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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 internode throughout the bamboo plant is surrounded by a tubular sheath, basally attached to the node, terminating in a flatter section, the 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, is best avoided in bamboo descriptions. It leads to a search for a leafy culm sheath, which will not have the correct features that distinguish the more differentiated culm sheaths. The different origins for grass and bamboo blade and auricles postulated below also highlight the potential pitfalls of trying to standardise strict terminology across bamboos and other grasses.

 

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, 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 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

auricles and oral setae on Phyllostachys edulis
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

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 correspond to the leaf blade of other grasses. Evidence for this is 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 by analogy to non-homologous structures in other grasses, as oral setae, which can be supplemented by marginal ciliation or fimbriation, akin to the oral hairs seen in other grasses.

In temperate 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 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
auricle as reduced lateral blade section  with setae as reduced veins?
of vestigial reduced organs. It is only when they are exaggeratedly large, as in species such as Ampelocalamus scandens, A. actinotrichus or Yushania brevipaniculata, that their true identity, as hypothesised here, becomes apparent, to me at least. 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.

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 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 boliana

                        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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