Inflorescences
Inflorescence structure
Bamboo inflorescences vary greatly in overall form, and in the finer details of the individual spikelets and florets that they contain. They are mainly categorized according to the presence of bracts and buds, the density and extent of branching, and number of stamens in each floret. As in all grasses the actual flowers, if any real parallel to the standard concept of a flower exists at all, are very small and inconspicuous. They are sessile (stalkless) and largely hidden inside 2 chaffy bracts, called the palea, and outside it, the lemma, which can open to allow pollination, like the beak of a bird. Inside them, a ring of 3 tiny membranous bracts, the lodicules, is thought by some to represent miniature petals of a flower. In the centre of these are the ovary and stamens. This ‘flower’ combined with its palea and lemma, is called a floret.
The florets are themselves sessile on alternating sides of a stalk, the rhachilla, and one to many florets are combined into a small spike, the spikelet. Descriptions of spikelet and particularly floret details are generally the same as those for other grasses. As the floret is so small with basic bracts, there being little or no adaptation for animal-related seed dispersal in bamboos, there are few characters for description. The number of characters found on the other sheaths, the culm sheath and the foliage leaves, along with the culm and rhizome, is much greater. That is one reason why grass taxonomists, who are trained to classify grasses mainly from the flowers, tend to lump many bamboo species together, not to mention the genera as well. Fleshy fruits are found in some bamboo genera, while most have a small dry caryopsis.
Historically, the bamboo inflorescence has received less critical scientific study than the inflorescences of other grasses, because of the infrequency of flowering and also the scarcity of bamboo plants in the western world. This is unfortunate as the unitary construction of the bamboo plant, and the evident homologies between vegetative and floral structures, provide insights into ontogeny, homology and phylogeny largely unavailable in other grasses.
It has been appreciated that certain bamboo inflorescences (described as indeterminate or iterauctant) have structures that are not found in other grasses, and that the spikelets (often called pseudospikelets) can develop in a different manner. There is usually an ability for the spikelet to branch from basal buds and thus produce further spikelets. This, along with the presence of bracts on inflorescence branches, is sometimes assumed to be ancestral, and sometimes assumed to be derived. Four main characters are involved, presence of buds in the spikelet, growth of those buds, presence of subtending bracts in the inflorescence, and presence of prophylls in the inflorescence.
It has been suggested that bamboo inflorescences can be sorted into 2 categories, either semelauctant or iterauctant, and that all spikelets in iterauctant inflorescences should be called pseudospikelets. However, the 4 characters given above are not always well correlated, and which of the 4 characters should be used as the criterion for placing an inflorescence in one group or the other is contentious. Moreover, spikelets and pseudospikelets are clearly homologous. Therefore it is arguable that the terms semelauctant, iterauctant, and pseudospikelet have become confused and misleading, and are probably best avoided altogether.
In tropical Asian bamboos, inflorescences are generally fully bracteate, with all axes subtended by a large bract, and each axis bearing a prophyll at the point of branching. Many of these bracteate inflorescences also have further axes or viable buds in the axils of the bracts and prophylls, and also in basal bracts in the pseudospikelet, allowing indeterminate (iterauctant) growth, which can lead to dense spherical clusters. In temperate bamboos the bracteate inflorescences, such as those seen in Fargesia, Thamnocalamus, Phyllostachys, and Chimonobambusa generally have few if any viable buds, and are not capable of indeterminate growth. It would seem best to describe them as determinate (semelauctant), but even better is the use of all the 4 characters rather than these two categories, determinate/semelauctant and indeterminate/iterauctant, which are more like syndromes than clearly defined categories.
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Examples of inflorescencess illustrated and explored
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Detailed inflorescence structure, including an analysis following the topologies and terminology of Troll and colleagues, has been considered elsewhere, and covered in depth in Stapleton (1997). The bamboo inflorescence is interpreted there as a modification of a simple polytelic synflorescence. The florescence is a component of the spikelet, with a zone of inhibition representing the glumes. Lateral paraclades with coflorescences together with the main florescence constitute the synflorescence. Following this critical analysis, which would make the constituent parts of the grass inflorescence homologous with inflorescences in other families, there are problems of conflicting usage of terminology, for example there can be no panicle or pedicel, as grass panicles and pedicels are not homologous with panicles and pedicels in other families. The substantial revision of terminology thus required means that new terminology is not likely to be adopted for the foreseeable future in grass circles, and a more traditional usage of terminology is followed here, going back to inflorescence, spikelet, pedicel and panicle in the traditional sense used in the grass family.
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Further Reference
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Characters of bamboo inflorescences
Inflorescence branch structure
bracts at points of branching
prophylls (prophyll = simply the 1st sheath on any axis)
prophylls at points of branching eg Bambusa, Neomicrocalamus,
Thamnocalamus, Phyllostachys
distant from branching and considered “absent” eg Racemobambos,
Arundinaria, Yushania, Pseudosasa
prophyll width
narrow, 1-keeled (not just split), eg Dendrocalamus
broad, 2-keeled, eg Bambusa
sheaths/bracts subtending branches (subtending = bearing in its axil, lit. ‘stretching under’)
all present, eg Bambusa, Phyllostachys
some to many substantial bracts remaining, eg Thamnocalamus, Fargesia
bracts all severely reduced to hairs or ridges or absent,
eg Arundinaria, Yushania, Pseudosasa
branching extent
racemose, strictly 1 order of branching, eg Fargesia
racemose-paniculate, 1 to 2 orders of branching, eg Thamnocalamus
paniculate, 2 or more orders, eg Dendrocalamus, Gaoligongshania
branch density
capitate, many orders of branching, eg Dendrocalamus
spicate, few orders of branching, eg Bambusa
compressed, eg Fargesia, Thamnocalamus, Himalayacalamus
open, eg Yushania, Drepanostachyum
branch grouping
fascicled into small clusters, eg Drepanostachyum
not fascicled, eg Yushania
branch orientation
spikelet insertion unilateral, eg Fargesia,
spikelet insertion not unilateral, eg Dendrocalamus
branching angles
branches becoming deflexed, axils pulvinate, eg Gelidocalamus
branches remaining erect, axils not pulvinate, eg Pseudosasa
surface
glabrous, eg Drepanostachyum
pubescent, eg Bashania
angles scabrous, eg Oligostachyum
Spikelets
position
sessile, eg Phyllostachys
borne on promontory, ‘pedicellate’, eg Yushania
ramification
basal bracts (glumes) with buds
buds developing (infl. iterauctant, spikelets often called pseudospikelets), eg Bambusa
buds not developing (inflorescence semelauctant), eg Thamnocalamus
basal bracts with or without buds present, some developing, eg Chimonobambusa
basal bracts without buds, eg Drepanostachyum
glumes (sterile lemmas)
number gemmiferous, number empty, size
number of florets
termination
complete floret, incomplete floret, rhachilla extension
rhachilla
internode length, pubescence
disarticulation
Floret (standard description as used in other grasses)
fertile lemma: pubescence, marginal ciliation, apex
palea: pubescence, marginal ciliation, keels, keel ciliation, apical division, apex
lodicules: number, shape, fimbriation
ovary: shape, pubescence
stigma: length, division, stigma number and shape
stamens: number, filament separation, anther length, anther apices
fruit: dry caryopsis vs fleshy fruit, shape, size, length of beak (persistent stylar base)
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