Leaves and sheaths
In woody bamboos, rhizome bracts, culm sheaths and foliage leaves are 3 different well distinguished forms of the monocot leaf. Rhizome bracts and culm sheaths are protective and supportive, while foliage leaf blades are responsible for photosynthesis. Culm sheaths and their blades are very important for species identification, those from the lower mid-culm being most distinctive. Foliage sheaths and blades are also important, but are rather more variable. Important parts for identification include ligules, auricles, oral setae, and blades. The sheathing section itself varies in density, nature, pattern and colour of hairs, and cilia along the margins. The persistence of the culm sheaths is also important, and they may be more persistent in the centre so that they hang for a while before falling completely.
Leaves as sheathing organs with blades
Each internode throughout the bamboo plant is surrounded by a tubular sheath, basally attached to the node, terminating in a blade. As in all grasses, these are all technically leaves, a confusing concept when applied in the bamboos, where the sheaths and blades are much more differentiated and specialized. Leaves are by definition flat organs capable of photosynthesis, yet the culm sheath may be tubular and not even green. Therefore the term leaf is best avoided away from the foliage, with the flattened apical section of all ‘leaves’ being called a blade, whether leafy or not. Sheaths and their blades are adapted for different functions according to the location of the internodes they surround, and in woody bamboos they are generally referred to as either rhizome bracts, culm sheaths or foliage leaves.
The rhizome bract is purely protective in function, tightly and permanently wrapped around the rhizome axis. The culm sheath is both protective and also thickened so that it can support the soft, weak elongating culm internode. It is usually deciduous, revealing the normally green culm internode inside, which can then also photosynthesize. The foliage leaves have a protective and supportive sheath in which more distal blades are wrapped, and a very much larger photosynthetic blade.
Culm sheaths are the most important part of a bamboo for identifying different species. The variations in shape, hairs, marginal cilia, and the characteristics of various apical appendages are most evident when the sheaths are young, and bamboos can be very difficult to identify properly without them. They vary substantially along the culm, and it is important to look at those in which the characters are best developed. Ignore those that look like leaves from the upper part of the culm, and those that look too much like rhizome bracts from the very base. Those at the base have few appendages, while those at the apex are not only more similar to leaf sheaths, usually being narrower, but they also have atypically stronger blade development, the terminal culm sheath blades often being considerably larger than even any foliage leaf blade. To standardise for descriptions, culm sheaths from about one quarter the height of the culm above the ground are most commonly examined and cited, as the important characteristics are expressed most comprehensively at that point along the culm. In the larger bamboos, that height is fortunately also much more accessible, and culm sheaths from eye-level can be used. A good bamboo description normally covers the culm sheath and foliage leaf details in depth, listing the characteristics for each separately.
The importance of inspecting the least ‘leafy’ of the culm sheaths, rather than those with well-developed blades is another reason why the term culm leaf, widely used in other grasses, should not be used in bamboo descriptions. It leads to a search for a leafy culm sheath, which will not have the correct features.
Ligules
At the apex of each culm sheath or leaf sheath, where the blade is attached, a thin membrane is found extending upwards along the apical margin, pressed against the underlying culm internode, culm sheath or foliage leaf. This ligule, literally a little tongue, varies in height, lateral breadth, hairiness, edge shape in terms of serration, and edge ciliation, and these characteristics are very important to distinguish species.
On the outside of the leaf sheath apex are one or two small less well developed flaps of tissue, usually known as the exterior ligule, sometimes called callus. This character is not always described as it varies less between closely related species, but the ciliation on their margins can be important. Very rare on the culm sheath.
Blades
The distal part of culm sheath and leaf sheath beyond the ligule is the blade, shorter and thicker in culm sheaths, thinner, longer and more flattened in leaf sheaths. The lower side of a leaf is called the abaxial side as it faces away from the culm when growing upright and parallel to the culm. The upper side is called adaxial, as it would face the culm instead. Overall shape varies substantially in the culm sheath, less in foliage leaf blades. Surfaces vary in hairiness. Venation is variable in leaf sheath blades, especially presence of transverse veinlets.
Vein patterns can be different, notable is the presence of transverse veinlets in temperate species. A large longitudinal midrib runs up the centre of the blade, and is accompanied by two to several dominant parallel veins on each side, with minor parallel veins between them. Reinforcing these parallel veins in temperate bamboos are short, minor, transverse veins crossing between the finer longitudinal veins. Together the longitudinal and transverse veins produce a reticulate network of tessellate venation. The standard shape and dimensions for a bamboo foliage blade are linear-lanceolate with length c. 10 times the width, and a shortly acuminate tip. Relative to this they can be broad or narrow, more linear or more lanceolate, and the tip can be acute as in Fargesia nitida to long-acuminate as in Cephalostachyum and Neomicrocalamus. The blade is joined to the leaf sheath by a narrow cylindrical to flattened neck, the pseudopetiole, which can bend and twist, to make the blade turn to face the light. It is often waxy or pubescent on one or both surfaces. I do not use the term petiole, as I consider the familiar petiole of a dicot leaf is probably homologous to the entire sheath in a woody bamboo.
The pseudopetiole in woody bamboos is a distinctive and diagnostic character. It can facilitate re-orientation of leaf blades to variable light sources in the forest understorey that change as tree canopies develop through the year. It would seem likely that it evolved in primitive grasses and has been retained in bamboos, while most other grasses, which have evolved out of the forest understorey, have lost this ability to re-orientate their leaf blades.
Auricles and oral setae
Where the blade and sheath connect, lateral projecting lobes may be found on each side. These are called auricles, meaning literally ‘small ears’. Their size and shape are very good characters for separating bamboo species, especially those on the culm sheaths, while on leaf sheaths they can be more variable.
Projecting bristles, borne on the auricles if any auricles are present, may also be seen. These are the oral setae, meaning literally ‘mouth bristles’, a term adopted from other grasses, where the circular apex of the sheath is sometimes referred to as the mouth.
Origins of bamboo auricles and oral setae
Woody bamboos differ from other grasses in the manner of connection of the blade to the sheath in their foliage leaves. In the bamboos, these are joined only by a narrow, articulating pseudopetiole. How this junction of the foliage leaf blade relates to the connection of blades to sheaths in other grasses is best understood by looking at the culm sheaths of a large tropical bamboo, in which the development into an efficient photosynthetic organ is secondary to the need to support and protect the massive but vulnerably soft, elongating culm. In many of these bamboos the blade is often as broad as the sheath itself, as in other grasses, with three often only partially separated sections. These are the larger, central blade, and two smaller, lateral, ear-like lobes, called auricles. It is hypothesised here that together these correspond to the leaf blade of other grasses, as evidenced by the parallel leaf veins that travel from the sheath into both blade and auricles. These largely vestigial veins cross the auricles and are often extended beyond the margin, forming the stiff bristles known as oral setae, which can be supplemented by marginal ciliation or fimbriation.
In many other woody bamboos the three sections of the blade are more separated in the culm sheaths as well as in the leaf sheaths, and less functional. In many species the two lateral sections, or auricles, are lacking altogether. Their sporadic occurrence or reoccurrence is typical
of vestigial reduced organs. It is only when they are exaggeratedly large, as in species such as Ampelocalamus scandens or Yushania brevipaniculata, that their true identity, as hypothesised here, becomes apparent. The spreading oral setae can then readily be seen as a kind of leaf skeleton, as produced by the decomposition of a fallen dicot leaf, except that the predominantly lateral venation of these monocotyledonous leaves gives a fringed effect rather than a reticulate pattern.
The remnants of the much finer transverse leaf veins may even be evident in species with tessellate venation, in the scabrid nature of the oral setae. There are even species in which very elongated leaf sheath auricles bear a strong resemblance to the pseudopetiole, eg Sarocalamus racemosus, Fargesia dracocephala. Nevertheless, the origin of the woody bamboo auricle as a reduced sections of a lobed leaf blade has not apparently been suggested before. It would suggest a primitive bamboo ancestor with a leaf blade having three sections and two divisions, possibly more like those seen in blades of Monstera deliciosa (Swiss Cheese Plant) rather than than the three lobes with prominent midribs as seen in palms or Arisaema triphyllum (Jack in the Pulpit). No such plants in the Poaceae are of course extant today. The adaptive advantage of loss of the lamina of the lateral lobes would be the efficiency gained by more separated rotating blades, as the sections would shade each other whenever a pseudopetiole rotated a blade to intercept the light.
Comparison with auricles in other grasses
In other grasses the sheath of some species have lateral naked membranous projections below or beside the ligule, which are also called auricles. Such appendages are seen in some bamboo species as well, where they are borne on the shoulders of the sheath apex, away from the blade. The fact that such auricles are not homologous with bamboo auricles is shown by the existence of both types of auricle together in Dendrocalamus hookeri. The culm sheaths of this species often bear an ovate auricle with oral setae attached to the blade margin, and a second, more acute, naked auricle on the shoulder of the sheath, and these two distinct forms of auricle overlapping side by side to give a partially double ‘auricle’.
Length vs width vs height vs breadth
As the acute auricles of other grasses usually extend further laterally rather than in the same orientation as the blade, their length is by convention given as their lateral extent, while their height is referred to as their width. On the other hand, as acute auricles in the bamboos are more parallel to the longitudinal axis of the sheath than the lateral axis, their length is usually given as the longitudinal dimension, and their width is the lateral dimension. This would be supported by their origin as a lateral strip of the blade, rather than a projection of the sheath instead, as in other grasses. However as the length of anything is by definition its longest dimension, it is best to refer to the longitudinal dimension as the height, and to the lateral dimension as lateral breadth. If in doubt as to what is meant, the best solution is a picture.
Characters
Culm sheath characters
(in addition to basic descriptions of shape, thickness, indumentum (hairs, marginal ciliation etc), colour, texture, venation)
persistence
quickly deciduous, eg Phyllostachys
slowly deciduous, eg Fargesia nitida
firmly attached at the centre, eg in Semiarundinaria fastuosa
persistent, disintegrating in-situ, eg in Pleioblastus
ligule
height (=length)
quantify, eg to maximum central length
outline
level, eg Himalayacalamus falconeri
arching, eg Yushania vigens
rising in centre, eg Dendrocalamus hamiltonii
asymmetrical, eg Bambusa tuldoides
margin
entire
serrate, eg Dendrocalamus hookeri
dentate, eg Dendrocalamus hamiltonii
margin ciliation
glabrous, eg
ciliate (quantify cilia length), eg
fimbriate (quantify fimbria length), eg Drepanostachyum falcatum
auricles
presence/shape
absent, eg Fargesia murieliae
rounded, eg Dendrocalamus hookeri
triangular, eg Dendrocalamus hamiltonii
linear, eg Bambusa tuldoides
falcate (=lunate), eg Yushania brevipaniculata
height (=longitudinal extent), perpendicular to ligule, quantify
breadth (=lateral extent), parallel to ligule, quantify
oral setae
length, colour
orientation
erect, eg Borinda papyrifera
radiating, eg Yushania brevipaniculata
straightness
straight
curving
angular
surface
glabrous
scabrous
blade
blade shape
needle-like, eg Neomicrocalamus
very narrow, eg Chimonobambusa
linear, eg Borinda papyrifera
lanceolate, eg Thamnocalamus spathiflorus
leafy, eg Cephalostachyum
long-triangular, eg Bambusa tuldoides
approximately triangular, eg Dendrocalamus hookeri
surface indumentum, eg adaxially densely brown-pubescent
marginal ciliation, eg margins basally angular-setose
Foliage leaf characters
sheath indumentum (hairs, marginal ciliation)
sheath shoulders
height
level, eg
elevated, eg Phyllostachys sp., 0048
ligule
height (=length), quantify and categorise
very short, eg Pleioblastus viridistriatus
short, the common situation, eg Fargesia nitida
long, eg Drepanostachyum falcatum
very long, eg Thamnocalamus chigar
outline
truncate, eg Pleioblastus viridistriatus
asymmetrically truncate, eg
rounded, eg Himalayacalamus falconeri
projecting, eg Thamnocalamus chigar
margin
entire, eg
serrate, eg
lacerate, eg
margin ciliation
glabrous
ciliate (quantify cilia length), eg
fimbriate (quantify fimbria length), eg Ampelocalamus patellaris
external ligule
not prominent
prominent
ciliation if present
auricles
presence/shape
absent, eg Borinda papyrifera
rounded, eg
oblong, eg Fargesia dracocephala ‘Rufa’
falcate (=lunate), eg Drepanostachyum intermedium
height (=longitudinal extent), perpendicular to ligule, quantify
oral setae
length, colour
orientation
erect, eg Borinda papyrifera
radiating, eg Yushania brevipaniculata
straightness
straight
curving
angular
surface
glabrous
scabrous
blade
blade shape (examples)
linear, eg Pleioblastus linearis
narrowly linear-lanceolate, eg Fargesia nitida
linear-lanceolate, eg Thamnocalamus spathiflorus
broadly linear-lanceolate, eg Shibataea kumasaca
proportions
narrow, length > 11 x width
broad, length < 9 x width
surface, eg abaxially shortly white-tomentose, proximally pilose near midrib
marginal ciliation, eg margins distally sparsely scabrous
parallel venation
quantify # main veins each side of midrib, eg 2–4-paired
transverse venation
present, venation tessellate, eg Fargesia nitida
absent, venation not tessellate, eg Bambusa vulgaris
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