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Rhizomes

Pachymorph rhizomes Leptomorph rhizomes Homologies & evolution Characters

Rhizome is simply an underground stem bearing roots. 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 different rhizome forms.

Two major categories of rhizome exist in the woody bamboos. One kind is pachymorph, literally meaning ‘thick kind’, and the other is leptomorph, simply meaning ‘thin kind’.

Pachymorph rhizomes

These have a root-bearing section that is thicker than the culm. They turn up to form culms at their apices. 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 thinner joining section is called a neck. This is usually short, so that the bamboo plant grows in a single well-defined clump. In some bamboos, however many rhizomes start off with a long, thin, rootless neck before the thick section. This allows some pachymorph-rhizomed bamboos to spread nearly as far as those with leptomorph rhizomes.

Leptomorph rhizomes

These are uniformly thin, long, and usually stay underground, with erect culms or further horizontal rhizomes branching off them at intervals. 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 roots at every node.

Homologies and evolution

Relationships between the two forms of rhizome are unclear. It may be that bamboos currently with pachymorph rhizome have descended from ancestors that lost their leptomorph rhizomes and relied on thicker, tillering culm bases instead. It may be that some pachymorph rhizomes lost their terminal negative geotropic response and remained underground, as modified culms. Given that only physiological changes are required for such alterations in a plastic, segmented plant, either scenario, or both, would be perfectly plausible. More likely though is that both evolved from ancestors with tillering, thin-based culms.

Evidence for the evolution of leptomorph rhizomes from culms is found in the early stages of establishment of seedlings of genera such as Phyllostachys and Sarocalamus. In these genera, seedlings initially only have erect, tillering culms and no well-distinguished rhizome. Each year, the culms from these have progressively longer, more horizontal proximal sections, as the mother culms become taller. Eventually they stay underground, and have truly become leptomorph rhizomes. While this ontogeny cannot presumed to reflect phylogeny per se, it demonstrates a mechanism by which leptomorph rhizomes could have evolved. Close proximity to large culms appears to be a necessary stimulus for diageotropic shoot growth in Phyllostachys and other genera with leptomorph rhizomes.

Not surprisingly, anatomical evidence has also shown that the leptomorph rhizome and the culm are very similar indeed, while the culm bases of bamboos with leptomorph rhizomes are anatomically similar to pachymorph rhizomes. 

This anatomical similarity between culm bases in pachymorph-rhizomed bamboos and the pachymorph rhizome, and the anatomical dissimilarities between leptomorph and pachymorph rhizomes, has been used to suggest that pachymorph rhizome should not be called rhizome at all, but culm base instead. However, this logic could also lead to leptomorph rhizomes not being called rhizomes either, just culms. That brings us back to the fact that rhizome only means rooting underground stem anyway.

The adaptive value of long rhizomes of one form or another for dispersal in a rarely flowering plant with no seed dispersal mechanism is obvious. They are only found in areas with sufficient rainfall, well-distributed through the year, however, as a longer growing season is required for their proper development.

Further Reference  Stapleton, C. M. A. (1997). The morphology of woody bamboos. In G. P.  Chapman, (ed.), The Bamboos: 251–267. Academic Press.  Ding, Y. L., Tang, G. G. & Chao, C. S. (1997). Anatomical studies on the culm neck of some pachymorph bamboos. In G. P. Chapman (ed.), The Bamboos: 285–292. Academic Press.  Stapleton, C. M. A. (1998). Form and function in the bamboo rhizome. J. Amer. Bamboo Soc. 12(1): 21–29.

Characters of rhizomes

Leptomorph rhizome (an adaptation of a whole horizontal culm)

  presence

                     present, eg Phyllostachys

                     absent, eg Bambusa, Neomicrocalamus

Pachymorph rhizome (an adaptation of the culm base)

  presence

                     present, eg Bambusa, Chusquea fendleri

                     absent, eg Phyllostachys

  neck

            length

                     short to moderate, consistent, eg Bambusa,Fargesia, Borinda

                     short to very long, highly variable, eg Yushania

                     usually very long, eg Melocanna, Pseudostachyum

            solidity

                     hollow (including nodes), eg Yushania microphylla

                     solid, eg Yushania maling