The long and short of it - the story of internodes

The structural units of a higher plant consist of a node,
an internode, and one ore more buds and leaves at the
node.  These units can be repeated indefinitely to form
a shoot, or modified in various ways to alter the
architecture of the plant.

Plant stems are organized into repeated units consisting of nodes and internodes.  Nodes seem to be where all the action is.  Leaves are attached at the nodes, and most often there is also a bud at the base of each leaf that has the potential to develop into a new shoot.  The new shoot also will develop as a series of nodes and internodes.


Internodes are essentially the sections of stem between nodes. Ho humm - end of story.  Not quite!  The internode in fact is a dynamic part the plant, the part responsible for most actual growth in height.  You can see the lengthening of internodes most dramatically in vines, which are adapted to extend themselves as rapidly upward as possible to reach the light zone.  In vines, cell division and expansion continue in the internodes well below the tip, propelling several sets of expanding leaves ahead of them.  Once reaching the light zone, internodes will end growth sooner, resulting in a bushier crown suitable for gathering light.  Tree saplings also show dramatic internode elongation in the shade, and many more humble plants will send their flowers rapidly upward by means of strikingly elongate flower stalks.

Internodes are quite distinct in most trees and shrubs (left), but are extraordinarily
elongate in vines like the Clematis (right).

The bamboo's elongate internodes are the secrets to its
height.

Bamboo shoots contain all the
nodes of a full stem packed close
together.  Expansion of all the
internodes, more-or-less
simultaneously, results in very
rapid growth.

Bamboos are grasses evolutionarily adapted to compete with trees, and they do so with the most extraordinary internodal elongation of all.   Each bamboo shoot begins as a compact bud, the "bamboo shoot" of culinary commerce.  Within that massive bud, the nodes and internodes for the entire shoot (culm) are preformed, but the internodes are extremely short.  When growing conditions are right, the entire bamboo shoot elongates through simultaneous elongation of all of its internodes, reaching full height in a matter of days.  The legendary "Chinese bamboo torture" consisted of strapping a prisoner over a bamboo shoot just beginning its expansion phase.  The sharp point of the shoot tip pressing against the back quickly became horribly painful.

This Pachypodium, in the Apocynaceae,
has virtually no internodes, but grows
slowly through the accumulation of
tissues at the base of each leaf.

This Neoregelia in the Bromeliad, forms a concise rosette
of leaves.

Many plants effectively have no internodes.  They are very short, or slowly build up trunks just from the bulk of the tissues around the leaf bases.  Palms and cycads are prime examples, as are many desert succulents.  Plants that hug the surface of the ground have short stems with leaves crowded into a rounded pattern referred to as a rosette (named for its resemblance to the crowded petals of a rose).  Strawberry plants, bromeliads, venus fly traps, sundews, and African violets are examples.

This tiny Drosera  rosette is dwarfed by its much larger flowers.  The flower
stalks consist of greatly elongate internodes, in contrast to the virtual lack
of internodes within the rosette of leaves.

The stalk of the Egyptian papyrus plant
(Cyperus papyrus) is a single elongate
internode.  The tassle at the top consists
of several dozen nodes packed closely
together through lack of any internode
elongation.

The growth of internodes is controlled by hormones, particularly gibberellin.  Dwarf forms of vegetables or other crops can be created by genetically limiting the plant's ability to produce or respond to gibberellin.  And plants that are normally compact, such as cabbages, can be stretched out like vines by applying the hormone to the young plants.

Gaillardia daisies are normally compact rosettes, but
under the influence of gibberellins can stretch out via
elongate internodes.

Internodes are therefore the unsung heroes of plant growth.  They determine whether a plant will be a skinny, stretched-out vine, a squat mound of leaves, or something in-between.

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 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.]