Buds
Culm buds can be very useful for identification. The shape of the bud, how it is covered, how many initials are visible within it, and where the bud is placed can all vary substantially. Some bamboos do not have any effective bud as the branches grow very quickly. Many have incorporated the first sheath on the branch axis, the prophyll, or more than one sheath, into a well-developed protective structure. Others have lost the main prophyll, and some of these, particularly those from S America, have many buds or show many bud-like initials that have presumably developed from higher order branches once contained within a normal single bud.
The prophyll and sometimes a second sheath can be thickened and fused, giving a bud that is closed or open, and membranous or heavily thickened. Buds are also positioned differently. A promontory may push them upwards, while in scrambling bamboos, the inverted internodes can seem to have buds below the nodal line.
The prophyll
The first bract or sheath on any axis is the prophyll, literally ‘first leaf’. Because of its unique position it is often different from other sheaths. It may be modified to protect a bud, and as it occupies a space between 2 axes at different angles, it often develops projections known as keels, which fill the gaps, keeping out predators and maintaining structural rigidity. It can vary in its width, so that it may be only narrow enough to have one keel, or it may be broad, encircling the branching axis, allowing it to have 2 keels. The palea of the floret is a prophyll. It is 2-keeled when the floret backs onto a continuing rhachilla axis with further florets. When it is from the upper floret and there is no rhachilla to back onto, then it has no keels.
Where there is no dormancy and branches grow very quickly, then the prophyll is not thickened, and not well distinguished from other sheaths, eg Phyllostachys. In bamboos where branch ramification is so fast and extensive that a multitude of fine branches is rapidly produced, then the prophyll is largely redundant. Just like other sheaths, it may become reduced, or disappear altogether. An array of multiple prophyllate buds within a bud is then presented, often around a thickened sheath containing the rest of a larger central primary branch, eg Chusquea.
The bud
The protective function of a prophyll is more important where dormancy of a lateral axis is required. Then the prophyll may close up completely, by fusion of the front edges of a broad prophyll that completely encircles the branch initials, eg Pleioblastus cf Arundinaria; Ampelocalamus cf Drepanostachyum. If the prophyll is very narrow, and cannot enclose the initials on its own, then it may fuse with the second sheath on the branch axis, to form a thickened ‘budscale’, closed at the back or the front or both, eg Yushania. A component of a branch complement may also remain dormant and exposed, and any sheath surrounding it can become thckened. This often happens for the distal part of the central branch, while the basal part and its laterals develop more freely. Either way a bud is formed, although the constitutent parts can be subtly or substantially different.
Buds are usually found at the base of an erect internode, positioned to allow upward growth. In some scrambling bamboos where culm and its branches are often horizontal of pendulous, upward growth of the branch initials from level or inverted internodes causes the nodal line to be distorted, so that the bud appears to be inserted at or below it, eg Dinochloa.
Promontory
The basic unit of bamboo structures is given above as a node/internode with sheath and bud, but that was a simplification as the bud is actually what is seen of a new axis, comprising further units. Where the bud is further above the nodal line than normal, borne on a thickened projection, that is termed a promontory. It is part of the internodal tissue, not the lateral branch axis. When found as a component of the inflorescence of many bamboos with open inflorescences, it pushes the prophyll (glume) of the axis (spikelet) well away from the mother axis. When this happens in other grasses it is called a pedicel, and has been assumed to have a somewhat different origin, involving instead amalgamation of many internodes, although there is no evidence of a segmented origin.
Main mid-culm bud characters
bud number
single, eg Bambusa, Arundinaria
multiple or apparently multiple, eg Chusquea, Rhipidocladum
shape
broader than height, eg Drepanostachyum, Ampelocalamus
ovate, eg Dendrocalamus
taller than breadth, eg Borinda, Phyllostachys
thickening
membranous eg Phyllostachys
thickened eg Borinda
front margin closure
always open, eg Arundinaria, Semiarundinaria, Drepanostachyum
initially closed, eg Pleioblastus, Sasa, Ampelocalamus, Bambusa
bud position
adjacent to sheath scar, eg Bambusa
well above sheath scar, often on a promontory
apparently below sheath scar, eg Dinochloa
Further bud characters
prophyll
absent, eg Chimonobambusa marmorea
narrow, 1-keeled, Yushania, Drepanostachyum
broad, 2-keeled, eg Arundinaria, Thamnocalamus
budscale origin
consisting solely of the prophyll, eg Sasa
formed from the prophyll and the 2nd sheath, eg Ampelocalamus
closure of front margins of bud at different nodes
initially closed at all nodes
initially closed only at basal nodes
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