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.]