Leaves and sheaths
 In most woody bamboos, bracts on the rhizomes, sheaths around the culm and foliage leaves represent 3 different well distinguished forms of the monocot ‘leaf’. Rhizome bracts and culm sheaths are mainly protective and supportive, while foliage leaf blades are responsible for photosynthesis. In other grasses there is less differentiation between the culm sheaths and the foliage leaves. Consequently some grass taxonomists refer to bamboo culm sheaths as culm leaves, which can be confusing, as there is often nothing leaf-like about them. Culm sheaths and the associated blades are very important for species identification, those from the lower mid-culm being most distinctive. Foliage leaf sheaths and blades are also important at the species level, but are rather more variable. Important parts for identification include the sheath, the ligule, auricles, oral setae, and the blade. The sheathing section itself varies in thickness, 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 unit of the bamboo plant is derived from one that bears a tubular sheath, basally attached to a node, terminating in a flatter section, the blade. As in all grasses, these are all technically ‘leaves’, but calling them all leaves can be confusing. Unlike other grasses where the leaves arising from the culm are all leaf-like, in bamboos the sheaths and blades on the culm are much more differentiated and specialised. Leaves are conceptualised as flat organs capable of photosynthesis, yet the bamboo culm sheath is mainly tubular and soon loses the initial green pigmentation. Therefore the term leaf is best avoided away from the foliage-bearing branches in bamboos. The culm bears culm sheaths, and as in all grass ‘leaves’ the flattened apical section is 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 by bamboo taxonomists as either rhizome bracts, culm sheaths or foliage leaves. The specialised bamboo culm sheath is a thickened, supportive and protective adaptation of the green leafy culm leaf that constitutes the foliage leaf of other grasses. Note that prophylls, glumes, paleas, lemmas, lodicules, anthers, and the ovary are also derived from leaves, but they are not called leaves as the specialised terminology is more useful and informative.
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 photosynthesise. The foliage leaves have a thinner and less protective or supportive sheath, a large photosynthetic blade, and are generally more persistent.
Well-specialised 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, which are most evident when the sheaths are young, provide consistent and informative characters, 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, bearing very large blades, from the upper part of the culm, and those that look too much like rhizome bracts from the very base. Near the base they have few appendages, while those at the apex are closer to foliage leaf sheaths, with longer, narrower blades, which towards the culm apex are often considerably larger than 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 described, as the important characteristics are expressed most comprehensively at that point along the culm. In the larger bamboos, that height is fortunately also the most accessible, and culm sheaths from eye-level are usually described. A good bamboo description must discriminate and cover the culm sheath and foliage leaf details in depth, listing the characteristics for each separately. A description in other grasses only gives one description, that of any leaf that comes from the culm, simply called a ‘culm leaf’.
The importance of inspecting the less ‘leafy’ of the culm sheaths, rather than those with well-developed blades cannot be overstated, and this is the main reason why the term culm leaf, widely used in other grasses, is best avoided in bamboo descriptions. It leads to a search for a leafy culm sheath, which will never have the correct features as expressed on the more differentiated culm sheaths lower down the culm, and to reiterate, it is exactly these features that are the most informative and useful in the separation of different bamboo species.
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, visible edge shape in terms of dentation or fissures, and edge ciliation. These characteristics are very important for separating species.
On the outside of the leaf sheath apex, between the shoulders or auricles and on each side of the base of the pseudopetiole are two small less well developed flaps of tissue, joining or nearly joining in the centre, usually known as the exterior ligule, sometimes called callus. This character is not always described as it is difficult to see and varies little between closely related species, but the degree of development, their length, and the ciliation on their margins can be important. These are only very rarely seen on culm sheaths, which do not have pseudopetioles, but they are sometimes present, for example in Melocanna.
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 differ mainly in the presence or absence of transverse veinlets, found mainly 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. Each margin is a parallel vein, and the blade margins are always visibly entire and never crenulate, serrated or dentate. Supplementing 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 known as tessellate venation. This conflicts with the inaccurate but universally repeated distinction between parallel venation in monocots vs. reticulate venation in dicots, and a better distinction would be dicots having dendroid venation in distinction to the parallel venation of monocots.
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.
Blades and pseudopetioles can have many different forms of indumentum or wax on their surfaces. Like sedges, they usually also have projections along or around the vein that forms the margin and on veins nearby, usually very short hardened points that make the edge feel rough (scabrous) or longer bristles towards the base (setaceous). These are denser on the margin that is external when the blade is rolled, and towards the leaf tip. Rich in silica, these deter grazing. In bamboos the blade margin always appears smooth, and so is entire, and never appears to the naked eye like symmetrical teeth (dentate) or the forward pointing serrations of a sawblade (serrulate), so common in dicot leaves, but only very rarely seen in grasses, eg Brachiaria serrifolia.
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 known as oral setae, meaning literally ‘mouth bristles’, a term adapted from other grasses, where the circular apex of the sheath is sometimes referred to as the mouth, and long hairs are often found in this general region, the ‘oral hairs’.
Origins of bamboo auricles and oral setae
Both the auricle and the oral seta may be formed in two different ways in the bamboos. Woody bamboos differ from most 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 apex itself, as in most 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 may correspond to the leaf blade of other grasses in some bamboos. Evidence for this is the parallel leaf veins that travel from the sheath into both blade and auricles. These largely vestigial veins can cross the auricles and be extended beyond the margin, forming the stiff bristles known by analogy to possibly non-homologous structures in grasses as oral setae. These can be supplemented by marginal ciliation or fimbriation, like the softer oral hairs seen in other grasses. Several fine cilia can also be combined to produce tougher bristles, in the same position and having the same function, also called oral setae, but more in the form of fimbriation, and more friable.
In temperate woody bamboos the three components are more separated in the culm sheaths as well as in the leaf sheaths, and less functional. In many bamboo species the two lateral sections, or auricles, are lacking altogether, and this is a specific not generic trait. This sporadic occurrence or reoccurrence is typical
of vestigial reduced organs. When they are exaggeratedly large, as in species such as Ampelocalamus scandens, A. actinotrichus or Yushania brevipaniculata, the possible origin as hypothesised above, seems more plausible. The spreading oral setae can then be more readily envisaged 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.
There are species in which elongated leaf sheath auricles bear a strong resemblance to the pseudopetiole, eg Sarocalamus racemosus, Fargesia dracocephala.
The origin of the auricles of woody bamboos as reduced sections of a lobed or compound leaf blade has not apparently been suggested before. I conjecture that it would suggest the existence of primitive bamboo ancestors with lobed leaf blades having three lobes or sections and two deep divisions, like those seen in leaves of Monstera deliciosa (Swiss Cheese Plant). This could have developed towards a compound leaf as seen in palms or the foliage of Arisaema triphyllum (Jack in the Pulpit). No such plants in the Poaceae are of course extant today, but the fossil record is incomplete for leaves, and a lobed or compound leaf fossil would hardly be recognised as graminoid anyway. The three separate sections could rotate to face scarce and changeable light sources better than a single grass blade as pseudopetioles developed, or the lateral lobes could shrink and make room for a swivelling central portion with pseudopetiole. Why the oral setae would persist is another question altogether, but there was probably some protective function.
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 or projections are seen in many bamboo species as well, where they are part of or are borne on the shoulders of the sheath apex, away from the blade. The possibility that different kinds of auricle are not homologous with each other 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 boliana
rising in centre, eg Dendrocalamus hamiltonii
asymmetrical, eg Bambusa tuldoides
margin appearance to the naked eye
entire
serrulate/denticulate, eg Dendrocalamus hookeri
serrate/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
structure
solid
compound, friable
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 abaxially densely brown-setose see separate illustrated key
marginal projections (thickened), eg margins scabrous or basally setaceous
Foliage leaf characters
sheath indumentum (surface trichomes - see separate illustrated key, marginal ciliation)
sheath shoulders
height
level, eg
elevated, eg Semiarundinaria sp.
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
structure
solid
compound, friable
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 indumentum, eg abaxially shortly white-pilose, proximally villous near midrib
see separate illustrated key
marginal projections, eg P. nigra one margin distally scabrous, one smooth
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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