Wednesday, March 28, 2012

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:

Monday, March 26, 2012

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.

Sunday, March 18, 2012

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.]

Thursday, March 1, 2012

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.