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.

Into the Ocean Without a Paddle - the Mystery of the Red Algae

Most red algae, like this
Batrachospermum, are complex,
multicellular seaweeds (though
this is a freshwater genus).  From
Oltmanns, Morphologie und Biology
der Algen. 1922, Fig.469. 

The term "algae" refers to a wide range of simple aquatic organisms that have chloroplasts.  The red algae are a distinct and natural ("monophyletic") group of algae with reddish pigments in their chloroplasts similar to those of the cyanobacteria.  They are believed to be directly descended from the first eukaryotes to incorporate cyanobacteria as proto-chloroplasts (see my posting "Plants, animals and kleptoplasts, oh my").  The green algae (which later would give rise to the land plants)  have different chloroplast pigments that are an adaptation for life in shallow water.  Though it is not totally settled, DNA evidence suggests that red and green algae are closely related and that the green algae are also directly descended from the first organisms with chloroplasts.  The two groups are believed to have split apart very early on.  The rest of this article assumes that this relationship is true, and explores the mysteries that result from it.


While green algae have varied growth forms, the red algae are mostly complex, multicellular seaweeds.  Seaweeds, like their photosynthetic cousins on land, are organisms that set down "roots" in a suitable location, and stay put for the rest of their life.  But being glued to one spot makes it even more important that they have a means to disperse their genetic information during their reproductive cycles.  In algae, that's typically a 2-step process: spores that travel long distances to mingle with other populations, and sperm cells to travel relatively shorter distances to seek out a suitable egg (see "The truth about sex in plants").  For that they need some means of locomotion, and for single-celled organisms that usually means slender whip-like organelles called flagella (or cilia, which are smaller, but of the same structure). 

They mystery of the red algae arises from the fact that neither their sperm cells nor their dispersal spores have flagella.   Such cells are literally adrift in the ocean "without a paddle."  Throughout this entire group there is no sign of flagella in any species or in any part of the life cycle.  There is no sign even of the flagella-supporting structures (centrioles), usually found in all cells (not just cells with flagella) in other algae and land plants.  It is as if the ancestors of red algae never had flagella at all.  That hypothesis is however complicated by the apparent relationship of red and green algae and their common descent from the first photosynthtic eukaryotes. 

In the unicellular Chlamydomonas, all cells (except for the
dormant zygote) have two flagella.  In multicellular green algae,
only the gametes and zoospores are so equipped. 
From Haupt, Plant Morphology, 1953, Fig. 16.

Flagella are found throughout the eukaryotic domain: in animal and plant sperm cells, in single-celled protists like paramecia and trypanosomes (vectors of sleeping sickness and other diseases), and in the zoospores and sperm cells of most (non-red) algae.  Flagella apparently evolved among the earliest eukaryotes, for their structure is fundamentally the same wherever we find them.  Most green algae produce sperm cells and zoospores with flagella exhibiting that ancient and universal structure.  So the common ancestor of red and green algae must have had flagella.


So the mystery has several layers, including a dilemma: red algae have the older form of chloroplast, and presumably came first, with the green algae evolving from them.  But green algae have the original mode of locomotion provided by flagella, and the flagella-less red algae must have evolved from them!  The simple solution to this is that the first algae had red chloroplasts and flagella, and subsequently split into two groups.  Red algae as we know them today have the original chloroplast but have lost their flagella.  The green algae evolved a new type of chloroplast but retained flagella.

But now we have to ask "why did red algae lose their flagella?"  How do their sperm cells find eggs without any means of self-directed movement?  Textbooks are vague on this issue, generally implying that the loss of flagella must have been accidental.  But it seems that the elimination of flagella and associated support structures, indeed all the genes involvd in producing flagella, was ruthlessly thorough and therefore must have happened for some adaptive "reason."


In red algae such as this Polysiphonia, tiny dust-like sperm cells, called spermatia, are released in great numbers from male plants. As the spermatia swirl around in the water, they are caught by the egg-producing structure on a female plant by long projections called trichogynes. Modified from Raven et. al, Biology of Plants, ed. 7, 2005.

I believe the reason was a change of strategy similar to the shift to wind-pollination in grasses or oaks.  Red algae sperm cells are tiny, barely more than a nucleus.  They are stripped down like this because they don't require the apparatus or energy reserves for locomotion.  They can be produced more cheaply and in much greater numbers.  By releasing huge quantities of tiny sperm cells, red algae are banking on the chance that some of them will randomly drift to the vicinity of egg-producing structures and get caught on them.  This is similar to the grass strategy of producing vast quantities of tiny pollen grains, a few of which get caught in the feathery stigmas of the female pistils.  Grasses exist in large populations in open, windy habitats where this strategy works well.  Presumably, red algae live where similar underwater currents produce the same results.  Green algae, on the other hand, may have evolved in quieter waters, where flagella were still necessary for successful dispersal.

How the grass leaf got its stripes

In monocot leaves, the veins are fundamentally parallel to one
another from the time they enter the leaf base from the stem. 
Some monocot leaves are distorted into other shapes, causing the
veins to bow out from one another.

[For a more complete story of the adaptations and evolution of grasses and other monocots, see Chapter 9 of my book, Plant life - a brief history]

A blade of grass, if you look closely, has long stripes running from its base to its tip.  These are technically the veins containing vascular tissues: xylem and phloem.  These veins conduct water and minerals up into the blade and sugar from photosynthesis down into the stem and roots.  The arrangement is what textbooks call parallel venation.  Each vein enters the broad leaf base independently, and remains parallel and separate from all the others up to the tip of the leaf. 

This anatomical feature of grass leaves speaks to the difference between the traditional division of flowering plants into dicots and monocots (of which grasses are perhaps the epitomy).  Though dicots are no longer recognized as a single distinct clade, they do exhibit a suite of characters by which they differ sharply from the monocots, and therefore can be referred to informally.  

Typical dicot leaves, for example, have a net venation, in which a few veins enter the leafblade via a narrow petiole and then branch repeatedly into a netlike pattern. 

The leaves of dicotyledonous trees, shrubs and herbs have a
 netted pattern of venation developing from the branching
of one or a few primary veins. 
From Kerner & Oliver.  The natural history of plants.  Gresham. London. 1904. 

These contrasting leaf types are the result of a different pattern of growth.  Dicot leaves form as miniatures, with just a few veins, then expand in all directions, adding finer veins between the main veins as they go.  Monocot leaves, on the other hand, produce new leaf tissue at the base, which pushes the older part of the leaf upward.  The many parallel veins appear when the leaf is young and very short, then new tissues are added to the bottom of each as the leaf gets longer.

The leaves of this Liquidambar tree, a dicot,
form in miniature in the terminal bud, then
expand rapidly.  The net-like pattern of
venation developes as ever finer veins are
added between the earlier veins.

The distinctive pattern of growth of the grass leaf is found throughout the monocots, at least in their seedlings, and it appers to have begun in the ancestral monocot as an adaptation to growth from an underground stem.  The tips of the monocot leaves become mature and stiff when they are very short, so as to be able to push through the soil, while the softer, growing bases are nestled below ground among the bases of older leaves.

Many monocots retain this subterranean existence with their primary stems taking the form of rhizomes, bulbs, and corms.  Those that don't, such as single-stemmed palm trees, still have seedling leaves that push up from below the ground.  

The leaves of most dicots, on the other hand, are adapted to form at the tips of exposed twigs, with the miniature versions forming within buds, then expanding rapidly to full size.

The Amaryllis plant consists of an underground bulb, with new leaves arising from the central bud.  One can demonstrate the basal growth of the leaves by making a mark near the base of a new leaf.  The mark rises as new tissues are produced below it.

Seedlings of dicots ( a and b) develop leafy shoots above ground, with the terminal bud and new leaves exposed to the elements, while the seedlings of monocots (c and upper right inset) have highly compressed shoots that keep the terminal bud below ground.  Leaves arising from the buds then push up through the soil, adding new tissues only at the base.

Keeping the main stems, buds and leaf bases below ground is advantageous in variety of conditions.  Plants can survive winters, dry seasons, fires, and grazing by remaining dormant below ground.  This is where grasses excel, giving them mastery over plains and savannas.  The tops of the grass blades can be chewed to the ground, and quickly restored by the basal growth.  But it also serves semi-aquatic plants well, by allowing leaves and shoots to rise above the water level through basal growth.

A number of dicots, such as
this aquatic pennywort, have
petioles that can extend through
basal growth to lift their blades
above water.  Something like
this may have preceded the first
monocots.

It is therefore not certain whether monocots were initially adapting to aquatic conditions or to harsh seasonal conditions.  They may have been preceded by aquatic dicots that also have the ability to extend their leaves upwards through basal growth of their petioles (leaf stalks).  One theory is that in the ancestral monocots, basally growing petioles eventually lost their conventional leaf blades, and the petiole became wider to become the new kind of blade.


From basic monocots with underground rhizomes, bulbs, and corms, many specialized forms have evolved, including tree-like members of the palm, pandan, and Dracena families. Epiphytic orchids, aroids and bromeliads have rhizomes that creep along tree limbs.  Banana plants use basal growth to lengthen their cylindrical leaf sheaths into a false stem, while Egyptian papyrus lifts a globular crown above water through basal growth of its flowerstalk.  And finally, the giant grasses known as bamboos rapidly reach tree height by regions of basal growth in each of their internodes.

Why are there no moss trees?

The land plants can be grouped into two broad categories:  vascular plants and non-vascular plants (better known as bryophytes).  Most of the plants we encounter in everyday life -trees, shrubs, flowering plants, ferns, etc., are vascular plants, while mosses, liverworts, and hornworts are non-vascular bryophytes, and quite humble in their habits.   

Vascular plants, such as this giant Sequoia,
 can get quite tall because they
possess xylem for upward water transport

 You may have heard the word “vascular” in reference to the vessels of our own circulatory system, and vascular tissues in plants perform a similar function. They consist of parallel sets of pipes: xylem vessels for conducting water upwards, and phloem tubes for moving dissolved foods from one part of the plant to another. Without such tissues, in particular the xylem, water cannot rise very high, and significant upward growth is not possible (see my posting "how does water get to the top of a redwood tree?").  Vascular plants have them, and bryophytes obviously don’t. 

Bryophytes cover every available surface
in the temperate rain forests of Washington
State

Mosses mostly do not exceed more than
a few centimeters in height. The leafy portion
of the plant produces gametes, and a fertilized egg
then develops into the spore-producing plant, which is
just the stalk and sporangium.  From W.H.
Brown, the Plant Kingdom, 1935.

Most bryophytes are prostrate, forming mats on the soil, trunks and branches of trees, rocks, and sometimes tombstones. Those that grow more upright are typically only a few centimeters high, but there are some giants among them that tower above their cousins to dizzying heights of about half a meter!

Liverworts lie flat, except for their reproductive structures.  From A. W.
Haupt,
Plant Morphology, 1953. 

Hornworts are similar to liverworts, but
with different form of reproductive
structures.  From G. M. Smith, Cryptogamic
Botany, 1938.

It seems strange at first that there are no larger mosses.  Nearly every group of organisms has both large and small members.    A common response, even in botany textbooks, is that mosses can't get any taller because they don't have vascular tissues.  More technically it is said, they "failed" to evolve lignin, a resin-like material that strengthens the walls of xylem vessels. 

Xylem vessels serve the same function as drinking straws.  Water is sucked up through them by the force of transpiration.  Imagine a straw made of silk.  It would collapse under the slightest suction and be useless.  The same applies to unreinforced cell walls.

I would suggest that if mosses really needed xylem, it would have evolved.  But we don't even have to pursue that argument because as it turns out, bryophytes couldn't get any taller even if they "wanted to," and it's not for lack of lignin or motivation.


For the real reason, we have to recall that plants have alternation of generations of gamete-producing plants (gametophytes) and spore-producing plants (sporophytes) (see my posting on "the Truth about Sex in Plants").   There are two alternate forms of every sexually-reproducing plant, one that produces spores and one that produces gametes. One is usually large and long-lived, the other small, short-lived, and generally unnoticed. 

In bryophytes, the main plants - the green mats that spread and live for many years - are the gamete-producing generation, just like their algal ancestors.  They cannot  get very tall, because their ultimate task is to release sperm cells and position eggs to receive them. Sperm cells can swim only a short distance but must reach an egg on another plant - a difficult proposition for fragile cells produced on a tree top.  Sperm cells produced on a large  gametophyte tree would be left literally "high and dry."


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Tree ferns are vascular plants, and
 their spore-producing generation is the main plant
that can get quite tall.

The spore-producing plant of a moss, its sporophyte, is a small, ephemeral structure that remains attached to the parent plant - just a slender stalk and a single sporangium.  It gets as tall as it can without toppling over or placing excessive demands on the gamete-producing plant - a few centimeters at most. 

But suppose that tiny spore-producing plant of the moss were to sprout its own roots and start growing on its own.  Then it could get as tall as it wants, because there is an advantage to dispersing spores from greater heights.  Well something like that did happen in the ancestors of the vascular plants, and their spore-producing generation became the dominant conspicuous one, inventing lignin and xylem as a means to become ever taller.  Voila, trees!

The gamete-producing generation of the fern resembles
that of a liverwort, but is even smaller and very short-lived.
From A. W. Haupt, Plant Morphology, 1953.

So in bryophytes, which are indeed well-adapted to creeping around in the shade, the gametophyte is the dominant plant, while the sporophyte is tiny, but in the tall-growing vascular plants, the sporophyte is the dominant plant, while the gametophyte is tiny.