Bamboo is a woody grass. In addition to providing the basis of the world's grazing pastures and therefore effectively our animal protein, the grasses give us not only edible bamboo shoots but also our cereal crops such as rice, wheat, maize, millet, oats and barley as well as sorghum and sugar cane. An impressive lineage.
As a generality, bamboos may be divided into two groups to accord with differing growth habits and natural origin. Tropical species are more likely to be of the "clumping" habit. That is, new growth springs up from close to the base of the mother plant to form, typically, isolated clumps of culms (as the upright stems are called) radiating from a center and growing outward in all directions to form a circular mass of vegetation. By contrast, temperate zone bamboos are characterized by the development of an open network of horizontal underground stems (rhizomes) from which the new culms grow. This invasive habit has earned this group the name of "running" bamboos. Intermediate or composite forms exist and habit may be modified by limitations imposed by the plant's immediate environment.
Culms emerge from the soil and grow to full height within as little as two to three months. Exceptional growth rates have been recorded of as much as 1.2 metres in one day but the individual rate depends, of course, on the species and the growing conditions. The quantity of new culms, their height and diameter varies within a particular species depending on soil and air temperature, the availability of water and especially upon the age of the plant itself.
True equatorial environments encourage almost continual shoot production but where any seasonality is introduced the growth of fresh culms occurs immediately prior to the rainy season and continues until such time as conditions prove less favourable. Temperature, water and nutrient availability are critical determinants with regard to the number of shoots.
As a bamboo plant matures, the height and diameter of new culms increases each growing season until the optimum for that species and site is reached. However, as all bamboo culms lack the ring of growing material typical of tree trunks and their branches, there is no subsequent individual increase in girth from the late bud stage, only a pronounced, telescopic elongation. Perhaps the most noticeable characteristic of bamboo is the segmentation of the culm into distinct nodes or joints with intermediate smooth sectors or internodes. It is the elongation of these internodes that provides the basis for culm extension. As might be expected, tropical bamboos attain the largest size with culm lengths reported in excess of 35 metres and diameters to 30 centimetres.
Peripheral culm tissue is a dense matrix of elongated cellulose plant fibres cemented together by lignin to provide a strong and very flexible frame. Such a high degree of flexibility is essential in a plant that grows to a significant height yet is secured only by a shallow rooting system as is the case with bamboo. Internal wall tissues are soft and serve as the path for the transference of water, nutrients and growth regulators through the length of the culm.
The culms and their branches carry aloft the leaves to gain sunlight and to deny it to competitors: during the reproductive phase they bear the flowers and fruits: they hold the canopy beyond the reach of many would-be seed and leaf eaters and they serve in unison to trap and contain leaf litter as a future nutrient resource.
It is a common mistake to visualize each culm with its head of leaves as being an individual plant. They are in reality simply single shoots of a parent plant. The role of the culms has been elegantly described as being that of the fingers of a hidden hand. That hand, the true foundation of the bamboo, is the underground rhizome system. Just as the culm has many functions so too has the buried rhizome. As it is the only part of the plant in contact with the soluble salts of the soil it is, by virtue of the roots that spread from the nodal points, primarily a nutrient and water gathering organ. The rhizome also acts in a food storage capacity. Importantly, it is an anchor for the towering culms above and the nursery of future growth.
Bamboo root systems are seldom to be found penetrating any deeper than one metre and most of the underground growth is contained within 50 centimetres of the soil surface. Overall plant stability is obtained partly by the flexible structure of the culm, its particular method of attachment to the rhizome and also partly because the underground architecture, though shallow, is formed into a dense and firmly interwoven matrix. This applies to both the clumping and running types of bamboo.
In the former, the rhizomes spread by the formation of lateral culm buds immediately adjacent to the base of an existing culm. Expansion is therefore both progressive and multidirectional following, typically, a circular pattern of colonization.
The runner type of bamboo has a quite different root mass structure consisting of fast growing and far ranging rhizomes bearing buds that may variously develop into culms or secondary runners. Their seasonal rate of growth depends upon the totality of the physical and chemical soil characteristics and is typically between two and five metres although this growth may not be unidirectional. The effect of this root elongation habit is to enable the plant to quickly invade surrounding areas and so to gratuitously locate and exploit any favourable localised resource. The repeated lateral branching of the rhizomes creates a strong and highly interwoven network of fibrous growth that supports the plant, stabilizes the soil and discourages competition.
All bamboo rhizomes are configured in the same nodal format as are the culms, the internodes typically hollow in runners and always solid in those of the clumping species.
At each node of the underground rhizome and of the aerial culms and their branches, protective sheaths cover the sensitive growing cells. In most cases this sheath falls off as tissues mature but in some instances is retained as an on-going protection.
Branches arise from the nodal points of culms. The number of such branches, of associated dormant buds (if any) and the height along the culm at which branches do or do not emerge depends upon the habitat niche to which the particular species is adapted. Bamboos that naturally form a dense and dominant canopy maintain uncluttered culms up to the level that sunlight can effectively and efficiently be employed in photosynthesis. Understorey species or those growing in more open environments produce branches along a greater length of the culm. Several bamboos bear modified branches in the form of thorns which commend them as hedging plants.
Branches are primarily the leaf bearers. Bamboo is an evergreen plant whose many leaves are continually being replaced and so tends towards the accumulation of a substantial and, for competing plants, stifling ground litter. Leaf fall is perhaps greater in the early spring when kinder growing conditions enables young growth to replace those leaves that have survived the winter. As with everything else bamboo, there are wide variations in the size of leaves between species although their general arrangement is sufficiently familiar as to be easily recognizable. Unlike other grasses, bamboo leaves have a stalk and are therefore not a simple continuation of the branch stem sheath although they arise from it.
One of the most vexing yet fascinating aspects of bamboo study is the issue of bamboo reproduction. Bamboos are flowering plants. Their mode of flowering, the structures of the flowers and the fruits are not dissimilar to those of other grasses. With one significant difference. Bamboos mostly flower sporadically and may do so in a periodic pattern that can span anywhere between twelve months to over a hundred and fifty years. As if by some built-in clock, a species of bamboo after years of vegetative growth begins to bloom, not just in the one grove but across districts, countries and whole regions. Then, either exhausted by or satisfied with this momentous effort, the plants in unison wither and die. Entire forests are destroyed. Juvenile cuttings, recent growth and ancient stands alike are reported to all join in this flowering and dying phenomena. It is called gregarious flowering and is the most discussed and perhaps least understood issue in bamboo science.
Inevitably, such a dramatic natural occurrence affecting so many important aspects of Asian daily life is indelibly woven into the common folk lore but the true periodicity, the extent of the flowering, the degree of loss to established groves and the boundaries of the areas affected are seldom as well defined or complete as is perhaps reported. However, gregarious flowering certainly does occur and the mechanism controlling it remains as yet unidentified. The commercial implications of this wholesale population decline are substantial.
Between these periods of flowering and sexual reproduction, bamboos actively propagate themselves by vegetative means. Clumping varieties, by establishing new culm growth at the perimeter of their particular stand, progressively encroach upon and suppress other plants that are competing for resources in the immediate vicinity. However, if the stand is damaged or broken up, the nodal buds on the separated culms have a very high chance of forming new plants although the original fragmented rhizomes are less likely to survive and regenerate.
The opposite is true for runner bamboos. In these it is the remote underground rhizome that gives rise to new culm growth and the invasive habit of this root system ensures a rapid occupation of territory either by-passing or overcoming other plant communities as opportunity provides. If damaged and the area substantially disturbed, it is the rhizomes that survive and generate clonal regrowth. Even in the nursery the culms of runner bamboos resist vegetative propagation and are not likely to substantially contribute to re-establishment in the natural environment.
If the point has not been sufficiently laboured it is necessary to reiterate that bamboos, although sharing many common features, are not by any means all alike and that even within a particular and identified separate species considerable variances may present themselves.
Much of this variation may be a reflection of the effect of long intervals between sexual reproductive generation. Cross fertilization, as an essential part of the sexual reproduction process, introduces homogeneity into a population. Protracted periods of clonal propagation without the averaging effect of cross breeding invites the survival of genetic diversity. Other apparent anomalies within a species may be the result of incorrect classification by botanists unable to check their results by reference to flowering material.
It is primarily by the particular design of the inflorescence that the flowering plants are traditionally and most reliably identified. In the absence of suitable data, bamboo species have been cautiously and variously described on the basis of growth habit, the frequency and arrangement of branches, buds, protective sheath details, culm size and colour, leaf size and arrangement as well as the presence or otherwise of hairs, thorns and other commonly occurring features. The degree of uncertainty regarding bamboo classification may be judged by the tentative estimates given in the scientific botanical media that there may be between thirty to ninety genera and in the order of five hundred to fifteen hundred discrete species! Varietal diversification by adaptation to discrete, local habitats is very much in evidence.
The rapid generation of biomass of which all bamboos are capable suggests that it is a rich or at least a very efficient feeder. Soil conditions vary substantially over the large geographical range that bamboo occupies but since most tropical forest soils are high in humus, are friable, moisture retentive and slightly to mildly acidic then such a soil condition is likely to represent the reasonable average for bamboo generally. Some bamboos are drought resistant but typically they exhibit far better growth and condition where water is plentiful and the soil well drained. Salt tolerance is not a feature so far identified in any bamboo.
Durnford Dart - September 1994