Culms and branches
The height of bamboos varies among species, ranging from lower ground cover species, not growing taller than 20-50 cm, to the giant bamboos of 20 m or more. The height of a plant is determined in the first place by the height of individual culms, but the growth habit is also important: the culms of some tropical clambering bamboos, scrambling over other vegetation, can be as long as 120 m.
Bamboo culms develop in one period of growth by a delicately timed process of cell elongation, with some cell division as well. When the new culms appear, they already have attained their ultimate diameter. Unlike trees, bamboos have no secondary thickening growth.
When bamboo is planted in the garden, or for determination of the species, some definitions are valuable or even necessary.
Nodes & Internodes
Bamboo culms, branches and rhizomes are all segmented axes, built of similar modular building blocks. One modular unit consists of a nodal zone (node), and the internodal zone between nodes (internode). At the nodal zone, buds, protecting leaf organs and eventually roots, are found.
The internodes are hollow (with the exception of the South American genus Chusquea) and at the nodes diaphragms are found which separate the hollow internodes. The culm wall is thick or thin, depending on the species. The length of the internodes varies, even within a single plant. In Phyllostachys aurea for example, the basal internodes are shortened while the upper ones can be longer than 20 cm.
The node of a culm is a more or less well defined zone, on which the culm leaves are found which protect the young culm or branch, as well as the buds they subtend, on elongation. On the lower part of the nodal zone the leaf organs and buds are implanted at the nodal line. The upper part of the nodal zone is very often delimited in a rather spectacular way by a prominent supranodal ridge.
On the nodes buds appear which can develop into branches and branch complements. The number of buds and branches on the node depends on the species. For example, Phyllostachys has branch complements with 2 (sometimes 3) branches which are equal or subequal. Bambusa has branch complements with many branches, one being more dominant. Sasa has only one branch, almost as thick as the culm, and only in the upper parts of the culm. Fargesia's have 3 or more branches. Like the mother culm branches are segmented axes built of the same modular units as the culms.
The growth of young culms is initiated underground, invisible to the eye. Rhizomes, or subterranean stems, take care of the growth of the plant. They transport water and nutrients, they store nutrients and they stabilize the soil. And most important: from the rhizome, buds develop into new culms or shoots. The leaves (and to some extent also the culms) produce sugars from sunlight and CO2, the subterranean part takes care of the distribution of nutrients and water, and control all of it. Rhizomes can be generally classified into 2 different types, pachymorph and leptomorph rhizomes.
➔ A pachymorph rhizome is generally thicker than the culm, and buds of the rhizome develop into new rhizomes. The apical bud of the rhizome will develop into a culm. The rhizome and the culm are connected to the rest of the plant through a rhizome neck. While the rhizome proper has roots and buds, the rhizome neck has no such appendages. The rhizome neck is generally short, but can be long, resulting in widely spaced culms.
➔ A leptomorph rhizome is thinner than the culms, grows diageotropically as a modified stem and some of its axillary buds develop into new culms. The nodes of a leptomorph rhizome all bear buds and roots or root initials. The culm is connected with the rest of the plant with a culm neck.
For a complex notion as rhizome the transition between branching and rhizome is not always straightforward. Transitional forms exist, not only in seedlings, prior to rhizome establishment, but also in stress situations or after regeneration of flowering. The growth habit and the spacing of the culms alone, cannot give an absolute guarantee about the nature of the rhizome. Culms can grow spaced, in one clump, or in more dispersed clumps.
The microclimate and the precise environmental conditions in which bamboos grow, are also important. In certain conditions Phyllostachys aurea behaves almost like a clumping bamboo, but when given more nutrition, or in warmer climates, this species can be very invasive. New culms can show up meters away form the mother plant. It is very important to take the power of bamboo growth into account when planning bamboos in your garden.
Every segment is protected by a leaf organ that is implanted at the nodal line. In bamboos leaves can differ very much both in appearance and function. The leaf organ is made up of the sheath and appendages, that are more or less developed depending on the place on the culm or branch, the lamina or leaf blade, auricles with or without oral setae and a ligule. Most important is the blade, which is connected to the sheath either directly (in culm or branch leaves) or via a petiole in foliage leaves. The blade can be elongated as in foliage leaves, or reduced as in culm leaves. On rhizome sheaths the blade is almost completely reduced.
On new elongating culms the blades are relatively small, without a petiole. These are the culm leaves, and their function is to protect the young growing culm during elongation. During and after elongation these leaves dry out. They are either deciduous or can remain attached to the plant for a long period. At the terminal part of culms and branches foliage leaves develop, with an expanded leaf blade, attached to the sheath through a petiole. Their function is to provide energy to the plants through photosynthesis. These leaves remain functional and green for an extended period. Bamboos often renew their foliage by developing new smaller twigs or branches, especially after the winter period. The weight of these leaves can cause a culm to bend over.
Depending on the species, leaves can be very delicate or strong and may range from sizes of about 7 cm long up to giant blades of 50 cm and 8 cm wide. In bamboos a lot of leaf variegations are found, ranging from white over golden to green variegations.
Culm leaves are especially important in determining the precise name of many bamboos. An accurate identification is therefore only possible during the growing season. The auricles and oral setae, pubescent or glabrous, the orientation of the blade (erect or reflexed), the shape (long, short, flat, wrinkled, etc.) and color are all very important characteristics.
In fact growth habit, culm, rhizome and leaves are all important characteristics for the precise identification of a bamboo species. Since flowering in bamboo is unpredictable, it is necessary to have the botanical characteristics at hand to identify bamboos in your garden or in their natural habitat. And this certainly is not a simple task.