Branches
Woody bamboos have substantial development of culm branches. These vary considerably between genera, and so can be very useful for identification at the generic level, especially in temperate bamboos. Suitable material for inspection is required however, and full-sized living plants are generally required to make an accurate assessment. The characters can vary from node to node and over the life of the culm. Therefore to compare branching patterns of different bamboos the characters are standardised on the branches produced in the first year of branch growth from the mid-culm region of a culm from a mature clump.
Branching patterns and characters
Most bamboos produce a complement of several to many branches from each culm node. The simplest way to categorise these is by an inspection of a mid-culm node in the first year of branch growth. Branch growth forms can be sorted into the basic patterns given below, or it can be examined in depth to reveal a number of more detailed characters.
Some common mid-culm 1st yr branching patterns:
usually a single branch, eg Sasa, Pseudosasa japonica
unequal pairs, eg Phyllostachys
larger central and a variable series of 2 to 4 smaller laterals, eg Thamnocalamus, Bashania
larger central and consistently two smaller level basal laterals, eg Chimonobambusa, Sinobambusa
central with 2 to 6 laterals all subequal and more or less level, eg Fargesia, Yushania
7 to 100 or more subequal and more or less level or in several rows, eg Drepanostachyum, Chusquea
large central branch with 6-many subequal laterals surrounding it, eg Neomicrocalamus, Chusquea subgen Rettbergia
In-depth branching analysis
Close inspection of good material is required to analyse branching in depth.
For bamboos to have several or many branches all originating close to the culm, the basal branch internodes have to remain very short, without intercalary growth. As extension of basal internodes is minimal on lateral branches as well as the central branch, the results after several years can be a highly complex complement of branches close to the culm, which can be difficult to examine, and may number several hundred branches.
The numbers of branches that are produced close to the culm depends upon the number of compressed basal internodes on the primary branch, how many of these have buds, and whether the resultant lateral branches have further compressed basal internodes and buds themselves. Which internodes have buds also varies, some bamboos having laterals on the most basal nodes, others having them further up the central branch.
In many bamboos the compression of basal internodes is so intense that several lateral second order branches are presented side by side, with a subequal primary branch in the centre. As the compression develops, the need for separating sheaths diminishes and sheaths on lateral branches may become completely absent. Thamnocalamus and Fargesia are quite similar alpine genera, but Thamnocalamus has retained all its sheaths, while Fargesia and several related genera have lost these sheaths, presenting several branch bases together. Other genera that often lack some of the sheaths in their complements of branches include Pleioblastus, Semiarundinaria, Chimonobambusa and Chusquea. The particular sheaths that are lost vary between these genera, suggesting the process has evolved independently. Cross-sections of the branch system are required to analyse and compare such patterns of reduction. The production of branch complements from more than one primary branch bud is only possible if derived from other meristematic tissue as well as the main branch. I have hypothesised that incorporation of 2 nodes together allows this, as does differentiation of root primordia into shoots rather than roots.
The time of growth of branching also varies. Some bamboos such as Phyllostachys have precocious branch development as soon as the culm internodes have elongated. Drepanostachyum, Himalayacalamus, and many S American bamboos also develop branches very quickly, and they are unusual in the bamboos in that the branches grow extravaginally, splitting open the culm sheath. Other bamboos, such as Fargesia nitida produce culms one year, and branches only in the following year. In scrambling bamboos small subsidiary branches often develop quickly, and a much larger central branch axis remains dormant for years. This may be associated with a thickened tightly closed budscale protecting the dormant branch, and/or a persistent culm sheath may remain protecting it, with a specialised thinner basal girdle splitting to allow extravaginal growth of the small branches.
Branch development can vary substantially along the culm, buds at lower nodes often remaining dormant. Larger, central branches can remain dormant for many years, only the laterals growing from the outset. This has to involve two phases of lateral branch development, which is the normal procedure in tropical bamboos such as Bambusa. The central branch produces 2 lateral branches from 2 basal internodes, and often 2 branches grow from their basalmost internodes as well, and so on for higher orders of branching. These all grow simultaneously in the first year. A series of 1–4 buds distal to these remain dormant on the central branch and also on the laterals, and they only develop in the second year or later. Internodes without buds often separate the nodes from which the two phases of branching occur.
Dormant central branches are common in scrambling or heavily pendulous bamboos, eg Neomicrocalamus and Dendrocalamus. If apical dominance is lost by the culm apex hanging out or down, or it being damaged, then the central branches can develop, sometimes growing nearly as large as the original culm. When central branches are very large, their bases can also be rhizomatous, eg in Dendrocalamus, and they can root if the culm falls down, giving these bamboos an alternative dispersal mechanism.
Advanced branching characters
internodal compression on central branch base
no compression at all, eg Phyllostachys
variable compression of basal internode, eg Sarocalamus
basal internode consistently compressed, eg Pseudosasa japonica
several internodes compressed, eg Thamnocalamus
many internodes compressed, laterals arcuate not level, eg Rhipidocladum
several internodes compressed, laterals level, eg Fargesia
many internodes compressed, laterals level, eg Drepanostachyum, Chusquea
presence of buds on compressed basal node
buds always present, eg Phyllostachys, Thamnocalamus
bud presence variable, eg Indocalamus tesellatus
bud usually absent on basal nodes, Sasa, Pseudosasa, Hibanobambusa
sheathing reduction
all sheaths present, eg Phyllostachys
some sheaths occasionally absent, eg Semiarundinaria, Pleioblastus
several sheaths consistently absent, eg Yushania, Drepanostachyum
time of central branch development
immediate, eg Phyllostachys
after dormancy, eg Fargesia nitida
time of basal lateral branch development
one phase, eg Yushania
two phases, eg Bambusa
manner of branch development if prior to sheath fall
all usually extravaginal, Himalayacalamus, Drepanostachyum, Ampelocalamus
all intravaginal, eg Sarocalamus, Pseudosasa japonica, Pleioblastus
subsidiary branches extravaginal, through tall girdle at base of sheath,
eg Melocalamus, dominant central branch intravaginal, often much later
Further information on branching systems in temperate bamboos is given in a draft publication
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