but an essential piece of information for the botany-geek-wannabee.



In the sweet potato (Ipomoea batatas),
adventitious roots from the vine swell to
become storage organs. Photo by H. Zell,
Wikimedia commons.

A sweet potato (Ipomoea batatas in the Morning Glory Family, Convulvulaceae) is also a root, but not a taproot as in a carrot. A sweet potato forms from an adventitious root that emerges from the vine creeping along the ground. Similar are the tuberous roots of Dahlias.  Regular ("Irish," "white," "Idaho") potatoes, on the other hand are specialized stems called tubers. The "eyes" on a potato are buds that can sprout into new leafy shoots.

The potato, Solanum tuberosum, is a
swollen underground stem with many
buds ("eyes") that can develop into new
leafy shoots. Photo by Donna, Wikimedia
Commons.

A ginger rhizome (Zingiber officianale), with a new rhizome
section developing (upper right).

The most widespread kind of underground stem is the rhizome.  This horizontal stem creeps along under the ground, or just above it, branching to form new rhizome sections, and sometimes expanding into an extensive colony.  A ginger or bearded iris is a good example of a rhizome that is also a food storage organ.



A new shoot is developing on top of the corm of this
Amorphophallus titanum. It will form a new corm at its
base, while the old corm withers. Photo by stickpen,
Wikimedia commons.

A more specialized type of underground stem is the corm.  Often confused with a bulb (see below), a corm is a stem filled with solid storage tissue.  Found mostly in the Iris and Aroid Families, but also in water chestnuts (Sedge Family). Corms are short and fat, with a single dominant bud facing directly upward.  As that bud begins growth, the base of the new shoot swells to form a new corm on top of the old one, and sends out new adventitious roots.  The old corm gradually decays, and the newest corm is pulled down to replace it by the roots, which physically contract and shorten (contractile roots).


An onion, Allium cepa, is a bulb
made up of the swollen, concentric leaf
sheaths of regular foliage leaves. Photo
by Amada44, Wikimedia Commons.

Finally, a true bulb is actually a big underground bud, made up of a cluster of modified leaves or leaf bases that swell up with food.  An lily, onion, or amaryllis bulb is a good example.   The bulb of the onion or amaryllis is made of of the concentric fleshy leaf sheaths of ordinary leaves. As each new leaf forms in the center of the bulb, it produces a long photosynthetic blade that emerges from the top, while the cylindrical sheath slowly expands within the bulb.  The collective fleshy sheaths ("onion rings") constitute the dormant bulb when the leaf blades wither at the end of the growing season. The oldest leaf sheaths on the outside of the bulb eventually dry up, becoming thin and paper-like.

In the true lily (Lilium), on the other hand, the bulb is a loose cluster of short, modified leaves, swollen with food and water. Like the vegetative leaves that develop on the elongate aerial shoot, these leaves have a relatively narrow base, rather than a cylindrical leaf sheath.

In the bulbs of Lilium (left) the food-storage organs are short, modified leaves, rather than the
sheaths of vegetative leaves.  From Brown, the Plant Kingdom, 1935.

License for photos from Wikimedia Commons: http://commons.wikimedia.org/wiki/Commons:GNU_Free_Documentation_License_1.2

Whatever Became of the Snapdragon Family?

For centuries, botanists have grouped similar plants into the taxonomic category of family.  Plant families are a very important way of recognizing relationship and predicting useful characteristics.  For example, if we find a valuable medicinal property in one member of a family, we are more likely to find similar properties in other members of the same family than in species outside the family.   Recently, I wrote about the important Grass Family, and the similar edible properties of many grain species in the family.  Other familiar families include the Rose (Rosaceae), Tomato (Solanaceae), Legume (Fabaceae), and Sunflower (Asteraceae) Families.

Butter-and-eggs, or Linaria vulgaris, is a
close relative of the snapdragon, and
no longer in the Scrophulariaceae.

The Snapdragon Family (Scrophularaceae, also known as the Figwort Family) used to be a happy but rather large family that included snapdragons, mullein, monkeyflowers, foxglove, veronica, torenia, Indian paintbrush, calceolaria and many other attractive garden and wild flowers.  It is not a major family for medicinal plants, but the foxglove is the source of the important heart stimulant, digitalis. Many other members of the family (e.g. snapdragons themselves) provide ornamental bedding plants worth millions worldwide.

Sadly the snapdragon family has suffered a nasty divorce, and what we now call the Scrophulariaceae no longer contains the snapdragons.  The old Scrophulariaceae has in fact been split into at least six smaller families (Olmstead, et. al 2001). The pieces, it was found, were not closely related to each other, some pieces were actually closer to other families, including the mint, gloxinia, and verbena families.

The family is a victim of the more precise analytic tools of the late 20th century and the new phylogenetic taxonomy. The similarities among the members of the old Scrophulariaceae were superficial, it turns out. The general flower shape and form of the seed capsule evolved many times from different ancestors (convergent evolution) because they were adapting to similar pollinators and seed dispersal strategies. In sum, the old family was polyphyletic. 

The story involves all the drama and complications of modern plant taxonomy.  For two centuries, botanists have worked to define more "natural" families.  Natural families contain genera that share fundamental characteristics, including similar structural features of flowers and fruits.  In the late 19th century the word natural took on the new meaning of evolutionary relationship.  Following Darwin's lead, biologists interpreted fundamental similarities within families, genera, and other categories as due to common ancestry.  For the next century or so interpretations of natural similarity and evolutionary history went hand in hand.  More and more information from anatomy, chemistry and genetics became available, often confirming, but sometimes toppling earlier assumptions about relationship.  The information available became so huge that putting it altogether into a master classification of plants relied on the experience and gut feelings of a handful of the leading taxonomists.  But the classifications of these wise old men often differed markedly, as much of the information could be interpreted in different ways.

In the middle of the twentieth century, there were many attempts to decipher evolutionary history (or phylogeny) with rigorous and objective statistical tools.  The most successful of these was cladistic analysis, a process in which similarity among a group of organisms was distilled into distinct and narrowly defined characters, and the organisms sorted out based on the number of shared characters.  Organisms with the most shared characters came out close together on a tree-like cladogram.  The branches of the cladogram were then grouped into taxonomic categories, and the cladogram was also interpreted as representing the evolutionary history of the group (i.e. as a phylogenetic tree).  Cladistic analysis,now routinely applied to structural, chemical, and genetic data, remains the core of systematic biology.

This lousewort, Pedicularis bracteosa,
 is now in the Orobanchaceae.

So then, where did the snapdragons end up?  The snapdragon genus (Antirrhinum), along with foxglove (Digitalis), Penstemon, Veronica, and butter-and-eggs (Linaria) ended up fortunately together, in what might now be called the new Snapdragon Family, or technically the Plantaginaceae.  The very similar-looking monkeyflowers (Mimulus) are only distantly related and have been moved to the family Phrymaceae.  Indian paintbrush (Castilleja) and Pedicularis now belong to the Orobanchaceae, which contains mostly parasitic species. Torenia is in the Linderniaceae, and Calceolaria is in the Calceolariaceae.  What remains in the Scrophulariaceae are the figworts (Scrophularia), Mullein (Verbascum) and some other original members, with the addition of  some genera that used to be in other families, including Myoporum and Buddleja. 

Nothing is more snapdragonish than a
monkeyflower, but this common western
American genus is now in a family
no one has heard of - the Phrymaceae.

Confused?  Don't worry, most professional botanists and horticulturists are as well.  Many websites are still using the name Scrophulariaceae in its original broad sense.  It's a trainwreck out there.

Plantago is a genus of weedy,
wind-pollinated herbs, that is
unfortunately atypical of the
new Snapdragon Family. Photo by Bernd Haynold
Source:http://upload.wikimedia.org/
wikipedia/commons/2/29/
Plantago_lanceolata_040807.jpg

The name of the new Snapdragon Family, Plantaginaceae, is a real irritant to many of us old-timers.  It is based on the genus Plantago, a genus of small, inconspicuous, wind-pollinated herbs known as plantains (not the big cooking bananas!).   Because of its highly reduced flowers, its relationship to snapdragons, etc., was not recognized, and it had been in its own family, Plantaginaceae, for centuries. 

Veronica was the basis for the proposed
family name Veronicaceae.  This name was
preferred by many, because at least it was
more symbolic of the types of flowers found
in most members the family. Plantaganiceae,
however had priority.

The name Veronicaceae was originally suggested for the new family (Olmstead et al. 2001), but according to the rules of taxonomy, an existing family names take precedence over a new family names, and since Plantaginaceae has been recognized as a family much longer than the Veronicaceae, the older name must prevail, and so the Snapdragon Family is now officially the Plantaginaceae.  Plantago represents a tiny specialized twig of the snapdragon family tree, and it is too bad to name this family of colorful, distinctively shaped flowers for such an atypical member. This tail-wagging-the-dog result seems counterintuitive and hard to accept, but for now that's the way it is.

The old Scrophulariaceae was polypheletic, and in order to create families that represented genuine evolutionary relationship, it had to be broken up.  Other traditional families have suffered a similar fate, including the Lily Family, Liliaceae.  Daylilies, for example, are no longer in the Lily Family - but that's another story.

Reference: Olmstead, et. al. 2001.  The Disintegration of the Scrophulariaceae.  American Journal of Botany 88:348-361.

The first "bamboos"

Earlier ("The grasses that would be trees," March 18, 2012), I described the unique pattern of development that results in the tall, lightweight, and very strong stems of bamboos.  The key to the rapid growth of bamboos is a combination of lightweight, hollow construction, plus a process of growth involving intercalary meristems in each internode that all elongate more-or-less at the same time.  Before there were grasses, before in fact there were any seed plants, a group of spore-bearing relatives of ferns discovered virtually the same growth form.  These were the horsetails, formally known as the Sphenophytes. 

Like bamboos, horsetails send up new shoots from buds
on underground rhizomes.  Each bud contains a complete
compressed stem, with many nodes and internodes packed
closely together. Intercalary meristems within each internode
become active at the same time, adding new tissues to each
and raising the stem rapidly.  The true leaves are modified
into toothed, cup-like structures that protect the tender
growing region of each internode.  From Kerner and
Oliver, The Natural History of Plants, 1904, Fig. 190.

Very few of these sphenophytes survive today, but you can see the bamboo-like form in the stems of modern horsetails.  Like bamboos, the horsetail stem is hollow and its wall fortified with fibers.  Also like bamboos, the young horsetail shoot forms as a condensed bud, with nodes and internodes of the entiren stem crowded together.  A basal intercalary meristem in each indernode begins expansion in coordination with all the others in the shoot, resulting in rapid upward growth.  Leaves at each node are reduced to stiff bracts that protect the tender growing region at the base of the internode. 

Giant horsetails, commonly referred to the
genus Calamites, grew like bamboos and
dominated the coal-forming swamps of
the Carboniferous Period. From Smith,
Cryptogamic Botany, 1955,  Fig. 151.

From the late Devonian, Carboniferous and Permian periods, some 350-300 million years ago, giant tree-like horsetails, growing up to 100 feet high, dominated early forests, sprouting from underground rhizomes, just like modern bamboos. They most likely elongated fairly rapidly, but develeped a modest amount of wood to support their large crown of branches.  

A modern horsetail, growing in a ditch beside the
road in Washington State, is just as at-home in the
21st century as its ancestors were 300 million years
ago.  It continues to compete with neighboring
vegetation through its rapid growth from preformed buds
in the spring.  The true leaves are modified into
 bracts that protect the growing  tissues above each node.
Photosynthesis is conducted by tissues in the main
stem as well as by the whorls of slender stems at each node.

Modern horsetails are for the most part fairly modest in size, living in shaded moist areas alongside the descendents of their other ancient companions, ferns and clubmosses.  The largest, up to 8 ft or more in height, are found oddly in moist streamsides in dry areas of Central and South America.  For an image, click on the link below, or if it is no longer active, do a simple web search for Equisetum giganteum: http://www2.fiu.edu/~chusb001/GiantEquisetum/Images/NorthernChile/LlutaRailroadScale2.html

The "root" of the root problem

Most plants have roots.  We take that for granted. These important organs grow into the soil (or sometimes tree bark) to provide anchorage and absorb water and minerals.  Some, like carrots and sweet potatoes, swell with food reserves and are edible.  Their structure and function is pretty much the same across the plant kingdom.  So what's the "problem?"

One of my pet peeves as an educator is the clutter of vague and confusing terminology often used to describe the different parts of plants, and the failure to relate terminology to the bigger pictures of plant development and evolution.  One particularly muddy area concerns the kinds of root systems in plants.  Whoa, you're thinking, Botany Professor is way out in Botanygeekland this time.  Bear with me, because there are just two basic kinds of root systems in plants, and they tell us a lot about the fundamental strategies of plants for survival and perpetuation.

Adventitious roots are most easily seen in
an epiphytic orchid.  Though these stems
are more upright, they are modifications of
creeping rhizomes. Roots, stems, and leaves of
orchids are all ephemeral, and periodically
replaced by new organs.

In most ferns, club mosses, and other ancient lineages of spore-bearing plants, as well as in modern monocots, waterlilies, and some eudicots, the main stems, or rhizomes, are horizontal and creep through the soil, putting out new roots as they go.  Their roots have two properties: they are adventitious and they are ephemeral.  Adventitious roots arise individually from the stem of the plant and occasionally from leaves), rather than through branching off of earlier roots.  Ephemeral means that they are temporary, i.e. disposable.  In fact, no part of a fern plant is permanent or woody.  Older parts of the rhizome, as well as older leaves and roots disintegrate as newer organs are generated. 



New adventitious roots emerge from
a young, growing section of rhizome.
The apical bud is to the right.

 







Such a creeping body plan has been called bilaterally symmetrical by Francis Halle.  Most animals have this kind of elongate symmetry in which the body can be split into two, mirror-image halves, with paired locomotory and sensory organs on either side of the animal.  Such organisms have a front (anterior) end, a rear (posterior) end, a right and left side, a backside (dorsal) and a belly side (ventral).  Think of a centipede creeping along with its many legs on either side. In animals, this is a set of adaptations for efficient forward movement and prey capture, with eyes, brain, and mouth at the front end.

A creeping rhizome can be described in the same terms.  The front end is the apical meristem, the rear end is the decaying part of the rhizome.  Leaves emerge from the top or dorsal side of the rhizome, while roots arise mostly from the belly of the rhizome.  Such a plant is actually mobile, moving slowly through the soil with each year's growth.  They also can branch, creating large colonies of outward-expanding rhizomes segments. 

A rhizome, like this of a Solomon's Seal (Polygonatum sp.), creeps along the ground.  The newest growth, including the primary apical meristem, is to the left.  The round scars, and the structure labeled "1940,"  represent the locations of ephemeral leaf and flower-bearing shoots.  The older growth from 1937 to 1939 is to the right.  Adventitious roots have emerged over time directly from the stem tissues of the rhizome.  From Transeau et al.,  Textbook of Botany, 1940, Fig. 91. 

Why this matters is that early seed plants, which were mostly trees and shrubs, evolved a very different symmetry: a radial symmetry.  This is more like the symmetry of a sea anemone.  From the top, the organs of the animal radiate out from a central point.  A tree, shrub, or cycad has a similar organization.  This kind of organization is more suitable to organisms that stay put, i.e. are sessile.  A tree does not creep along the ground like a fern or ginger rhizome, because, by definition, its growth has been redirected skyward.  It also needs a more massive root system to support that massive upright growth.  The root system of a tree develops through the branching of the original primary root directly beneath the trunk.  It becomes woody over time like the trunk and branches above it, so it is neither ephemeral nor adventitious.  

A woody tree or shrub has a roughly radial
symmetry with an overall hourglass shape. It
is fixed to one spot for life.

A plant embryo consists of an axis with two poles. The primary root develops from an apical meristem at the "south" pole, while the primary shoot develops from an apical meristem the "north" pole. Through branching, the entire trunk and branch system develops from that original embryonic shoot, and the entire root system develops through branching of the primary root.  There remains a relatively narrow zone where the shoot system and the root system meet, giving a tree or shrub an overall hourglass shape.

Though plants are endlessly varied in structure and in their adaptation to particular environments, most fall into one of these two basic symmetries.  There is not, however, a simple pair of terms derscribing the two types of root systems.  They are most often referred to in textbooks as a "fibrous root systems" and  "taproot systems."  This dichotomy is not only vague, but can be misleading as well.

A taproot by definition is a single dominant root, as exemplified most beautifully by a carrot.  While many woody plants begin with a taproot, which develops directly from the primary root of the seedling, most actually branch to the point where the original taproot can no longer be identified.  The term "axial root system" has been used in the past, and is much preferable for those that develop entirely through branching of the lower axis of the embryo. 

A fibrous root system is one consisting of many roots of similar length and thickness, and  forming a thick, broom-like mat.  The adventitious root system of a grass plant or onion bulb fits this model, but the adventitious roots of a climbing Philodendron are not so broom-like.  In some definitions, fibrous is equated with adventitious, but in others, a similar-looking cluster of roots resulting from multiple equal branching of the primary root would also be called fibrous.  In addition, the word "fibrous" is used in a very different context for the presence of strengthening fibers within leaves and stems.  Fibers are not generally present in ephemeral adventitious roots. So this is a poor choice for the typical adventitious root system of monocots and many herbaceous dicots.

Why not just call them "adventitious root systems?"  That, though better, is also somewhat misleading because the word adventitious refers to how a root forms (from a stem or sometimes even a leaf) rather than to its mature form.  Woody plants may produce adventitious roots, but these will typically become woody themselves.  A cutting  taken from a tree or shrub may produce adventitious roots in response to hormone treatment, but as the cutting becomes a new plant, one of the adventitious roots typically becomes a new woody taproot.  Members of the genus Ficus known as banyan trees produce adventitious roots from their main branches, which dangle to the ground and become new woody trunks. These are adventitious root systems, but very different from the ephemeral systems of monocots.

There does not appear to be a fully satisfying term for the "disposable adventitious root systems of  plants with non-woody creeping stems," but it is important to recognize them as a major and widespread  alternative to the permanent woody root systems of trees, shrubs, and carrots (carrots are another story; their thick taproots are woody except that their "wood" has been modified into food storage tissue).  Perhaps something like "fibrous/adventitious" would be the best compromise to accurately identify the distinctive root systems of  ferns and monocots.  If we are to continue to use the word "fibrous" alone for these systems, it must be carefully and consistently defined as adventitious in nature, and contrasted with "axial root systems," whether these are in the form of a taproot or something that superficially resembles a cluster of fibrous roots.

The grasses that would be trees

 Ecologically and economically, the Grass Family (Poaceae) is a one that the world could scarcely do without.  If all grasses were to suddenly disappear from the Earth, it would cause a mass extinction worse than the one that saw the end of the dinosaurs.  Grazing animals all over the world would starve into oblivion, as might the human species itself.  The lawn care business, employing millions at golf courses, hotels, freeway medians, and neighborhoods with HOA's, would be ruined! 

Though we might do without lawns, and could survive without the flesh of grazing animals or their mammary secretions (got soymilk?), could we live without wheat, rice, maize, oats, or other cereal grains?  Those too are from the grass family.  If we did manage to survive, probably in much smaller numbers, life would be duller without cane sugar to sweeten our drinks and desserts.  The next wave of billionaires might be Stevia farmers.

In some parts of the world, another type of grass would also be sorely missed - the bamboos.  These are the grasses that would be trees.  Where they grow, these giant grasses are the source of building materials used for everything from housing to water pipes, scaffolding, chopping boards and chopsticks.  In many applications the hard tissues of the bamboo stem are stronger, harder, denser, and more resilient than the wood of trees, yet they contain no wood at all. 

The hard wall of the hollow bamboo stem contains no wood, but
rather dense bundles of fibers with the strength of steel cables.

Wood, by definition, is fine layers of secondary xylem (water-conducting tissue) laid down annually by the vascular cambium, which increases the thickness of the trunk, roots and branches over time.   Bamboos, instead, are built of densely packed bundles of fibers that run up the stem like parallel steel cables.  They are endowed with these tissues during their primary upward growth, and do not increase in thickness after that.  Although it may live for many years, an individual bamboo stem (or "culm") develops its tree-like dimensons and matures within a few months, changing very little after that.   

Bamboo shoots arise from underground rhizomes, the same as in other grasses and monocots in general, and they can expand year after year into extensive clonal colonies. A bamboo shoot is the same structure as a cornstalk, a sugar cane, or even the slender, flower-bearing stalk of an ordinary lawn grass. Bamboos are an example of gigantism in a normally humble group of organisms. 

The rapid growth of their stems is the key to the success of these plants as they compete for real estate with more conventional trees.  Being hollow is part of the strategy - not as much tissue needs to be produced - but the other part is the presence of multiple centers of growth, or meristems, in a bamboo stem.  Bamboos add tissues not only at the tips (at the apical meristems), but also throughout the elongating stem, allowing them to grow as much as a foot a day. 

Bamboo stems consist of elongate
internodes between the ring-like nodes.
A bud at each node can develop into
a slender leafy shoot.  During develop-
ment, a sheath-like bract encircled the
node and protected the tender basal
intercalary meristem as it added
new tissues to the internode.

Internodes are the sections of stems between the nodes (the points where leaves and buds are attached).  In rapidly growing plants such as vines and tree saplings, the internodes between the upper expanding leaves continue to elongate for days or weeks, allowing these plants to stretch rapidly toward more brightly lit spots in the forest.  Strawberry runners also employ greatly stretched-out internodes to extend new plantlets away from the mother plant, creating clonal colonies. 

The stretching of these stems is due to the creation and expansion of new cells locally within each internode, not to the activity of the apical meristem. These supplementary areas of growth are called intercalary ("between") meristems, and greatly enhance the ability of young plant stems to increase in length compared with plants that grow from their apical meristems only.

A young bamboo shoot contains an entire compressed
stem, in which many internodes expand at the same time
to achieve rapid upward growth. Note the fibrous bracts
(modified sheath-like leaves) that surround the stem
at each node.

In bamboos, new stems appear as very compact "bamboo shoots," which are tender enough to eat because their fibers have not hardened yet. The young bamboo shoot contains a complete stem, with many nodes and internodes packed close together.  Within each internode is a dormant intercalary meristem.  When the time is right, the internodes of the stem begin expanding, beginning at the base, but overlapping so that many stem sections are elongating at the same time. This results in the extremely rapid elongation of the bamboo stem.  As each stem section approaches its final dimensions, the fibers within the wall gradually harden, with the tender, growing, meristem at the base the last to mature.

The infamous "Chinese bamboo torture" was based on this rapid growth. A prisoner who was reluctant to divulge information was persuaded to talk by being stretched over a newly emerging bamboo shoot aimed at his abdomen. The tip of the shoot was often sharpened, and if the prisoner were particularly stubborn, it would pierce his body and grow right through it.

Bamboos may not truly qualify as trees, but do a pretty good job pretending.  They reach tree height faster than any true tree, and through clonal growth can edge out all other vegetation to make their own forests.   In a seeming contradiction, their stems are hollow and lighter, but their tissues harder and stronger, than the wood of most trees.  They are advanced monocots at one of the leading edges of plant evolution, and provide an endlessly useful construction material for humanity.

[See also "How the Grass Leaf Got its Stripes" for more on the revolutionary adaptations of the monocots.]

More than just a flower

Flowers are not always quite what they seem.  Technically, a flower consists of 4 sets of organs: sepals, petals, stamens, and carpels.  Stamens (pollen-producing organs) and carpels (seed-producing organs) do the work of sexual reproduction, while petals do the all-important job of attracting pollinators with color, nectar, and fragrance.  Sepals typically provide a protective wrapper for young flowers, but sometimes assist in the job of attracting pollinators, or sometimes replace the petals altogether. 

The actual flowers of the Poinsettia are tiny and clustered
within green cups, which themselves are clustered within a
mass of bright red bracts.  Nectar is produced in the yellow,
mouth-like appendages on the sides of the green cups.

Many plants have adopted an alternate strategy in which a colorful display is created, not by the flowers themselves, but by colorful bracts (modified leaves) around them.  Probably the most spectacular example of this is the winter-blooming Poinsettia.  In its native tropical American habitat, the bright red bracts attract migrating hummingbirds to hidden cups of nectar.

In some cases, what appears to be a single flower is a dense cluster of flowers, with specialized, petal-like flowers arranged in a circle around them.  This is the highly successful strategy of the sunflower family.

In the sunflower family, the "flower" consists of a compact head
of flowers, in which the outer circle of flowers (ray flowers)
are most often stretched out into a long petal-like shape.  The
inner flowers (disk flowers) are small and trumpet-shaped, with
five lobes representing the actual petals. Usually these are the
ones that produce the seeds.

In the Anthurium, the actual flowers are
the tiny bumps on the long spadix.

In the equally successful Aroid family (Araceae), a large colorful bract (called a spathe) provides a backdrop for a fleshy spike (the spadix) of tiny crowded flowers.  Bright red Anthuriums are a popular example, as are Calla lilies and Spathiphyllums.

For additional examples of "false flowers,"  I refer you to my article of several years ago in Florida Gardening Magazine:  False Flowers

I also featured two genera of the  Araceae in Florida Gardening articles: Amorphophallus and Calla lilies.


For a nearly complete list of my Florida Gardening articles, and for a link to the website where you can find an index to all of their articles, go to Florida Gardening Magazine.  The magazine is devoted to gardening in Florida, but in my own articles, I often explore more botanical questions.  The magazine is also followed by people in similar subtropical climates around the world.