Rhizome is by definition essentially just an underground stem bearing roots.
In bamboos it can arise by culms simply growing horizontally instead of vertically. The normally aerial roots at each node develop better underground. It is then morphologically a largely unmodified culm. All that has really changed is physiological control of the direction of growth, and the capacity to continue apical growth without gravity calling an end to ambition. Such diageotropic growth of culms is witnessed in certain bamboos with pachymorph rhizomes under unusual environmental conditions, and it is easy to see how it could be readily derived during evolution. Molecular evidence suggests that it has indeed arisen readily on multiple independent occasions.
Alternatively the base of a culm can be much more radically modified with greatly thickened, shortened internodes bearing much more prolific roots and with abundant storage tissue, and also temporarily horizontal growth, to give a form more readily recognizable as rhizome.
Thus two major categories of rhizome exist in the woody bamboos. One category is pachymorph, literally meaning ‘thick kind’, and the other is leptomorph, simply meaning ‘thin kind’. Curiously there is a school of thought that only leptomorph rhizomes are really ‘true rhizomes’. This could be considered a bizarre manifestation of pedantic semantics, and it is not helpful in understanding, classifying or identifying bamboos. Both forms have some but not all of the characteristics that can be attributed to rhizomes, and thus both have an equal claim to being rhizome.
To examine rhizomes there is usually no alternative to digging. The rhizome form cannot be assessed simply from how the culms and clumps appear above the ground. Different forms of rhizome can give an identical above-ground appearance. There is unfortunately no direct correlation between ‘clumpers and runners’, and the two major rhizome forms of leptomorph and pachymorph. Similarly there is no consistent correlation between the two major rhizome forms of leptomorph and pachymorph, and their monopodial or sympodial growth habits, because of tillering of culms from leptomorph rhizomes. Confusion between morphological and developmental terms for bamboo rhizomes has produced, and will no doubt continue to cause, a chaotic variety of inadequate descriptions in the literature.
These have a root-bearing section that is thicker than the culm. Beyond this section they turn upwards to form culms. The root-bearing nodes are close together with short, thick, solid internodes. New rhizomes develop from buds on a mother rhizome. After emerging from the bud the rhizome becomes progressively thicker. The proximal, thinner joining section is called a neck. This is usually short, so that the bamboo plant grows in a single well-defined clump, a form of growth that is termed sympodial (feet together). In some bamboos, however many rhizomes start off with a long, thin, rootless neck before the thick section, a form of growth that in extremis can be termed monopodial (single foot). This allows some pachymorph-rhizomed bamboos to spread nearly as far as those with leptomorph rhizomes.
These are uniformly thin, long, and usually stay underground, with erect culms or further horizontal rhizomes branching off them at intervals. This form of growth can be termed monopodial (single foot). All nodes bear roots, and the nodes are well separated, without any of the thick, solid, compressed internodes found in pachymorph rhizomes. The rhizome looks like a thin horizontal underground culm, except that it has well developed roots at every node, and the sheaths all remain typical of those seen at the base of the culm, with shortened blades. Tillering of culms commonly leads to sympodial growth of culms from leptomorph rhizomes.
Characters of rhizomes
Leptomorph rhizome (weak adaptation of a whole horizontal culm)
present, eg Phyllostachys, Chusquea fendleri
absent, eg Bambusa, Fargesia
present, eg Semiarundinaria
absent, eg Phyllostachys (growing strongly under good conditions)
Pachymorph rhizome (a radical adaptation of a horizontal culm base)
present, eg Bambusa, Chusquea fendleri
absent, eg Phyllostachys
short to moderate, consistent, eg Bambusa,Fargesia, Borinda
short to very long, highly variable, eg Yushania
usually very long, eg Melocanna, Pseudostachyum
hollow (including nodes), eg Yushania microphylla
solid, eg Yushania maling