i rai , leaf spine (70). P: prickle (76). Se: stem emergence (116). NO Ne Ss: stem spine (124). Sts: stipular spine (56). Rs: root spine (106). 8 | Methods of description We have in our library a book entitled Natural Illustrations of the British Grasses edited by F. Hanham (1846). The preface tells us that the success of the book depends entirely upon the illustrations which are, in fact, dried specimens of actual grasses of which 62 OOO were collected for the edition. Actually, plants lose many morphological features when pressed and dried, such as colour, hairiness, and three- dimensionality [Corner (1946) points out that many plants with spiral phyllotaxy (218) have been recorded as having distichous leaf arrangement when studied from pressed specimens alone]. Nowadays it is customary to record aspects of plant morphology using a combination of illustrative methods. A photograph is an obvious choice, colour being preferable to black and white as the brain can have trouble deciphering grey tones. However, a photograph alone is not enough as it is likely to contain a great deal of distracting ‘noise’ both on the subject itself and in the background. It is better to augment or replace a photograph with line drawings (see for example 106 and 205b, 62b and 63b). These will range from accurate detailed representations of actual specimens to drawings in which line work is kept to a minimum in the interest of clarity (but not at the expense of accuracy), to various diagrammatic versions of the actual specimen or even hypothetical diagrams (such as 183, 253) of its construction (an example of these various possibilities is given for one specimen on pages 11 and 13). A particularly useful feature of a greatly simplified diagram is that it can take the form of a cartoon series to illustrate the developmental sequence of a particular morphological feature (e.g. 11b, c, d, e). The simplest form of this type of illustration can be termed a ‘stick’ diagram in which the thickness of the organ is ignored and a stem, for example, can be represented as a fine line with leaves and axillary shoots portrayed symbolically (9b). However, stick diagrams suffer the same limitations as pressed plants in that it is difficult to retain three-dimensionality. It is for this reason that the combination of photograph and/or accurate line drawing plus diagrammatic portrayal is most informative. In addition the relative juxtaposition of parts can be indicated by another type of diagram, the ground plan (or specifically floral diagram for flowers and inflorescences 150). The ground plan depicts a shoot system, or flower, as if viewed from directly above. The position of leaves, buds, and axillary shoots are located on the plan in their correct radial (i.e. azimuth) positions, the youngest, Fig. 8. Euphorbia peplus The inflorescence consists of a symmetrical set of cyathia (144, 151f), each resembling a flower but in fact composed of numerous much reduced flowers. -_——— ee eee ee le ee ——— distal organs are sited at the centre of the diagram and are best drawn in first, and the oldest (lowest, proximal) organs are at the periphery. All components are represented in a schematic fashion, there being a more or less conventional symbol for a leaf, for example. Two leaves which are in reality the same size, may appear as different sizes in the plan, the more distal, inner, being drawn the smaller. Conversely, a proximal, small, scale leaf may appear on the plan as a larger symbol than a more distal foliage leaf. Nevertheless the ground plan approach is a most valuable adjunct to the other forms of diagrammatic illustration as it reveals underlying patterns of symmetry (228) or their absence, in the construction of shoot systems (including flowers). The four types of monochasial cymes illustrated in ‘stick’ form (141s, t, u, v) are repeated here in ground plan form (9d, e, f, g). The power of illustration in conveying detailed information should not be belittled: ‘Artistic expression offers a mode of translation of sense data into thought, without subjecting them to the narrowing influence of an inadequate verbal framework; the verb ‘to illustrate’ retains, in this sense, something of its ancient meaning—‘to illuminate’ ”’ (Arber 1954). Fig. 9. a) Shoot drawn from life. b) ‘Stick’ diagram of ‘a’ indicating its component parts. c) Plan diagram of ‘a’ showing relative locations of parts. The same two leaves are labelled x and y in each case. d)-g) Plan diagrams of the four types of monochasial cyme shown in stick form in Fig. 141s—v. Each shoot symbol (a circle) has the same patterning as that of the leaf symbol in whose axil it is located. (a) (b) (f) Methods of description | 9 10 | Methods of description: example Philodendron Many species of Philodendron (Araceae) have a distinct morphology which presents a number of the features described in this volume. The shoot organization is not immediately apparent from casual observation, but can be interpreted by study of the plant as it develops (Ray 1987a, b). A description of this admittedly complex plant is given here in order to demonstrate the use of various descriptive methods. Figure 10 shows the general features of a young vegetative plant of Philodendron pedatum collected in French Guiana. The photograph gives an overall impression of the plant, but this is enhanced by the accompanying line drawing (1la) which eliminates confusing detail and background and allows the major features of the plant to be labelled. At first sight the stem of the plant appears to bear an alternating sequence of large scale leaves, represented by their scars except for the youngest, and foliage leaves. Close scrutiny will fail to find an axillary bud associated with the foliage leaf and it will be noticed that two leaves appear on one side of the stem followed by two more or less on the opposite side as indicated in Fig. 13e. If the shoot represents a monopodial system (250) then the plant must have an unusual phyllotaxis. Figure 13d illustrates this in a simplified manner and this is repeated in more simple ‘stick’ fashion in 13e. The adventitious roots (98) present at each node (11a) are omitted from these diagrams for simplicity. Close study of Philodendron shows that the sHoot axis is in fact sympodial (250) in its construction, each sympodial unit (shown alternately hatched and unhatched in Fig. 13b) terminates as an aborted apex (244) in this juvenile plant (in a mature plant these distal ends of sympodial units could be represented by inflorescences). The aborted apex is usually barely visible. In a sympodial sequence, growth of the shoot is continued by the development of a bud on the previous sympodial unit. In the case of this Philodendron species, the bud that develops is one of two buds (accessory buds 236) in the axil of the first leaf of each sympodial unit (i.e. the prophyll 66), with the second leaf, the foliage leaf, subtending no bud. (Continued on page 12.) Fig. 10. Philodendron pedatum Young plant. Latest developing leaf is emerging from the protection of a prophyll (66). Methods of description: example Philodendron | 11 (a) Fig. 11. a) Philodendron pedatum Young plant (cf. 10). b)-e) Developmental sequence indicating how the upper bud (hatched) of the pair in the axil of a prophyll displaces the shoot apex to become the next sympodial unit. Compare with Fig. 13b. Ab: accessory bud. Adv: adventitious root. H: hypopodium. L’: leaf lamina. P: prophyll. Pe: petiole. Ps; prophyll scar. Sa: shoot apex. YI: young leaf. (b) (c) (d) 12 | Methods of description: example Philodendron continued Fig. 12. Philodendron pedatum Close up of stem showing prophyll (66) scars and adventitious roots (98). Shoot apex bent over to upper right. This photograph is represented by the upper half of Fig. 13a. The developmental sequence of events in Philodendron (page 10) is shown in Fig. 11b, c, d, e. The upper, hatched, bud in the axil of the prophyll develops rapidly displacing the shoot apex to one side and leaving behind the second, lower bud in the axil of the prophyll. This bud can be seen in Fig. 12. The sympodial nature of the axis is represented in Fig. 13a, b, c. Figure 13a gives a stylized appearance of the shoot, 13b indicating the locations of the successive sympodial units, and 13¢c (a stick diagram) giving a truly diagrammatic representation of the shoot construction. Stick diagrams such as this are extremely useful in conveying plant construction with a minimum of background noise. However, it is not easy to illustrate the three-dimensional aspect of the morphology by this means; the relation of one leaf to another can be portrayed in the form of a ground plan diagram (see page 8), being the vegetative equivalent to a floral diagram (150). Figure 13f indicates the juxtaposition of parts of the sympodial specimen (13c) and 13g the plan that would be found if the shoot was monopodial (13e). The precocious elongation of each sympodial unit results in a substantial bare length of stem between the point of attachment of this side shoot on its parent, and the node bearing the propyll (‘H’ in Fig. 13a). Such a portion of stem, proximal (8) to the first leaf on a shoot is termed a hypopodium (see syllepsis 262). So in this young state, this particular species of Philodendron has a sympodial shoot system, each unit of which bears just two leaves and two buds (both in the axil of the first leaf). However, it is recognized that plants can go through a sequence of morphological forms as they develop, each of which can be described as an age state (314). The seedling Philodendron probably undergoes a period of monopodial establishment growth (168) before switching to the sympodial sequence described here; this is deduced from the activity of the second prophyll bud on this plant which, if it develops, does so monopodially at first (and thus represents a reiteration 298). Furthermore the sympodial sequence shown, with prophyll scale leaf followed by a single foliage leaf, itself gives way to a different sympodial sequence in which the second leaf aborts when about 1 cm in length, whilst the hypopodium is greatly extended (66a). In this state the plant climbs rapidly. It has yet to reach a mature stage with enlarged foliage leaves and a reproductive capacity. Details of the very precise sympodial sequences found in the family Araceae are given by Ray (1988). (a) Ab Ps (b) H Pe Pp L2 (c) S Sa LS p \ e SS SS SS SS S S S p2 Pe Methods of description: example Philodendron continued | 13 1 | Fig. 13. Shoot construction of Phi/jodendron pedatum (cf. 11). a)-c) Alternative methods of depicting the sympodial sequence. d), e) Diagrams showing the superficial monopodial appearance. f) Plan view of ‘b’. g) Plan view of ‘d’. Ab: accessory bud. H: hypopodium. L: foliage leaf. P: prophyll. Pe: petiole. Ps: prophyll scar. Sa: shoot apex. The specific features labelled in the lower half of b) (Ab', L', L?, P’, P?, Sa’) have corresponding labels in the ground plan f). 14 | Flowering plants: monocotyledons (M) dicotyledons (D) This book describes the morphology of flowering plants. Taxonomically these represent seed producing plants (Spermatophyta) as opposed to spore producing plants, and further represent plants in which the seeds are contained within a fruit (154) (Angiospermae, or Angiosperms, from the Greek ‘angeion’—a vessel, i.e. container, +sperma, seed) in distinction to the Gymnospermae in which the seeds are naked (e.g. principally conifers). The flowering plants fall naturally into two categories: the dicotyledons and the monocotyledons (D or M in the index). The differences between these two groups are marked and the botanist can usually tell the one from the other at a glance even when meeting a plant for the first time. There are however, also plenty of species whose monocotyledonous or dicotyledonous affinities are not at first sight apparent. Monocotyledons include palms, gingers, lilies, orchids, grasses, sedges, bananas, bromeliads, and aroids; dicotyledonous plants include most trees and shrubs, and herbaceous and woody perennials. As their name suggests, monocotyledons have one cotyledon (seedling leaf 162) whereas dicotyledons almost invariably have two. The flowers of monocotyledons usually have components in sets of three whereas dicotyledons very rarely have flower parts in sets of three; four or five being more typical. The principal difference between the two groups of a morphological nature involves their mode of growth. The stems of monocotyledons, with few exceptions, lack the ability to increase continuously in girth, i.e. they lack a meristematic cambium (16). Very many dicotyledons do possess this tissue and their stems and roots can grow in diameter keeping pace with the increase in height. Increase in stature in a monocotyledon takes place by means of establishment growth (168) in which each successive internode (4) or sympodial component (250) is wider than the last. One consequence of this difference and of a difference in vascular anatomy is that the leaf of a monocotyledon is usually attached more or less completely around the stem circumference at.a node, whereas a dicotyledon leaf is more often attached on one relatively narrow sector of the stem circumference. More fundamentally, the first root (radicle 162) of a dicotyledon is quite likely to increase in size as the plant above grows. This also means that the proximal (4) end of the root system increases in size as more and more lateral roots develop distally; no bottleneck or mechanical constriction is formed. This cannot occur in the majority of monocotyledons and the roots initially developing from the embryo are soon inadequate in diameter to serve the growing plant. All monocotyledons develop an adventitious root system (98), i.e. numerous additional, but relatively small roots extend from the stem of the plant. This is particularly well seen in rhizomatous (130) or stoloniferous (132) monocotyledons which are usually sympodial (250) in construction and in which each new sympodial unit will have its own complement of new adventitious roots. The lack of a cambium in monocotyledons is also reflected in the limitation of their above-ground branching. When a bud of a monocotyledon develops into a shoot it must usually do so by the progressive increase in diameter of each successive internode as in the establishment of a seedling (169c); the whole base of the new plant or branch cannot grow in diameter as is possible in dicotyledons. The consequences for aerial branching would be a mechanically unstable constriction at the point of attachment of the branch. Monocotyledons that do branch aerially either have very slender branches (e.g. bamboos 192) or branches supported by prop roots (100) or gain support by climbing (98) or form a mechanically sound joint by precocious enlargement of the side branch at the time when the parent stem is itself still growing, i.e. the two develop in unison (11b, c, d, e). A commonly stated ‘rule of thumb’ to distinguish a dicotyledon from a monocotyledon is that a dicotyledon leaf probably has a petiole (40) and is reticulately (net) veined (34) whereas a monocotyledon leaf usually lacks a petiole and is parallel veined. However, there are innumerable exceptions to both these sets of generalizations (e.g. 21b, 35). Flowering plants: monocotyledons (M) dicotyledons (D) | 15 (b) Fig. 15. a) Setcreasea purpurea Lb (Commelinaceae), a monocotyledon. b) Catharanthus roseus (Apocynaceae), a dicotyledon. Lb: leaf blade. Ls: leaf sheath. Pe: petiole. 16 Fig. 16. Cyclamen cv A developmental ‘mistake’ (270) in which a flower bud is joined to the stem axis (adnation 234) and is being pulled away from its subtending leaf so much that the flower stalk (pedicel 146) has snapped. Meristems and buds: basis of plant development All the various organs and morphological features of a plant are made up of cells, growth and development taking place in localized regions of active cell division and enlargement. Such regions are termed meristems (18, 94, 112) and typically there is a meristematic zone at the apex of every shoot (a shoot apical meristem) and every root (a root apical meristem) on the plant. The apical meristem of a shoot may be protected, particularly if in a resting stage, by older tissues and organs such as scale leaves to form a bud (264). However the shoot apical meristems of many plants undergo more or less continuous growth and do not rest in bud form, thus these axillary shoot apical meristems develop contemporaneously with that of the supporting axis (syllepsis 262). Lateral roots (96) and adventitious roots (98) develop from root apical meristems that arise deep in the tissue of existing roots or stems, respectively (94). Meristematic activity at shoot apices gives rise to new leaves. The first stage is the appearance of a leaf primordium (18), in which cell division of specific meristems results in a leaf shape characteristic for the plant. The leaf edge expands as a result of meristematic activity of the marginal and plate meristems of the leaf, for example (19c, d). In the majority of monocotyledons (14) and in many dicotyledons, the entire plant body is built up by cell division and enlargement at the apical meristem of shoot and root. This is referred to as the primary plant body. In numerous dicotyledons, a second form of meristematic activity can also take place which results in the enlargement of the existing primary plant body. Within the primary stem and root there is a cylinder of cells that retains its meristematic properties. This zone, the cambium, is sometimes referred to as a lateral meristem to distinguish it from an apical meristem. Cell division within the cambial cylinder leads to expansion in girth of the stem or root by the production of secondary tissue including vascular tissue. A constantly enlarging plant, such as a tree, is built up in this way. Reference should be made to an anatomical textbook (e.g. Esau 1953; Cutter 1971) fora comprehensive account. A few monocotyledons have a similar process producing secondary tissue by means of lateral meristem activity and form branched tree structures (e.g. Cordyline, Dracaena). Other monocotyledonous trees, such as palms, gain their stature following establishment growth (169c). A second type of lateral meristem, again in the form of a cylinder, may be present just beneath the surface of a stem or root; this is termed a phellogen, or cork cambium, and gives rise to the bulk of the bark (114). Meristems and buds: basis of plant development | 17 ugh the . Longitudinal section thro oleracea shoot apex. The leaves % 2 a 7) As] ~ se) ~ - 2 uw ation (149c). show crumpled vern 18 | Leaf morphology: development Fig. 18. Plumeria rubra Shoot apex before flowering showing sequence of leaf development. New leaves develop at the surface of the apical meristem of a shoot which is itself extending by cell division and enlargement (16). Thus each new leaf, termed a leaf primordium, is left behind on the flanks of the axis as the shoot extends. The most recent leaf primordium to appear at the apical meristem is the least developed, and successively older leaf primordia are progressively more elaborate due to the activities of meristems within the leaf itself. The primordium of a dicotyledonous leaf is usually confined to a relatively narrow sector of the shoot circumference whereas in contrast a monocotyledonous leaf primordium is initiated and therefore develops around most, if not all, of the shoot apex. Thus very young dicotyledonous leaves are peg-like structures (19a) and correspondingly young monocotyledonous leaves are collar-like structures (19b) surrounding or even arching over the shoot apex. The sequence in which new primordia appear at the apex will give the plant its particular phyllotactic arrangement (218). A leaf primordium will continue to grow in size and gradually attain its destined determinate size and shape. Increase in leaf size results from an increase in cell numbers followed by an increase in cell size. Cell division is loosely confined to identifiable meristematic regions (16) in the leaf and it is the differential activity of these regions that produces different leaf shapes. At first the apical meristem of the leaf is active and the leaf elongates, subsequently leaf elongation results from activity of the intercalary meristem (19c). This meristem can have a prolonged activity, in grasses for example (180). A horizontally flattened shape (bifacial or dorsiventral) will result if the marginal meristems become active (19c), leaf width being increased by division in the plate meristems (19d). If the marginal meristem is only active at sites dispersed along the leaf edge then a pinnate leaf (22) will result. Each leaflet of a pinnate leaf develops from an isolated patch of marginal meristem and will be organized in a similar manner to a whole simple leaf (19e). The midrib becomes thicker than the lamina due to cell division of the adaxial meristem (19d). If the adaxial meristem continues to contribute to thickness in this region and at the same time the marginal meristems are inactive, then the leaf will be flat in the vertical plane (lateral flattening) and result in an ensiform leaf (86). Between monocotyledons and dicotyledons there is a fundamentally different emphasis of meristematic activity towards either the base of the very young leaf primordium (lower leaf zone) or the apex of the primordium (upper leaf zone) (20). Also in some instances controlled cell death plays a part. This is responsible for the indentations and holes that appear early in the development of leaves of some members of the family Araceae (10) and occurs in the formation of compound leaves in the palms (92). Areas with meristematic potential . may remain on parts of a leaf and subsequently develop into vegetative (233) or inflorescence (75g) buds. In a few plants the apical meristem of the leaf remains active and the leaf can continue to grow apically for an extended period (90). Leaf morphology: development | 19 (a) (b) Fig. 19. a) Diagrammatic representation of the shoot apex of a dicotyledon, and b) of a monocotyledon. c) The meristematic zones of a simple developing leaf seen from above, and d) in section. e) The same components apply to the leaflet of a compound leaf. Adm: adaxial meristem. Am: apical meristem (of the leaf). Im: intercalary meristem. Lp: leaf primordium. Mm: marginal meristem. Pm: plate meristem. (c) (d) Adm 20 | Leaf morphology: upper and lower leaf zones Studies of the very early sequences of growth of leaf primordia indicate that the two ends of the primordium, the distal (apical) end and the proximal (basal) end, give rise to specific parts of the mature leaf (Kaplan 1973b). A fundamental difference is found in the development of monocotyledonous and dicotyledonous leaves. In many ‘typical’ dicotyledonous leaves the proximal end of the primordium (lower leaf zone) will develop into the leaf base which may or may not ensheath the stem (50) together with the stipules if present (52). The distal end of the primordium (upper leaf zone) develops into the dorsiventrally flattened leaf blade (21c) (lamina) or laterally flattened phyllode in the case of some Acacia species (44). Subsequent activity of an intercalary meristem and an adaxial meristem (18) may separate the base from the lamina by the development of a unifacial (i.e. more or less cylindrical) petiole (40). However, if the relative development of the lower and upper leaf zones of a dorsiventral monocotyledonous leaf are monitored, it is found that the whole of the leaf, sheathing base plus lamina (21e) and also the petiole if present (21b), is derived from the lower leaf zone. The upper leaf zone hardly contributes to the mature leaf structure at all but may be present in the form of a unifacial rudimentary ‘precursor tip’ at the apex of the leaf (20a, b). Some monocotyledons have unifacial leaves (86), the distal unifacial portion being substantially longer than the basal sheath (21a). Studies of development of these leaves show that the unifacial portion develops from the upper leaf zone and is equivalent to the precursor tip of bifacial monocotylendonous leaves and thus also in these cases equivalent to the lamina of a dicotyledonous leaf. Indeed the development of such a unifacial monocotyledonous leaf is virtually identical to the development of the unifacial leaf that occurs in some dicotyledons although the latter may show rudimentary pinnae (e.g. 21d). Conversely a few dicotyledon leaves are equivalent in their development to monocotyledon leaves in that the bifacial portion develops from the lower leaf zone (89c). In a heteroblastic sequence (29d) of leaves the change of leaf shape emphasizes changes in relative activity of upper or lower leaf zones. Fig. 20a. Musa sp. Precursor tip at distal end of unrolling leaf. Fig. 20b. Sansevieria sp. Terete (86) precursor tip at distal end of leaf. Leaf morphology: upper and lower leaf zones | 21 Fig. 21. Comparison of single leaves of monocotyledons (M) and dicotyledons (D). a) A//ium crepa (M), b) Monstera deliciosa (M), c) Ranunculus repens (D), d) Foeniculum vulgare (D), e) Rossioglossum grande (M). La: lamina. Pe: petiole. Pt: precursor tip. Rp: rudimentary pinnae. Sh: sheath. U: upper leaf zone. L: lower leaf zone. 22 | Leaf morphology: shape The shape of a leaf depends on its development, usually in terms of cell division and enlargement, but also due to cell death in some cases (18). There is a very precise and extensive terminology applied to the lamina shapes of simple (22, 23a, c, f) leaves and to the individual leaflets, each of which has its own stalk, of compound leaves (23b, d, e, g). Such terms refer to the lamina base, tip, the margin, and overall geometry. Thus a simple leaf may be described as widely ovate, apex caudate (with a tail) and base cordate (35b), i.e. heart-shaped with a long drip- tip. Definitions of these terms in common usage may be found in the glossary of any flora. A compound leaf may be simply pinnate, leaflets or pinnae being arranged in an alternate (124a) or opposite (27b) fashion, sometimes the one merging into the other (23b). There may be a single terminal leaflet (imparipinnate 57f) or this may be absent (paripinnate 27a, 23e) or represented by a pointlet (79). If the leaflets are of variable size the compound leaf is described as interruptedly pinnate (271h). The central stalk bearing the leaflets is termed the rachis. In a bipinnate leaf the rachis bears rachilla on which the leaflets (pinnules) themselves are inserted (23e). The stalk of each individual leaflet can be termed a petiolule. If all the petiolules are attached at one point the compound leaf is palmate (27e). More precise terms can be applied to palmate leaves having a consistent number of leaflets, e.g. bifoliate and trifoliate (23g). The term unifoliate can be applied to a simple leaf in which the lamina is articulated on the petiole Fig. 22. Calathea makoyana Translucent simple leaf with chlorophyll confined to specific areas mimicking a pinnate leaf. (49d). (Continued on page 24.) Leaf morphology: shape | 23 Fig. 23. Shapes of single leaves. a) Passiflora coriacea, simple; b) Sophora macrocarpa, simply pinnate; c) Banksia speciosa, pinnatisect, lobed to the mid-rib; d) Weinmannia trichosperma, simply pinnate; e) Rhynchosia clarkii, bipinnate; f) Grevil/lea bougala, pinnatifid, lobed nearly to mid-rib; g) Lardizabala inermis, trifoliate. R: rachis. St: stipule (52). Sti: stipel (58). 24 | Leaf morphology: shape continued More elaborate configurations exist and will be described in specialist works on taxonomy and systematics (e.g. Radford et al., 1974). All these terms apply to dorsiventrally flattened leaves. In addition leaves may be variously three- dimensional (24), laterally flattened or radially symmetrical (86), may bear various structures on their surface (74, 76, 78, 80), or may be represented by tendrils, hooks (68), and spines (70). Plant species exhibit a remarkable range of leaf shapes (22). Indeed the manner of development of a leaf (18) permits almost any configuration, subject to mechanical constraints. By no means all leaves are bilateral (86, 88), and many have a three-dimensional construction. If only an occasional leaf on a plant is a bizarre shape, it is likely to represent an example of teratology (270) or possibly gall formation (278). Leaves may bear other organs—epiphylly (74). The illustrations here (24, 25) depict just a very few of the leaf shapes that can be found; many other examples could be used. The base of the leaf and its juxtaposition with the stem (sheath 50) also presents a range of forms, again often of a three- dimensional construction. The petiole of the leaf (40) and the mid rib (rachis) of a compound leaf (22) may be winged, the manner in which the wings meet the stem varying considerably. Four of the more usual forms are auricled (25c), amplexicaul (29c), perfoliate (25b), and decurrent (24). The leaves on any one plant may have a range of shapes either of distinctly different forms (30) or in developmental series (28). Fig. 24. Onopordum acanthium The decurrent leaf bases extend down the stem as wings. SS 10 mm | Leaf morphology: shape continued | 25 Fig. 25. Shapes of single leaves. a) Sauromatum guttulatum, palmate; b) Montia perfoliata, perfoliate; c) Senecio webbia, auricled at petiole base; d) Othonnopsis cheirifolia, simple; e) Foenicu/um vulgare, multi-pinnate. 26 | Leaf morphology: symmetry Leaves of all shapes vary very much in their degree of symmetry. Asymmetry is much more pronounced in some species than others and may occur to varying extents on the same plant. Asymmetry in a given species can be precise and repeated by all leaves (243) or may be imprecise so that each leaf has a unique shape in detail (27d). Thus simple leaves are frequently asymmetrical at their base and then the shoot as a whole may or may not be symmetrical due to mirror imagery of leaves to left and right (32). In the Marantaceae, leaves are more or less asymmetrical about the midrib (22), the wider more convex side being rolled within the narrower straighter side in the young state. The wide side may be to the left or right viewed from above and this may or may not be consistent in a given plant or species. The arrangement can be antitropous (27h) or more frequently homotropous (27f). A theoretically possible alternative homotropous configuration (27g) does not seem to occur (Tomlinson 1961). Pinnate leaves frequently show a degree of asymmetry both in the apparent absence of some pinnae (45) and in the admixture of first order leaflets with second order rachillae (47c). Compound leaves with symmetrically opposite leaflets at their proximal ends may have asymmetrically alternate leaflets at their distal ends (69f) or vice versa (27 1h). Fig. 26. Manihot utilissima Palmate leaf. Leaf morphology: symmetry | 27 Fig. 27. Shapes of single leaves. a) Ca/liandra haematocephala, bipinnate; b) Azi/ia eryngioides, pinnate; c) Acacia hindisii, single pinna cf. 79; d) /sopogon dawsonii, bipinnate; e) Cussonia spicata, palmate; f)—h) Asymmetrical leaf arrangement in the Marantaceae (M); f) homotropous; g) not encountered; h) antitropous. (f-h after Tomlinson 1961.) 28 | Leaf morphology: heteroblasty, shape change along a shoot The leaves on a plant often vary greatly in size and shape, some may be foliage leaves, some may be scale leaves (64), and this general phenomenon of variability is described as leaf polymorphism or heterophylly, although the latter term is perhaps better retained to apply specifically to changes in leaf form induced by the environment. If the plant has two very distinctive types of leaf the condition is described as dimorphism (30). In other cases two leaves of different size or shape occur at the same node, this arrangement being described as anisophylly (32). In addition all plants show at some stage in their development a changing progression of leaf shape, this sequence being described as a heteroblastic series such as almost inevitably occurs along the seedling axis of the plant (28, 29a), and often is also present along any developing lateral shoot (29d). For example the first leaves on axillary shoots might be scale leaves, each leaf being slightly more elaborate than the previous. This might give way gradually to a sequence of foliage leaves, and then the sequence may revert back to the production of scale leaves similar to those at the proximal end of the shoot. Such a shoot might then terminate in an inflorescence, each flower of the inflorescence being subtended by a bract (62) which is in itself a form of scale leaf. Be Fig. 28. Albizzia julibrissin bipinnate foliage leaves. This seedling has the sequence out A heteroblastic sequence consisting of a pair of simple of step as the second bipinnate leaf is less elaborate than the cotyledons followed by a once-pinnate foliage leaf and two first. SS OOOO Leaf morphology: heteroblasty, shape change along a shoot | 29 Fig. 29. a) Alisma plantago, seedling; b) Kennedia rubicunda, seedling; c) Epidendrum ibaguense, single shoot; d) Prunus avium, leaf sequence on developing shoot. Co: cotyledon. Efn: extra-floral nectary (80). If: intermediate form. Sc: scale leaf. SI: simple leaf. St: stipule (52). TI: trifoliate leaf. ip Nee 30 | Leaf morphology: dimorphism, two distinct shapes on one plant One of the most obvious types of heterophylly (different leaf forms on the same plant 28) is that of dimorphism. This is the production of two totally different shapes of leaf during the life of the plant. The phenomenon is true of most plants in the sense that the cotyledons are usually distinct in form to subsequent leaves (cf. onion 163e) and likewise many plants bear scale leaves (64) on perhaps rhizome, bud, or in association with flowering (bracts 62). However, some plants illustrate an abrupt change of leaf form associated with environment such as occurs in water plants where there may be a submerged leaf form and an aerial leaf form. Similar abrupt changes in leaf form can occur in the aerial system of plants (31c) or between juvenile and adult portions of the plant (31a, b; 243). Fig. 30. Acacia pravissima A young seedling axis showing sudden transition from bipinnate leaves below, to simple phyllodes (42) above. Leaf morphology: dimorphism, two distinct shapes on one plant | 31 ee Fig. 31. a) Hedera helix, adult form; b) Hedera helix, juvenile form; c) Dracaena surculosa, aerial shoot; d) Cephalotus follicularis, seedling from above. FI: foliage leaf. PI: pitcher leaf (88). Sc: scale leaf (64). 10 mm 32 | Leaf morphology: anisophylly, Fig. 32. Urtica pilea One large leaf and one small leaf at each node. two distinct shapes at one node Anisophylly is a term commonly applied to a condition of heterophylly (28) in which different sized or shaped leaves occur at the same node (i.e. nodes with opposite phyllotaxy 219i). However, its use is appropriate in any situation where a difference of leaf form or size is repeated on a regular basis. On horizontal shoots with opposite and decussate phyllotaxy (219j) the leaves of the lateral pairs are likely to be of the same size whilst the leaves of the dorsiventral pairs will be unequal in size. There is evidence that such anisophylly can be primary, resulting from an irreversible difference in leaf primordium size from the start, or secondary, dependent upon shoot orientation at the time of development of the leaf pair. Anisophylly can also occur in horizontal (plagiotropic 246) shoots in which there is only one leaf per node. In this situation, leaves borne on the upper side of the shoot will be of a different size (usually smaller) than those on the lower side. This type of anisophylly is sometimes referred to as lateral anisophylly distinguishing it from the nodal anisophylly recorded above. Plants in which all leaf pairs are anisophyllous may show an overall symmetry of pattern within a branch complex which is particularly apparent if the leaves themselves are asymmetrical (33e). Frequently the activity or potential (242) of the bud or buds in the axil of the larger leaf of an unequal pair is greater or different compared with the bud in the axil of the smaller leaf (anisoclady 33a, b, c). Anisophylly can occur even at the cotyledon stage (anisocotyly 163f, 209). Leaf morphology: anisophylly, two distinct shapes at one node | 33 Fig. 33. a) Be/operone guttata; b) Eranthemum pulchellium; c) Monochaetum calcaratum, single node; d) Phellodendron lavallii, developing shoot pair; e) plan view of shoot system of B. guttata (a) indicating symmetry of large and small leaf at each node, e.g. Aa, Bb. Cl: compound leaf. SI: simple leaf. 34 | Leaf morphology: venation, pattern of veins The veins (i.e. anatomically speaking the vascular bundles) form prominent features of many leaves. The pattern of venation is often distinctive for a given plant species or may be characteristic for a larger taxonomic group. A classification for the venation of leaves, and also their shapes, is given in detail in Hickey (1973). Generally monocotyledons are described as having parallel venation reflecting the insertion of the leaf base all round the stem, and to lack free vein endings. Parallel veins are inevitably joined by numerous fine cross-connections (22). However, there are numerous exceptions (34, 35e). In contrast dicotyledons are described as having reticulate (network) venation (35a) and a considerably range of basic patterns is to be found. Nevertheless a number of dicotyledons have a parallel venation (35c, d) or very rarely dichotomous venation. The veins of an individual leaf can usually be categorized into primary veins, secondary veins, and so on; anda marginal vein may be prominent. A number of examples are illustrated here (35g). The areas bounded by the ultimate veins are termed areoles (35f) (cf. Cactaceae 202) and the blind ending veinlets entering these again make distinctive patterns. Fig. 34. Dioscorea zanzibarensis A monocotyledon with net-veined leaves. venation, pattern of veins | 35 Leaf morphology Ly ncn ~_- oO [@) bis] a BS) 2 — ® ~ 7) 3 aS) w if) —~ [@) ~~" = & & 5 3 — 1S) oc rer} ” 2 iL ig Plantago lanceolata, parallel veined (D); d) Plantago major; e) Smilax sp., net-veined (M); f) typical areole patterns showing ultimate veinlets; g) variations of one type of dicotyledon, (M) =monocotyledon. secondary vein layout. Dt: drip tip. (f and g after Hickey 1973). (D) 36 | Leaf morphology: ptyxis, folding of individual leaves Developing leaves are often confined within a protective structure, a bud (264), and at this stage will quite possibly have acquired more or less their final shape (mostly by cell multiplication) but not reached their final size (mostly due to cell enlargement). Depending upon the number, size, and complexity of the leaves in the bud, they are likely to be variously folded, the manner of folding being consistent for any given species. The phenomenon of leaf packing is referred to by a variety of terms. The contortion of a single leaf is called ptyxis (from the Greek for folding), the various modes of packing of leaves together is referred to as vernation (or prefoliation 38). The packing of perianth segments in a flower bud is very similar to that of vegetative leaves in a vegetative bud and is termed aestivation or prefloration (148). The different forms of ptyxis are frequently of diagnostic value in identifying a plant and as such have acquired an extensive range of terminology. The most common terms are illustrated here (37) in three-dimensional diagrams which convey more information than an over-simplified written description. The individual leaflets of a compound leaf may be folded in one manner, the leaf as a whole showing an alternative arrangement. The folding of the leaves of palms is particularly elaborate and is a function of their unique mode of development (92). An extensive discussion of ptyxis occurs in Cullen (1978). oor aee OTE EINE eg cy rag $ « e < « > 4 ts rs « ¥ Fig. 36a. Drosophylium lusitanicum An insectivorous plant with circinate ptyxis. Unusual in that the leaves are rolled outwards rather than inwards as in Fig. 37e. Fig. 36b. Nelumbo nucifera Peltate leaves (88) with involute folding (37b) Leaf morphology: ptyxis, folding of individual leaves | a7 Fig. 37. Types of individual leaf folding. a) curved; b) involute; c) revolute; d) supervolute (also termed convolute, cf. 39e, f); e) circinate (cf. 36a); f) supervolute/involute; g) conduplicate/involute; h) conduplicate/plicate; i) plicate; j) conduplicate; k) explicative; |) plane (flat). 38 | Leaf morphology: vernation, folding of leaves together in bud Individual leaves are variously folded (ptyxis 36) equitant (39g) or obvolute (half-equitant 39h). and variously packed in a bud (vernation or As a bud expands, distinctive patterns of colour prefoliation). The manner of packing is often or ridging may be seen on one leaf due to the distinctive and is a noticeable feature of the pressure exerted by a neighbouring enlarging perianth segments in the case of flower buds leaf. Such markings are quite common in (aestivation 148) to which the terms illustrated monocotyledons with linear leaves, such as here also apply. The form of vernation depends to grasses, and are termed constriction bands. some extent on the number of leaves at a node (phyllotaxis 218). In monocotyledons where there is only ever one leaf at a node, leaves are likely to be folded or rolled, if at all, in a manner consistent with the protection of each leaf by the sheath of the preceding or axillant leaf or by a more or less tubular prophyll (66a). In more condensed shoot apices, where the phyllotaxis is likely to be spiral, overlapping of adjacent leaves will be more elaborate. In dicotyledons, particularly those with two or more leaves per node, a variety of formats are found. The edges of adjacent leaves may not touch (open vernation 39c) or just touch but not overlap (valvate 39b). Two leaves at a node may face each other and then be appressed (39a) or opposite (39d). Overlapping leaves (or petals) are said to be imbricate (e.g. Fig. 149d-j for perianth segments). Care must be taken to identify the precise details of imbrication; sectioning of unopened buds, and close scrutiny of a series of buds as they unfold, may be necessary. A common form of imbrication in flowers, but rare in vegetative buds, is convolute (39e) (cf. the use of this term in ptyxis 37d). Convolution may occur even where only two leaves are involved Fig. 38. Rhizophora mangle (38, 39f). When the individual leaves are Convolute vernation (39f) of a pair of opposite leaves. conduplicate (37j) then the vernation may be Leaf morphology: vernation, folding of leaves together in bud | 39 Fig. 39. Folding of leaves together. a) appressed, b) valvate, c) open, d) opposite, e) convolute, f) convolute, g) equitant, h) obvolute. 40 | Leaf morphology: petiole, leaf stalk The leaves of many dicotyledons and some monocotyledons (e.g. Araceae) have a stalk or petiole separating the leaf blade from its base or point of attachment to the stem. The development of the petiole is different in these two groups (20). Leaves lacking petioles are said to be sessile. Occasionally the leaf blade is apparently absent and the petiole flattened laterally into a photosynthetic organ—a phyllode (42). Likewise in a leaf bearing a lamina, the petiole may bear wings along its side (49d). The petioles of a number of climbing plants are persistent, remaining on the plant and becoming woody after the lamina has become detached (41h), the abscission zone developing into a point. More frequently the petiole will fall with the leaf with or without an abscission joint (48). Petioles may be fleshy and swollen (41f) or sensitive to touch acting as twining supports in climbing plants (41e, h). The whole or part of a petiole may form into a permanent woody spine (40b, 41a, b, c, d). The orientation of a leaf lamina can be affected by movement in the petiole. This can take the form of twisting of the petiole (238) or may be due to the presence of one or more pulvini (46) or due to the presence of a pulvinoid (46). This latter structure is a swelling similar to a pulvinus but operates only once producing an irreversible repositioning of the lamina. The proximal end of a petiole frequently forms a protective cavity surrounding the axillary bud (265b) or the terminal bud (265b, d). The cavity formed by longitudinal folding of petioles of Piper cenocladium is inhabited by ants; food cells develop on the inner surface of this cavity. Similar food bodies occur on the petiole of Cecropia (78). In Vitellaria the petioles of the cotyledons elongate at germination forcing the radicle of the seedling underground (41g). Fig. 40a. Psammisia ulbriehiana Long-lived leaf with woody petiole being encroached upon by stem expansion. Fig. 40b. Quisqualis indicus Petiole spines (laminas shed). Leaf morphology: petiole, leaf stalk | 41 Fig. 41. a, b) Ribes uva-crispa; c,d) Fouquieria diguetii, spine formed from stem tissue adnate (234) to abaxial surface of petiole (d); e) Maurandia sp.; f) Zamioculcas zamifolia, single leaf; g) Vite/laria paradoxum, germinating seedling; h) Clematis montana, single node. Cp: cotyledonary petiole. P: petiole. Pl: plumule. Ps: petiole spine. Ra: radicle. 42 | Leaf morphology: phyllode, flattened leaf stalk Many leaves, especially of dicotyledonous but also of certain monocotyledonous plants, may be described in terms of sheath (50), petiole (40), and lamina (22), although the development of these structures in the two groups is different (20). A ‘typical’ petiole of a dicotyledonous leaf is a cylindrical, not necessarily photosynthetic, stalk. In a number of plants, however, the petiole is flattened and contributes considerably to the light interception area of the leaf. The lamina may appear to be correspondingly rudimentary (45c) or apparently absent as inferred by a transition series (45). Such apparently flattened petioles are termed phyllodes, and the flattening may be dorsiventral (53c) or more often lateral (43). Recent developmental studies suggest that the phyllode can represent a modification of the whole leaf, not just the petiole (44). A phyllode, being a leaf, will subtend a bud or shoot and may thus be distinguished from flattened stems, cladodes and phylloclades (126), which are themselves subtended by leaves. Fig. 42. Acacia paradoxa Each phyllode subtends a number of buds (236) one of which develops into an inflorescence. Stipular spines (56) present (cf. Fig. 43a, c). Leaf morphology: phyllode, flattened leaf stalk | 43 Fig. 43. a) Acacia paradoxa, single node; b) A. g/aucoptera, leaves on seedling axis; c) A. paradoxa; d) A. pravissima; e) Sarracenia flava, phyllodes and ascidiate (88) leaves. Ph: phyllode. Sts: stipular spine (56). 44 | Leaf morphology: phyllode interpretation In order to compare the nature of apparently similar structures in different plant species it is usually helpful, if not essential, to compare their development. This is particularly true in the case of leaf structure (18) and has aided the comparison of dicotyledonous leaves and monocotyledonous leaves (20). It has also proved useful in the interpretation of phyllode structure (42). Thus developmental studies of the phyllode of species of Oxalis indicate that the lamina is suppressed, more so in some leaves than others, and that the phyllode does indeed represent a flattened petiole. This is not true however for those Acacia species (43a, b, c, d) bearing phyllodes (Kaplan 1975). In these cases the whole rachis (22) of the leaf is involved in the formation of the phyllode, developing as a flattened structure due to the activity of an adaxial meristem (18). All the leaves on an Acacia plant (except the seedling leaves 30) may develop in the manner of phyllodes; in some species a variable range of ‘transitionary’ forms will be found. Thus the older (proximal) branches of Acacia rubida bear bipinnate leaves (45k) whilst the younger (distal) branch ends will bear phyllodes (45a, b). In between a range of intermediate types may be found (45c-j). Adaxial meristem activity can occur anywhere along the petiole/rachis axis and is combined with a variable reduction in the activity of the primordia responsible for leaflet production. The compound leaf of A. rubida is paripinnate (no terminal leaflet 22) but this leaf, and the phyllodes, and the intermediate forms, bear a minute terminal ‘pointlet’, representing the distal oldest end of the leaf (45j). For the Acacia spp. therefore, at least, the phyllode does not represent a flattened petiole, but rather a flattened rachis as determined by developmental studies. Fig. 44. Acacia rubida Phyllodes and bipinnate leaves. A ‘transitional’ leaf at centre. Leaf morphology: phyllode interpretation | 45 (a) (b) Fig. 45. A selection of leaf forms from one tree of Acacia rubida. a, b) are adult foliage types; k) is the juvenile form; c)-j) are intermediate types. Efn: extra-floral nectary. Ph: phyliode. Po: pointlet. Rh: rachis. 46 | Leaf morphology: pulvinus, swelling at junction of leaf parts ioe? peer Localized swellings of leaf or leaflet stalk are common occurrences in both dicotyledons (47) and monocotyledons (220). Usually such swellings act as hinges, allowing more or less reversible movement between the parts of the leaf (46a, b, 47a, a’). Such hinges are referred to as pulvini (singular pulvinus) and may be found at the base of the petiole (80b), or at the junction of petiole with lamina (220), and/or at the base of each leaflet in a compound leaf (47d). They also occur on stems (128). Displacement of the leaf with respect to light or gravity will cause a compensatory reorientation of the leaf parts as the cells at one side of the circumference of the pulvinus swell or shrink due to water gain or loss. Abscission joints (48) and pulvinoids often closely resemble pulvini. Pulvinoids form irreversible growth joints reorientating a leaf or leaflet once only or forming a clasping aid to climbing (41h). Abscission joints locate the point of weakness where a leaf or leaflet or portion of petiole or rachis will eventually break, and are not capable of reorientation but are usually identified by an associated annular groove (49a). As viewed in a transverse section, the anatomy of a pulvinus will differ from that of a pulvinoid or an abscission joint: pulvinus—reversible movement possible, vascular bundles located centrally and often lignified; pulvinoid— irreversible movement, no groove present, vascular bundles peripheral and not lignified; abscission joint—no movement possible, groove present. The swelling at each node on the culm (180) of a grass plant is in fact a pulvinus at the base of the leaf sheath inserted at that node. Fig. 46a, b. Mimosa pudica a) Undisturbed plant; b) 5 seconds after disturbance. Pulvini at base of leaves and leaflets have distorted, acting as hinges causing the leaves to collapse. Most pulvini operate much more slowly. HATING : Ky yS , OW) Vt Leaf morphology: pulvinus, swelling at junction of leaf parts | 47 Fig. 47. a, a!) Oxalis ortgeisii, day and night leaf arrangement; b) Derris e/liptica, end of compound leaf; c) Leea guineense, single leaf; d) Acacia heterophylla, single leaf. Pu: pulvinus. 48 | Leaf morphology: articulation, abscission joint The construction of a leaf is often articulated, i.e. jointed, and the leaf will eventually fall apart at the points of articulation—the abscission joints (or struma). Such joints are frequently swollen (49a, c) and usually also bear an annular constriction groove marking the location of future breakage (49a). Abscission joints may occur at intervals along the rachis of a compound leaf and/or at the base of each individual leaflet, or simply near the base of the leaf itself. Abscission joints resemble pulvini (46) and pulvinoids (46) but do not reorientate the leaf or leaflet. Pulvini may be present in addition to abscission joints and then the pulvinus may be shed with the leaf or left behind (48). The point of breakage resulting in leaf fall (i.e. the abscission zone) is not necessarily recognizable externally as a swollen abscission joint. Indeed in some trees the leaf is only forced to fall by the increase in girth of the stem bearing it (40a). Similar zones of breakage occur in stems (see cladoptosis 268). Fig. 48. Philodendron digitatum A palmate leaf, two leaflets detached, breakage occurring at the mid-point of the pulvini. Leaf morphology: articulation, abscission joint | 49 Fig. 49. a) Mahonia japonica, single leaf; b) Hevea brasiliensis, part of leaf; c) Schefflera actinophylla, single leaf; d) Citrus paradisi, single node. Ab: abscission joint. An: annular groove. 10 mm J 50 | Leaf morphology: sheath, base of leaf Fig. 50. Hedychium gardnerianum The leaf sheath to the left of the fruit clasps the stem; the one below is pulled away from the stem. The structure of a leaf often lends itself to description in terms of leaf blade (or lamina 22), leaf stalk (or petiole 40), and leaf base (or sheath). These terms are applied equally to dicotyledonous and monocotyledonous leaves, although the development of leaves in these groups is fundamentally different and the petiole of one is only equivalent to the petiole of the other in a purely descriptive sense (20). Sheath is perhaps the least well defined of these descriptive terms and has been applied to any structurally distinctive portion of the leaf at or near the point of insertion of the leaf on the axis. Sheaths sometimes bear stipules (52). Leaf sheaths range in structure from barely noticeable enlargements of the base of the petiole (33c), to prominent elaborations clasping the stem (50, 51b,c). A sheath may partially or totally protect the axillary bud (51c, d). Either the proximal or the distal end of the sheath can be modified into a pulvinus (46), or may become persistently woody or fibrous (51b) as in many palms. A leaf sheath is a particularly conspicuous feature of most monocotyledonous leaves, encircling the stem due to the mode of development of these leaves (18). An aggregate of such concentric leaf bases forms a pseudostem of, for example, a banana (Musa sp.). In some instances the leaf base forms the bulk of the photosynthetic surface of the leaf (89c). Leaf morphology: sheath, base of leaf | 51 Fig. 51. a) Dianella caerulea, top of aerial shoot; b) Rhapis excelsa, top of aerial shoot; c) Fatsia japonica, single leaf; d) Smyrnium olusatrum, single node. Axb: axillary bud. En: ensiform portion of leaf (86). L: lamina. P: petiole. Sh: sheath. Ey ow Ss Te. = A Sse Z Z Ss x Sf pe Ss ob SS e/ irr, L RE = . 52 | Leaf morphology: stipule, Fig. 52. Liriodendron tulipifera The pair of stipules at the base of each leaf petiole protects the next youngest leaf here seen silhouetted inside. outgrowth at base of leaf stalk A stipule is an outgrowth associated with the base of a leaf developing from part of the leaf primordium in the early stages. Plant species are termed stipulate or exstipulate (with or without stipules). Stipules are not common in monocotyledons where they usually occur one per leaf (55b) or very occasionally two per leaf (57b). Stipules of dicotyledons are paired typically one on either side of the point of insertion of the petiole on to the stem (53b). However, there are many positional variations often including fusion of structures (54). Stipules may be relatively small and insignificant (53c, d), often scale-like (61b, 80b), and may fall off early in the life of the leaf leaving a scar (78). They often protect younger organs in the bud (52), and then fall when the bud develops. Conversely stipules can be very conspicuous and leaf-like (55a, 57e) or resemble entire leaves (55d, 69e) from which they may be recognized by the absence of associated axillary buds. It is quite possible that in some cases structures traditionally described as stipules (as in many members of the Rubiaceae (55d), see for example Rutishauser 1984) in fact represent whole leaves. The structures in question themselves bear outgrowths (colleters 80) which could be rudimentary stipules and thus all the members of the whorl of ‘leaves’ at a node would be foliar in origin, some with axillary buds, some without. Stipules may be modified into a number of structures (56), especially spines (43c, 6). These are lignified (woody) and usually persist after the rest of the leaf has fallen. Leaf morphology: stipule, outgrowth at base of leaf stalk | 53 Fig. 53. a) Bergenia sp., single leaf; b, b') Pelargonium cv., single leaf and side view node; c) Lathyrus nissola; d, d') Oxalis sp., single leaf and top view node; e, e') Potamogeton sp., leaf rosette and single leaf. Axb: axillary bud. Ph: phyllode (42). Pu: pulvinus (46). St: stipule. 54 | Leaf morphology: stipule location Fig. 54. Reynoutria sachalinensis The stipule, an ochrea, completely encircles the stem. Adventitious root primordia (98) visible just below the node. The position of a pair of stipules in dicotyledonous plants relative to the leaf base, petiole, and insertion of the leaf on to the stem, varies considerably. The stipule may be located at the extreme proximal end of the petiole (53b, 55f) or be borne actually on the stem apparently detached from the leaf base (55g). The stipules can also be found at the junction of the leaf base (sheath) and petiole (55h, i). The stipule is described as adnate if it is fused along part of the length of the petiole (55f, k). A stipule may be attached to the side of the stem, 90 around from the point of leaf insertion (interpetiolar 55i) and may then be fused with the corresponding stipule of a second leaf at that node (55c). Stipules are occasionally found on the side of the stem away from the point of leaf insertion (55m), a situation described as ‘abaxial’ or ‘counter’, or more precisely as leaf opposed. Conversely a single stipule may be found in a truly adaxial position between the petiole and the stem (55a, j): a median or intrapetiolar stipule. A similarly located single structure often described as a stipule is found in a number of monocotyledonous plants (55b, 53e), that of Eichhornia being particularly elaborate bearing an additional terminal structure, the stipular lobe. If the single stipule encircles the whole stem it is described as an oc(h)rea (55e, n, 54). Leaf morphology: stipule location | 55 Fig. 55. a) Melianthus major, single leaf at node; b) Eichhornia crassipes, single leaf; c) Manettia inflata, \eaf pair at node; d) Ga/ium aparine; e) Polygonum sp.; f) Rosa sp., base of leaf petiole; 0) Ficus religiosa, young end of shoot. g)-n) range of stipule locations. Abst: abaxial stipule. Ist: interpetiolar stipule. Mst: median (intrapetiolar) stipule. Och: ochrea. St: stipule. Stl: stipular lobe. 56 | Leaf morphology: stipule modification Members of the genus Smilax (Liliaceae) are most unusual amongst monocotyledons in that each leaf bears two structures in a stipular position which are modified into tendrils (57b). Stipular structures in other monocotyledons occur singly and are usually membraneous or otherwise rather insignificant (53e). In dicotyledonous plants the stipules may persist as long as the rest of the leaf or may fall a long time before or after the leaf or not at all. Such persistent stipules are usually modified into the form of woody spines lasting for many years (202b, 119f). The stipular spines of some species of Acacia are hollow and inhabited by ants (205a, 6). One of each pair of stipular spines in Paliurus spina-christi is straight and the other is reflexed. Stipules also occur that are modified as extra-floral nectaries (56), or are represented by a fringe of hairs (57c). In many plants the scale-like stipules of leaves aggregate in a dormant bud performing a protective role (265c). Fig. 56. Bauhinia sp. Stipules represented by extra-floral nectaries (80). Fig. 57. a) Acacia hindisii, woody stipular pairs; b) Smilax lancaefolia, single node; c) Anacampseros sp., single leaf; d) Impatiens balsamina, leaf pair at node; e) Pisum sativum, single leaf at node; f, f') Robinia pseudacacia, single leaf at node and close view node (compare 119f). Sth: stipular hairs. Stg: stipular glands. Stp: photosynthetic stipule. Sts: stipular spine. Stt: stipular tendril. 58 | Leaf morphology: stipel, outgrowth of leaf midrib : * Fig. 58. Phaseolus coccineus A pair of small stipels located on the leaf petiole just below its junction with the lamina. Occasionally the individual leaflets in a compound leaf have small outgrowths at their bases resembling stipules (52). These structures are referred to as stipels (or secondary stipules, or stipella) and are usually uniform in size on the leaf (59c) or may vary considerably. They are most frequently met with in members of the Leguminosae (58, 59). Many compound leaves and some simple leaves (25c) have irregularly placed small leaflets, interspersed between major leaflets (interruptedly pinnate) which resemble stipels in appearance but lack the precise location at the proximal end of each main leaflet. A number of other structures can be found on a compound leaf in a similar position to those of stipels but not of a membraneous or leaf-like appearance. Examples include prickles (774d, e) and extra-floral nectaries (81d). Leaf morphology: stipel, outgrowth of leaf midrib | a9 Fig. 59. a) Cassia floribunda, single leaf (cf. 81c); b) Erythrina crista-galli, single \eaf; c) Phaseo/us vulgaris, single leaf; d) Butea buteiformis; e) Wistaria sinensis. E: emergence (76). St: stipule. Sti: stipel. In the compound leaves of some dicotyledons, the proximal pair of leaflets is positioned very close to the point of insertion of the leaf at the node and thus appears to be in the stipular position. True stipules may also be present (61b) in which case the nature of the basal leaflets is apparent. If not, these leaflets are sometimes referred to as pseudostipules. These leaflets are also termed pseudostipules if the family to which the plant belongs is predominantly exstipulate (60). The pseudostipules may have a different shape to other leaflets on the same leaf (61a). A careful study of the development of the leaf primordia may indicate the relationship of the lowest pairs of leaflets to those located more distally, revealing that they are indeed pseudostipules rather than stipules. In some cases the nature of the vascular supply to the leaf and pseudostipules may be of value. Stipulate leaves often have three leaf traces, exstipulate leaves often only one. The single prophyll of some Aristolochia spp. (67c) is sometimes referred to as a pseudostipule. 60 | Leaf morphology: pseudostipule, basal pair of leaflets Fig. 60. Mutisia acuminata The proximal, i.e. lowest, pair of leaflets of the pinnate leaf is located in a stipular position. Family Compositae—mostly exstipulate. Leaf morphology: pseudostipule, basal pair of leaflets | 61 Fig. 61. a) Cobaea scandens, end of climbing shoot; b, b', c) Lotus corniculatus, c) portion of shoot, b) single leaf, b') close view of minute stipule. Le: leaflet. Lt: leaf tendril. Ps: pseudostipule. St: stipule. 62 | Leaf morphology: bract, bracteole, leaves associated with inflorescences Leaves located in association with flowers are frequently modified or reduced in size relative to vegetative leaves on the same plant. Such leaves are referred to as bracts (or hypsophylls cf. cataphylls 64). Any leaf, modified or not, that subtends a flower can be termed a bract although there are many instances in which flower buds are found without associated subtending leaves. Conversely the stalk (pedicel) of an individual flower may bear a bract (typically one in monocotyledons and two in dicotyledons, 66) which may or may not subtend its own flower. Such a leaf is termed a bracteole. Thus the Fig. 62a. Cephaelis poepiggiana A pair of coloured bracts beneath an inflorescence bracteole of one flower may be the bract of another flower (63e). If a number of flowers are borne in a condensed inflorescence, their individual bracts will occur in a tight whorl or involucre (144). However, an involucre may be associated with a single flower (147e). A compound umbel (141m) will display an involucre of bracts at its base and an involucel beneath each distal flower cluster. Each individual bract of an involucre can be called a phyllary. One or several of the bracts associated with an inflorescence may be relatively large and conspicuous (62a, 63a, d). Such bracts may Fig. 62b. Barleria prionitis Young flower buds, in axils of foliage leaves, surrounded by bracts in the form of spines (i.e. leaf spines 70). assist in the wind dispersal of fruit or fruits (235e). Bracts are a conspicuous feature of many monocotyledonous inflorescences (63a, c) and form distinctive features of the grass spikelet (186). Generally, bracts may appear leaf-like, are frequently scale-like, may be massive as in many palms, or modified into spines (62b), hooks (161b), or persistent woody structures surrounding fruits (157h, 1550; the fused woody bracts—cupule—as in oak for example). Leaf morphology: bract, bracteole, leaves associated with inflorescences | 63 Fig. 63. Portions of inflorescences incorporating bracts. a) Heliconia peruviana, b) Barleria prionitis, c) Tradescantia sp., d) Leycesteria formosa, e) Silene dioica. B: bract. Br: bracteole. Bs: bract spine. Fb: flower bud. FI: foliage leaf. Fw: flower. 64 | Leaf morphology: cataphyll, scale leaf Fig. 64. Agave americana Scale leaves on the extending flowering axis (see Fig. opposite the Introduction for mature inflorescence). A great many plants are dimorphic (30), bearing membraneous scale leaves in addition to relatively large foliage leaves intercepting light. These ‘cataphylls’ are sometimes devoid of chlorophyll, and often perform a protective role surrounding vegetative or floral meristems (64, 62b). Underground stems of rhizomatous plants commonly bear scale leaves (65a, e, but cf. 87c) which may or may not subtend axillary buds. Successive leaves located along a shoot may demonstrate a heteroblastic series (29c) from a simple scale leaf to a more or less elaborate foliage leaf. A similar heteroblastic sequence occurs in relation to flowering shoots, the foliage leaves at the proximal end of the inflorescence merging into scale leaves at the distal end. Scale leaves associated with an infloresence are termed hypsophylls or more commonly bracts and bracteoles (62). Particularly in monocotyledons, the first leaf on a shoot (the prophyll 66) is often represented by a cataphyll and differs greatly in size and morphology to more distal leaves on that axis. Scale leaves are typically smaller in size than the corresponding foliage leaves of a particular plant, although small is a relative term, the protective scale leaves (bracts) of the inflorescence of some palms being massive woody structures over 1 m in length. Leaf morphology: cataphyll, scale leaf | 65 Fig. 65. a) Cyperus a/ternifolius, developing aerial shoots; b) Casuarina equisetifolia, distal end of shoot; (c) Asparagus densiflorus, single node (cf. 127a); d) Raphia sp., fruit; e) Costus spiralis, rhizome; f) Fatsia japonica, scale leaves beneath shoot apex. Cl: cladode (126). Fis: foliage leaf sheath (51c). Lsp: leaf spine (70). Sc: scale leaf. Scs: scale leaf scar. —————— SS SS SSS —— —_— S—a SS ——— —— —<—w~ —SSS> —$——= | A | ) | f, \ Ki, TM ANA | SS ee ae ead ta ae (—— PSS a YO Cons ee ARK &. ek. TT 2S S \ =z SS Sen See Sa 66 | Leaf morphology: prophyll, first leaf on a shoot Fig. 66a. Philodendron pedatum Same plant as Fig. 10, later development stage. The pale coloured prophyll is about to fall, having protected its axillary shoot (an elongating hypopodium 262) The term prophyll is applied to the leaf or leaves at the first (proximal) node on a shoot. The leaves in this position are often but by no means always represented by cataphylls (64) whether or not subsequent leaves are similarly modified. The single prophyll of many monocotyledons can be a particularly distinctive scale (66a), often appearing double with a double tip (bicarinate). It is almost always found in an adaxial (4) (or adossiete) position, i.e. on the top of the lateral shoot. Single adaxial prophylls also occur in some dicotyledons (67c, d). In dicotyledons prophylls of a pair are usually positioned laterally (66b, 67a, b); if only one is present it is not necessarily adaxial. A bracteole (62), being the first leaf on a shoot, is thus also a prophyll. The palea of a grass spikelet (186) and the utricle of a sedge (196) are likewise prophylls because of their positions. The prophyll on the shoot system forming a female inflorescence of Zea (190) occurs as the first of a series of large protective ‘husks’. Prophylls are occasionally persistent and woody, represented by hooks, spines (203b, 71c), or possibly modified into tendrils (123e). The prophyll(s) may be involved in bud protection (264). In the case of sylleptic growth (262) the prophyll is separated from the parent shoot by a long hypopodium (263a). However, if the prophyll is inserted on the side shoot in a proximal position very close to the parent axis, then the bud in the axil of the prophyll can also develop very close to the parent axis and this process may be repeated giving rise to proliferation (238). Fig. 66b. Simmondsia chinensis Each axillary shoot bears two small lateral prophylls at its base (one of each pair visible from this viewpoint) which protect the axillary buds (264). Leaf morphology: prophyll, first leaf on a shoot | 67 Fig. 67. a) Esca/lonia sp., b) Leycesteria formosa, prophyll pair at base of side shoot; c) Aristolochia cymbifera, single adaxial prophyll; d) Liriodendron tulipifera, single \arge prophyll. Abc: accessory bud complex (236). Pr: prophylil. St: stipule. 68 | Leaf morphology: tendril - BX mt Fig. 68a. Bignonia sp One of the three leaflets of each leaf forms a persistent clasping woody tendril Climbing plants exhibit a considerable range of morphological features that prevent the shoot system falling. The stem may twine, may develop adventitious clasping roots (98), or tendrils and hooks which represent modified shoots (122) or inflorescences (145b), or may possess leaves all or part of which develop in the form of tendrils or hooks. In one genus only (Smilax) the stipules of the leaf operate in this manner (57b). The leaf petiole may be a twining organ (41e, h). Leaf tendrils themselves are found in a variety of forms. The distal extremity only of a simple leaf may be elongated forming a twining tendril (68b, 69g), or the whole leaf may be involved (69e). Alternatively either the terminal or one or more lateral leaflets of a compound leaf will occur as a tendril (69a, b, c, f). In the case of compound leaves, the proportion of leaflet tendril to ordinary leaflets may be flexible in a given species or the leaves may be very consistent in this respect. Tendrils show pronounced movement and will twine around a support once contact is made, usually due to faster growth rates on the side away from the support. In some species the encircling portion of leaf will subsequently become enlarged and woody and permanent (68a). A tendril may operate in a dual fashion, acting as a grappling iron before commencing to twine, frequently the extreme distal ends of such tendrils, which may be branched, form very small recurved hooks (61a) or occasionally suckers (229b). Once a tendril has become anchored at any point, the remaining portion may continue to twist resulting in a spring shape. Such twisting may be clockwise over one portion of the tendril and anticlockwise over the remainder. A leaf tendril will usually have a bud (or shoot) in its axil, a stem tendril (122) will be subtended by a leaf (or its scar). However, interpretation is not always easy; the tendril of the Cucurbitaceae, which appears to be a stem tendril in the axil of a leaf, may in fact represent the prophyll of the bud in the axil of that leaf (122). Fig. 68b. Mutisia retusa Each simple leaf terminates in a tendril. Leaf morphology: tendril | 69 Fig. 69. a) Bignonia sp., shoot apex; b) Bignonia ornata, single leaf at node (second not shown); c) Pyrostegia venusta, single leaf; d) Ti/landsia streptophyl/a, whole plant; e) Lathyrus aphaca, shoot apex; f) Mutisia acuminata, single leaf; g) Littonia modesta, single leaf; h) Clematis montana, single leaf at node (second not shown). Lt: leaf tendril. Ltt: leaflet tendril. Rt: rachis tendril. St: stipule. TI: leaf tip tendril. 70 | Leaf morphology: spine The whole or part of a leaf may be represented by a woody and more or less persistent spine (spine, thorn, prickle 76). A leaf spine can usually be recognized as such as it subtends a bud or shoot. Conversely, stem spines (124) will be in the axil of a leaf or leaf scar (6). However, care must be exercised as an apparent stem spine may in fact be formed from the first leaf or leaves of an axillary shoot (e.g. 71c, e, 203b). The petiole only of the leaf may in whole (40b) or in part (41a, b, c, d) become woody and pointed after the detachment of the lamina or the leaf may bear stipular spines (57f). Occasionally a few leaflets only of a compound leaf develop as spines as in the case of climbing palms (71f). Alternatively the whole leaf (possibly including stipules if present) takes the form of a spine (71c) or spines (71a). In such cases the plant is dimorphic (30) having two distinct leaf types (i.e. spine and foliage leaf in this case) or all the leaves on the plant may occur as spines (most Cactaceae 202). A distinctive form of spine that is foliar in origin occurs in some palms (such as Zombia 70b) in which the leaf sheath persists after the loss of the petiole and lamina and the veins in the distal portion of the remaining sheath form spines radiating out apparently from the trunk of the tree (70a). Fig. 70a, b. Zombia antillarum a) The spine covered stem; b) the sheath of each leaf is splayed out at the junction with the petiole into a fan of spines. Leaf morphology: spine | 71 Fig. 71. a) Berberis julianae, portion of shoot; b) Parkinsonia aculeata, single young palmate/pinnate leaf; c) Microcitrus australasica, shoot apex; d) Ulex europaeus, shoot apex; e) Citrus paradisi, single leaf at node; f) Desmoncus sp., distal end of leaf. E: emergence. Lf: leaflet. Lfs: leaflet spine. Ls: leaf spine. Ps: prophyll spine (66). Shs: shoot spine (124). Ss: stipule spine (56). 72 | Leaf morphology: traps, insectivorous plants ae The leaves of plants in a limited number of 7 families (Droseraceae, Cephalotaceae, Lentibulariaceae, Nepenthaceae, Sarraceniaceae, and Dioncophyllaceae) form structures that trap insects and other similar-sized animals. Once caught the insect will be digested and absorbed over a period of time. The classical descriptions of insectivorous plants are to be found in Darwin (1875). Leaf traps are of two general types: sticky leaves (73a, b, 36a, 81g) with or without elaborate glandular tentacles, the leaves usually curling up to enclose caught insects; and epiascidiate (88) leaves, i.e. leaves forming a container into which the insect falls (72, 31d, 89c, 43e), flies, or is sucked (73e). The mode of development of pitcher-type leaves is described in section 86, these leaves frequently have deposits of loose wax flakes around the inner rim of the trap opening, which become stuck to insects’ feet and speed the fall into the container. The epiascidiate leaf of Utricularia (73e) differs in that it is active in its action: the container has a lid which opens inwards in response to tactile stimulus of hairs at its entrance, and the structure of the bladder is such that water plus insect is instantaneously sucked in, water pressure being greater outside the trap than inside (Lloyd 1933). Rapid response to stimulation is also seen in Dionaea muscipula (7 3f) in which repeated pressure on hairs of the adaxial side of the leaf results in the two halves of the leaf snapping together. A complete account of insectivorous plants is given by Juniper et al. (1989). Fig. 72. Nepenthes cv. The lamina of each leaf is modified into a hollow chamber. This is a form of epiascidiate (88) leaf development. Leaf morphology: traps, insectivorous plants | 73 Fig. 73. a) Pinguicula /anii, \eaf rosette from above; b) Drosera capensis, seedling; c) Nepenthes khasiana, seedling; d) Cepha/otus follicularis, single leaf; e) Utricularia minor, portion of shoot; f) Dionaea muscipula, seedling. 74 | Leaf morphology: epiphylly, structures developing on leaves It is conventional to interpret flowering plant structure in terms of four categories of organ— leaf, stem, root, and trichome (206). However, in many instances strict rigid adherement to this scheme creates major problems (206-212) or conflicting opinion of interpretation (4, 122). Classically, a leaf is expected to be a determinate (90) lateral appendage on a stem and not itself to bear other leaves or stems. Nevertheless, epiphylly (growth on a leaf) is not uncommon (Dickinson 1978). Many species are to be found which bear inflorescences or vegetative buds located on leaves in a variety of positions (75h-n). Such an occurrence will usually be a regular and normal feature for the given species, regardless of its apparent inconsistency with conventional morphological ‘rules’ although a range of epiphyllous structures arise in response to attack by mites in some plants (Ming et al. 1988). Conventionally a bud, be it potentially an inflorescence or a vegetative structure, is expected to be located in the axil of the leaf (4), not ‘carried up’ and positioned out on the leaf petiole or blade. There are a number of ways in which an epiphyllous structure may develop. One theoretical explanation is that the axillary bud has become fused (post-genital fusion) on to its subtending leaf after the independent growth of both. This is rarely observed. A second developmental explanation involves ontogenetic displacement. In the earliest stages of growth, cells below both the young bud primordium and subjacent leaf primordium divide actively and the bud and leaf grow out as one unit, i.e. they never have a separate existence. This sequence of intercalary growth undoubtedly occurs in many instances and will result in an inflorescence or vegetative bud apparently sitting on a leaf and qualifying for the traditional explanation of adnation (74). Epiphylly can result from a second developmental phenomenon with or without the occurrence of ontogenetic displacement. One (or more) area of cells on a leaf primordium retains its meristematic ability, initially common to all the cells of the primordium, and subsequently becomes organized into an independent shoot system. This is referred to as heterotopy. A good example is that of Streptocarpus (208). Such heterotopies (‘other place’) are in direct conflict with the classical interpretation of plant growth which will have to dismiss them as ‘adventitiou: structures (98, 178, 232). Nevertheless, heterotopy is well documented and produces in conjunction with ontogenetic displacement, inflorescences on leaves (75h-n), leaves on leav (Maier and Sattler 1977), vegetative detachable buds with roots on leaves (75a, c, e) and even apparently embryo-like structures on leaves (Taylor 1967). During the development of leave: of Bryophyllum species, patches of meristematic Fig. 74. Spathicarpa Sagittifolia A row of flowers, representing an inflorescence spike (141¢ remains attached to the subtending bract (or spathe) during development, as in Fig. 75k. cells stop dividing at intervals along the leaf, giving the leaf at first an indented margin. Subsequently these heterotopic areas recommence development to produce the detachable buds (233, cf. 227). A number of other apparently extraneous structures may be found on the leaves. These include adventitious roots (98), galls (278), glands (80), food bodies (78), emergences (76), and stipels (58). Recognition of an epiphyllous structure is not always clear; the Pleurothallis sp. shown here (75d) has a conventional morphology with a terminal inflorescence located very close to the distal foliage leaf, i.e. it is not an example of epiphylly. _ Fig. 75. a, b) To/miea menziesii, single |eaf and close view lamina/petiole junction; c) Bryophy//ium tubiflorum, end of shoot (cf. 227); d) Pleurothallis sp., end of shoot (apparent epiphylly only); e) Bryophy/lum diagremontanum, single leaf (cf. 233); f) Po/ycardia sp., single leaf; g) Tapura guianensis, single leaf; h)—n) epiphyllous locations, after Dickinson (1978). Adr: adventitious root. Db: detachable | bud. Fl: flower(s) Pe: petiole. Sc: scale leaf. St: stem. Leaf morphology: epiphylly, structures developing on leaves | 75 VIZy 76 | Leaf morphology: emergences, prickles Spiny structures are quite common features of above-ground plant parts. The terminology associated with these features is not consistently utilized, the terms spine, prickle, and thorn, being found more or less interchangeably. In this book ‘thorn’ is not employed; a ‘spine’ represents a modified leaf (leaf spine 70), stipule (56), stem (stem spine 124), or root (root spine 106), and ‘prickle’ is applied to sharp usually woody structures that develop from a combination of the epidermis of an organ plus subepidermal tissue (and also sharp structures on a leaf edge 7). The general term applied to a structure with this epidermal/subepidermal origin is emergence (stem emergence 116, leaf emergence 77), the consistent feature of an emergence being that it will not develop in the expected location of a leaf or shoot primordium (cf. phyllotaxis 218), representing as it does an additional form of organ. Leaf emergences vary considerably in size and shape and may be confined to the leaf margin or to either the upper (adaxial) or lower (abaxial) surface of the lamina or petiole. In compound leaves emergences may be found on the rachis between adjacent leaflets (77d). Emergences are not always haphazard in their location, in Acacia seyal (117d) a prickle occurs very close to each stipule. The stipules themselves are ephemeral and soon drop leaving a small and easily overlooked scar. A casual glance would doubtless suggest that the plant has spiny stipules. Fig. 76. Solanum torvium Emergences on leaf surface. They are also present on the stem (116). Leaf morphology: emergences, prickles | 77 Fig. 77. a) Centaurea sp., inflorescence; b) Laportea sp., single leaf; c) Rubus australis, single leaf; d) Acacia sp., portion of leaf; e) Aralia spinosa, portion of leaf. B; bract. E: emergence. Pe: petiole. Rh: rachis. 78 | Leaf morphology: food bodies Fig. 78. Cecropia obtusa On the abaxial side of each petiole base is a pad of tissue producing a constant supply of food bodies. On the opposite side of each node, just below the stipule scar, is a weak spot that is excavated by ants to provide an entrance to the hollow internode nesting site. A range of structures commonly referred to as ‘food bodies’ occur on the surface of some plant leaves and anatomically can represent either trichomes (80) or emergences (76) which are secreting usually edible proteinaceous substances. Unfortunately, as each new example of this phenomenon has been discovered the food body has been given a specialist term. A number of such food bodies are listed here. (1) Beltian bodies (after Belt)—these are food bodies occurring at the ends of leaflets in Acacia species (79); (2) Millerian bodies (after Miiller)—food bodies born on a swelling (trichilium) at the base of leaf petioles of Cecropia species (78); (3) Beccariian bodies (after Beccari)—found in various locations on leaf and stipule of Macaranga;. (4) Pearl bodies on Ochroma (on the leaves and stems); (5) Food cells on Piper species found in domatia (204) in the petiole. Similar structures usually secreting oil (oil bodies or elaiosomes) are found on the seeds of many plants and then usually act as ant attractants. Small structures found on the surface of leaves of many Passiflora species mimic butterfly eggs. Leaf morphology: food bodies | 79 ANN yy) Fig. 79. Acacia hindisii, single leaf. Do: domatium (204). Yay hy Efn: extra-floral nectary. Fb: food body. Pi: pinnule. Po: N Chi NN Ng é é ; i ANN We pointlet. Rh: rachis. Ril: rachilla. St: stipule. Ke ‘ IN i) sae \\ LAC DU N yy LT} S Dyys oe ow SATS Ws SAAN eA (ut tik iY f ue ELLER Rh NIM Ny pS engage nS SONATA! al ANNU LOVIDOD UND DUDYIIVY AA a dsudedlec son) ' AN AE (A DOD LURE We usa SANVUANIOURUDNUUNNS SOON Me £07 Ea (fete LOL MLUOUCLLN Mh eee MONI YY LLLLLP (" UV VQUY WTTTTT WY TWN WAV Nit AMM ee ROTTER WATTS Sou LI UAW llc 7 V\\ RI YW / sie acca M4 S$ cantar hh, Why DOI TAA inne SMA Wy TTT TERE ee Efn \\ SS SMM l77p UTI RR i < VITTUIS YL YS WH 7 SSP TUT WK me STO DV CG WOO hy WIS i Ly 7700 WU VOW K€ YDS 5) eed “pee ! VOT URS tlic IE 10 mm Bs 4’ AY 80 | Leaf morphology: trichomes, glands, hairs, and nectaries Various structures are to be found developing on the surface of leaves, stems, and roots. These include galls (278), nodules (276), adventitious buds (232, 178), and epiphyllous organs (74). In addition many plant parts bear hairs (trichomes) which may be unicellular or multicellular and which are epidermal in origin, and emergences (76) which are usually more substantial and of epidermal plus subepidermal origin. The range of trichome anatomy is largely outside the scope of this book, but the more bulky glandular types can be very conspicuous. Some glands are undoubtedly of subepidermal origin and therefore are strictly emergences, but for convenience these are illustrated here. Emergences of a woody nature are described elsewhere (76, 116). Glandular structures may secrete salt (salt glands), or water (hydathodes), or sugar solutions (extra-floral nectaries 81d, e). A review of the range of morphology and terminology associated with nectaries is given by Schmid (1988). Glands of many insectivorous plants (36a, 73b, 81g) secrete a very viscous substance. Solid secretions are referred to as food bodies (78). Two types of glandular trichome are associated with the protection or unfolding of buds. In just two superorders, the Alismatiflorae and the Ariflorae, glandular trichomes occur in the axils of vegetative leaves. These trichomes are termed squamules. Similar glandular trichomes are associated with the buds of many other plants where they are referred to as colleters. Fig. 80a. Passiflora glandulosa Fig. 80b. Acacia lebbek % Ant feeding at extra-floral nectary on the surface A cup shaped extra-floral nectary on the upper (adaxial) surface of the of the flower bud. leaf petiole at the edge of the pulvinus (46). Dead stipules (52) about to fall. Leaf morphology: trichomes, glands, hairs, and nectaries | 81 Fig. 81. a, a') Osbeckia sp., flower bud and single sepal; b) Dendrobium finisterrae, single flower; c) Cassia floribunda, portion of leaf rachis (cf. 59a); d) /nga sp., end of shoot; e) Acacia pravissima \eaf at node (cf 43d); f) Laportea sp., single stinging hair (cf. 77b); g) Drosera binata, leaf tip; h) Impatiens sodenii, single leaf. Axb: axillary bud. Efn: extra- floral nectary. Gh: glandular hair. Gs: glandular stipel. Lf: leaflet. Lfs: leaflet scar. Ph: phyllode. S: sepal. T: trichome. ——— Se a 82 | Leaf morphology: succulency Fig. 82. Graptopetalum sp. The spirally arranged leaves of each rosette are fat and fleshy. Parts of plants are generally described as ‘succulent’ if they are particularly fleshy, not woody, to the feel and noticeably watery if squashed. Roots (111), stems (203), or leaves can store water and are associated with environments subjected to conditions of drought. The leaf bases of bananas forming a pseudostem (50) can be described as succulent, likewise the thick scale leaves constituting a bulb (84). More pronounced succulency is found in xerophytic and epiphytic plants (potentially dry conditions) and in halophytes (saline conditions). The fleshy leaves of such plants may be bifacial (83c), cylindrical (i.e. unifacial 83j), or approximately spherical in shape (83a). If internodes between leaves are very short, then successive leaves will be partly enveloped by older leaves. This is particularly pronounced if the leaves are in opposite decussate pairs (83i) and especially so if each pair is united around the stem (connate 234). ‘Stone plants’ (e.g. Lithops spp.) take this form (83b). Leaf morphology: succulency | 83 Fig. 83. a) Senecio rowleyanus, two leaves; b) Conophytum mundum, leaf pair; c) Co/eus caerulescens, end of shoot; d) Ceropegia woodii, portion of shoot; e) Haworthia turgida ssp. subtuberculata, leaf rosette from above; f) Adromischus trigynus, \eaf rosette; g) Trichodiadema densum, leaf rosette from above; h) Oscularia deltoides, end of shoot; i) Cheridopsis pillansii, leaf pair; j) Othonna carnosa, end of shoot. Clp: connate (234) leaf pair. Sf: single succulent leaf. St: stem tuber (138). 84 | Leaf morphology: bulb Fig. 84. Urginea sp. The base of each leaf sheath is swollen, the whole forming a bulb. In the axil of each leaf is a vertical row of accessory buds (236) developing as detachable bulbets. A bulb consists of a short, usually vertical, stem axis bearing a variable number of fleshy scale leaves. Its organization is usually imprecise in dicotyledons but more precise in monocotyledons and has acquired a considerable descriptive terminology. The outer scale leaves of the bulb may be membraneous rather than fleshy. They may develop in this way or represent the collapsed remains of a previous season’s fleshy leaves. Internodes between leaves rarely elongate and adventitious roots develop from the basal part of the stem (or ‘stem plate’); these roots are often contractile (107e). The bulb may produce inflorescences in the leaf axils in which case the monopodial main stem axis can bear a series of bulb-like structures (85d), the successive stem plates possibly remaining after the leaves have decayed. Alternatively, the inflorescence can be terminal in which case one or more axillary buds will develop as renewal (replacement or regenerative) daughter bulbs producing a sympodial series. Additional (increase or proliferative) bulbs smaller in size (bulbets) than the main renewal bulbs may be present, and form a mode of vegetative multiplication (170, 172). Green foliage leaves will develop usually at the distal end of the bulb axis, alternatively the bulb is constructed of fleshy leaf bases, each leaf then having a photosynthetic, temporary lamina. Loosely organized bulbs are typical of dicotyledonous plants. The majority of monocotyledonous bulbs have a more compact structure resulting from the concentric insertion of the leaves on the short stem plate, and the sequence of parts in a bulb @g can be precise (85e). For example a fixed number of concentric protective (i.e. membraneous and/or somewhat woody) scale leaves at the proximal end, being followed by a fixed number of fleshy storage leaves, possibly only one, in turn followed by a fixed number of foliage leaves. In Hippeastrum, the bulb is constructed sympodially and each sympodial unit bears four leaves and a terminal inflorescence. Axillary buds may be formed subtended by some or all leaf types and will develop into new bulbs or inflorescences if the inflorescence is not terminal. An axillary bulb may be physically displaced away from its parent bulb at the end of an elongating stolon (cf. dropper 174). The bulb of garlic (Allium sativum 85b) consists of a proximal series of membraneous protective scale leaves subtending no buds, a series of membraneous scale leaves each subtending a number of axillary buds, the ‘cloves’ (accessory buds 84, 236) and most distally a number of foliage leaves. The axis terminates in a sterile inflorescence. Each clove has an outer protective prophyll (85b), the second leaf is a storage scale leaf, the third leaf is a foliage leaf with little lamina, and subsequent leaves will be fully functional foliage leaves. It is customary and useful to display the construction of a bulb by means of a conventionalized ‘exploded’ diagram in which the internodes are elongated with successive leaves drawn as a nest of inverted cones (85d, e). The commercial importance of bulbs of various kinds has led to a wide range of terminology to describe their various features. The applied aspects of bulb construction can be consulted in Rees (1972). Leaf morphology: bulb | 85 (e) (d) Fig. 85. a) A//lium cepa, longitudinal section entire bulb; b) Allium sativum, longitudinal section single axillary bud, clove, of bulb; c) Bow/ea vol/ubilis, whole bulb; d) diagram of construction of typical monopodial bulb, e) of sympodial bulb, f) storage scale leaf; g) membranous scale leaf, h) protective scale leaf, i) foliage leaf. Adr: adventitious root. Fl: foliage leaf (yet to extend in b). la: inflorescence axis. Pr: prophyll. Sl: scale leaf. St: stem. Stl: storage leaf. | WAVAVAY (f) (g) (h) (i) 86 | Leaf morphology: ensiform, terete, laterally flattened and cylindrical leaves Active cell division and enlargement in the a | AY ~ an. Shy =. various meristems of a leaf primordium (18) can SS Ad ~~) result in a leaf of virtually any shape. A typical dorsiventrally flattened leaf (bifacial leaf 87f) with a ‘top’ (adaxial) side and a ‘bottom’ (abaxial) side results if the meristems along the edge of the leaf primordium are active. Increase in the number of cells at the centre of the adaxial side of the leaf (adaxial meristem 19d) will give rise to the thickening of the midrib. In some leaves where an adaxial meristem activity is marked, lateral extension is suppressed resulting in a more or less cylindrical leaf; such a leaf is termed unifacial as it is radially symmetrical (Kaplan 1973b) and does not have the two sides of a bifacial leaf. The unifacial leaf may remain cylindrical (terete or centric 87g) or subsequently become flattened bilaterally (isobilateral or ensiform 87h). The phyllodes of Acacia and other plants are formed in this way (42). The base of an ensiform leaf retains a bifacial form which is usually folded (conduplicate 37j) and the bases of successive leaves demonstrate equitant vernation (39g). The leaf of Dianella has a conduplicate base, an ensiform middle portion, and a bifacial distal end (51a). Terete (cylindrical) leaves result from the development of the upper leaf zone of the leaf primordium in both monocotyledons and dicotyledons and are therefore homologous (20). Localized subtleties of meristematic activity also give rise to peltate and ascidiate leaves in some species (88). Fig. 86. Tillandsia usneoides The adult plant has no roots, atmospheric water being absorbed by the fine terete leaves. Leaf morphology: ensiform, terete, laterally flattened and cylindrical leaves | 87 Fig. 87. a) Senecio sp., end of shoot; b) Oberonia sp., end of shoot; c) /ris pseudacorus, foliage leaves at distal end of rhizome; d) Ceratostylis sp., stem with distal terete leaf; e) Reichenbachanthus sp., stem with distal terete leaf, f) bifacial, g) terete, h) ensiform, i) peltate, j) epiascidiate, k) hypoascidiate. El: ensiform leaf. St: stem. TI: terete leaf. 88 | Leaf morphology: ascidiate, peltate, pitcher and circular leaves Activity of the various areas of meristematic cells present in a developing leaf primordium (18) commonly gives rise to a bifacial leaf with an upper (87f) surface (ventral, adaxial) and a lower surface (dorsal, abaxial). However, leaves flattened in the ventral plane (ensiform 87h) and cylindrical leaves (terete 87g) are not uncommon. Similarly, differential meristematic Fig. 88a, b. Norantea guyanensis The bract (62) subtending each flower is hypoascidiate initially developing as an inverted spoon shape (a) and then forming a hollow chamber (b) containing extra-floral nectaries (80). The final form is shown in the frontispiece. activity can give rise to a peltate leaf (87i) in which a more or less circular lamina has the petiole attached near the centre (36b, 89b, d). This shape can also occur as a teratology (peltation 270) in any leaf, particularly one that normally has basal lobes. The lamina of a peltate leaf is flat or slightly dished; if meristematic activity continues, the lamina can become funnel-shaped forming a container and the leaf is termed ascidiate. Normally the inside of the container is developmentally equivalent to the top of a peltate leaf, and the outer surface equivalent to the underside of a peltate leaf (epiascidiate). The distinctive leaf of a pitcher plant conforms to this arrangement (89c). The epiphyte Dischidia has two forms of leaf, bifacial on a climbing stem and ascidiate leaves developing near the branch of the supporting tree. Adventitious roots (98) grow into the opening of the ascidiate leaf which contains debris (89f). Bladder leaves of the Lentibulariaceae are ascidiate (73e), and are variously developed from highly dissected submerged leaves of these water plants which have no roots (cf. 9le). Very rarely an ascidiate leaf results from the development of a pouch in which the lower surface is inside—a hypoascidiate leaf. Bracts (62) subtending flowers of Pelargonium.can take this form, as do those of Norantea (88a, b, and frontispiece). Leaf morphology: ascidiate, peltate, pitcher and circular leaves | 89 Fig. 89. a) Cassia floribunda, abnormal leaf tip; b) Hydrocotyle vulgaris, stolon bearing leaves; c) Nepenthes x coccinea, single leaf; d) Umbilicus rupestris; e) Justicia suberecta, single leaf; f, f') Dischidia rafflesiana, single leaf and section of leaf. Adv: adventitious root. Ap: abnormal peltate development (peltation). Epa: epiascidiate leaf. Epl: epiascidiate lamina (upper leaf zone 20). Le: inrolled leaf edge (not peltation). Lz: lower leaf zone. P: peltate leaf.(f' after Massart 1921). 7 DoS Mee 1 90 | Leaf morphology: indeterminate growth Fig. 90. Guarea glabra A young tree. Each apparently woody slender stem bearing simple leaves is in fact a long- lived growing compound leaf (91f). A leaf, particularly on a woody plant, is generally found to be a temporary structure, developing relatively rapidly to a finite size (i.e. it is determinate) and persisting until dislodged by drought or frost or loss of vascular connection on an expanding stem axis (48). A branch system is seen to be more permanent. However, twigs and branches are often shed (268) and conversely some plants possess leaves that grow progressively for some time (i.e. they are more or less indeterminate). This results from a proximal intercalary meristem in the Gramineae (180) and other monocotyledons. In some dicotyledons, the distal end of a pinnate leaf retains its capabilities for cell division and the final length of the leaf is attained over an extended period by the periodic production of extra pairs of leaflets (90, 91f). Such structures, delayed in their appearance, can be preformed, i.e. the whole leaf develops initially but its parts mature in sequence (91a, b) from leaf base to leaf apex and the leaf is thus strictly speaking determinate. Alternatively, the leaf is truly indeterminate and the apical meristem of the leaf continues to function, initiating new growth periodically for several years as in Guarea (Steingraeber and Fisher 1986), (epigenesis 91c, d). The oldest, i.e. proximal leaflets, fall off in the meantime and the leaf rachis increases in girth due to cambial activity (such cambial activity is sometimes also found in the petiole of other long-lived but determinate leaves 40a). Indeterminate leaves often bear inflorescence primordia in association with the new leaflet primordia (epiphylly 74). The underwater leaves of Utricularia (cf. 206), are indeterminate in | | development and form an apparent much branched structure (91le). The unique | phyllomorph (208) of some Streptocarpus spp. behaves in the manner of an indeterminate simple leaf. | Fig. 91. a, b) determinate leaf developing over a long time interval from preformed leaflets; c, d) indeterminate leaf developing new leaflets from an apical meristem: e) Utricularia reniformis, end of indeterminate leaf; f) Guarea ; | glabra, distal end of compound leaf (90). Ab: axillary bud. \ i Am: apical meristem (of the leaf). Lf: leaflet. 5mm 10 mm 92 | Leaf morphology: palms Fig. 92a. Jubaea spectabilis Reduplicate attachment of leaflets to midrib of leaf. Fig. 92b. Phoenix dactylifera Induplicate attachment of leaflets to midrib of leaf. The leaves of the palms (Palmae) show a sufficient number of distinctive morphological features to warrant separate description. All palm leaves have a lamina, a petiole, and a sheath, the lamina being mostly of three general shapes— palmate (93a) lacking a rachis, pinnate in which leaflets are born on the rachis (93c), and costapalmate, an intermediate shape in which palmately arranged leaflets are born on a very short rachis or costa (93b). (A few palms have simple leaves; Caryota has a bipinnate leaf 93d.) The most distinctive feature of the palm leaf occurs in the development of the leaflets (Dengler et al., 1982, Kaplan et al., 1982a, b). These do not arise by differential growth rates in meristems along the leaf primordium edge (18). Instead, differential growth in the expanding leaf lamina causes the lamina to become plicate (37i), i.e. folded into ridges and furrows. There is then a subsequent separation of rows of cells between plications giving rise to the distinct leaflets. Strips of dead cells occur at the edges of palm leaves and are known as reins, or lorae; they form a conspicuous feature of some palms (93d). In palmate and costapalmate leaves the splitting may not extend all the way from the lamina edge to the centre; this is a specific variation. One effect of the splitting between plications of a palm leaf is that the attachment of an individual leaflet, or ‘finger’, to the rachis or petiole can take two forms. It may be reduplicate (92a) or induplicate (92b). Almost all ‘fan’ leaves (palmate and costapalmate) are induplicate; most ‘feather’ leaves (pinnate) are reduplicate and have a terminal pair of leaflets (paripinnate 23e). The few that are induplicate are imparipinnate (57f) having a single terminal leaflet. A ridge of ‘tissue, the hastula (93a’), is present at the _junction of petiole and lamina in some palmate and costapalmate leaves. It may be on the adaxial side, the abaxial side, or both. (A similar ‘structure occurs on leaves in the Cyclanthaceae.) The sheaths of palm leaves may persist on the tree for many years in the form of a fibrous mat (51b), or as stumps, splitting in the mid line due ‘to stem expansion, or forming a collection of ‘spines (70a, b), the spines representing the fibrous vascular bundles of a ligule at the junction of sheath and petiole. Non-spiny ligules occur in a number of palms. Spines also occur in the form of modified adventitious roots (106), as spines on long thin modified inflorescences (flagellum), or as emergences on leaf (71f) or stem. The leaves of rattans (climbing palms) often bear distal pairs of leaflets modified into spines or reflexed hooks on an extended rachis or ‘cirrus’ (71f). A full account of the morphology of palms ‘is given by Tomlinson (1990). _ Fig. 93. a, a') Livistonia sp., single palmate leaf and close ' view of lamina/petiole junction; b) Saba/ pa/metto, single d) Caryota sp., single bipinnate leaf. C: costa. E: emergence (76). H: hastula. R: reins. costapalmate leaf; c) Phoenix dactylifera, single pinnate leaf; Leaf morphology: palms | 93 (c) WY ; Hye Sy S 100 mm : \} se 94 | Root morphology: development Fig. 94. Pisum sativum A portion of root rendered semi- transparent showing the internal (endogenous) location of lateral root primordia. z 3 a A root develops from a root primordium, a group of meristematic cells originating below the surface of an existing root or shoot (endogenous development—produced within 94). The first root of the embryo and all subsequent roots increase in length due to cell division and enlargement behind the root apex. The region of the root apical meristem is protected by a permanent covering of mature cells, the root cap, which is particularly obvious in some aerial roots (95). The root cap can be replaced by the root apex if the cap is damaged. Apart from the root cap, a root apex does not bear any other structures and thus contrasts with the shoot apex (112) which bears leaf primordia and associated axillary buds on its surface (exogenous development—borne externally), the shoot apex being protected by its enveloping leaves or other means (264). Some distance back from the root cap and apex a root may bear lateral roots. These lateral roots commence development from meristematic areas, with root primordia occurring beneath the surface of the parent root pushing their way out through the parent root cortex. In addition to lateral roots, other structures may develop on a root away from its apex: nodules in association with bacteria (276), mycorrhiza in association with fungi (276), and root buds (i.e. shoot buds on roots 178) capable of developing into new complete shoot systems. Elaborate root systems can develop in two basic ways. The initial radicle _ (162) of the seedling will bear many lateral root primordia possibly in some orderly sequence (96). : The lateral roots can subsequently branch, and root cross-sectional area will increase due to -cambial activity (16) as new lateral roots are added. Secondly, root primordia can arise endogenously in stem tissue giving rise to an extensive adventitious root system (98). Such roots are often associated with the nodes on the stem. This type of system is found in the majority _of monocotyledons, the roots being incapable of extensive enlargement in girth. Adventitious roots in the Bromeliaceae can extend some | | distance in the stem cortex, growing parallel to the stem surface before finally emerging (intercauline roots). Root primordia present in an embryo before germination are referred to as seminal roots (162). : ialiemeeia o Root morphology: development | 95 Fig. 95. Pandanus nobilis. Tip of aerial prop root (cf. 103) showing massive root cap. 96 | Root morphology: primary root systems Root systems are generally recognized to be of two basic types. In the first type the whole system is derived by growth and lateral branching of the seedling radicle (162) and is termed a primary root system; this type is typically found in dicotyledonous plants. In the second type, the primary root system is supplanted by an adventitious root system and is ubiquitous in the monocotyledons. An adventitious root develops from a root primordium arising in a stem or leaf (98). (The term adventitious is also occasionally applied to roots developing late and out of sequence in a primary root system.) Some dicotyledonous plants possess both types. Attempts to describe the varieties of branching of primary root systems take three approaches: a description of the overall form of the branching system, an investigation of the location of lateral root primordia in the developing root system, and analysis of the branching system in terms of branch orders (284), geometry and topology (mathematical description of branching) (Fitter 1982). An example of the type of classification that can be applied to primary root systems is given in Fig. 97a-f modified from Cannon (1949). This system relies on the clear distinction of the vertical growth of the primary root and the various configurations of the first order lateral roots. First order laterals will bear second order laterals and so on. Four additional categories are applied to adventitious root systems (97g-4). Similar types of classification exist for tree root systems (100). Whatever the form of rooting, the details of the branching pattern depend on the location of the lateral root primordia. Root primordia result from the meristematic activity of patches of cells beneath the surface of existing roots (94). The sitings of primordia are not haphazard and varying degrees of orderliness can therefore be observed in the location of lateral roots (‘rhizotaxis’). Lateral root primordia are frequently initiated in longitudinal rows within the parent root, the position of rows being governed by the arrangement of the vascular tissue in the centre of the root. Rows vary in number from two, three (97k), and four to many. The greater the number of rows, the less precise lateral root initiation appears to be. There can also be a degree of regularity of primordium spacing along any one row. (Mallory et al., 1970). Fig. 96. Bignonia ornata A climbing plant in which the central leaflet of each trifoliate leaf is in the form of a three pronged hook (cf. Fig. 69b). There is a bud visible in the axil of each leaf. Also developing at this node is a pair of branched adventitious roots (98) visible just above each leaflet ‘claw’. Root morphology: primary root systems | oF Fig. 97. Adapted from Cannon (1949). a)-f) Variations of primary root systems (lateral roots developing on radicle); g)-4) types of adventitious root systems. Roots developing on vertical (g, h) or horizontal (i, j) stem. k) Section through root having three-rowed xylem arrangement. Ar: adventitious root. Arf: fibrous adventitious root. Arfl: fleshy adventitious root. Lr: lateral root. R: radicle. S: stem. Ss: stem scale leaf. X: xylem tissue. 98 | Root morphology: adventitious root systems Fig. 98. Philodendron sp. A number of adventitious roots develop at each node of the climbing stem. Some grow vertically downwards, others grow horizontally, wrapping around the support (the supporting plant demonstrates stem emergences, cf. Fig. 117c). Adventitious is an unfortunate adjective that literally means ‘arriving from outside’ and in morphology can be applied to any organ that is found in an atypical position. This is possibly appropriate in the case of an adventitious bud (232) occurring on the lamina of a leaf (74) because the vast majority of buds occur in the axils of leaves (4). Even then it is not necessarily unusual for the plant in question. The term is even more inappropriate in the case of adventitious roots, where it is applied to roots developing on stems or leaves, i.e. not forming part of the primary root system (96). In practically all monocotyledons the primary root system is short-lived and the whole functional root system of the plant is adventitious, the roots arising on the stem near ground level or below. This is particularly obvious in the case of rhizomatous monocotyledons (130). Similar elaborate adventitious root systems develop as a matter of course in many dicotyledonous plants having a rhizomatous or stoloniferous habit (132). In both cases, adventitious roots tend to be associated with nodes (i.e. they may be termed nodal roots) and the exact positions of development of the endogenous root primordia are governed by features of the vascular tissue at that node. This can result in quite precise patterns of nodal root arrangement, particularly in dicotyledons (96). Conversely, adventitious roots of climbing plants are often borne between nodes (99a). Adventitious root primordia may be formed in the meristematic region of shoot apices and then develop into roots immediately or possibly much later when the supporting organ is old, or they may arise in old tissue by dedifferentiation, i.e. the return to meristematic activity, of selected patches of cells. The development of these new or latent primordia in an existing primary root system gives rise to an additional root system to which the term adventitious is sometimes applied, particularly in the case of tree roots. Thus the term adventitious root can be found applied either to a root ‘out of place’, i.e. on stem or leaf, or a root developing from old organs including old roots. Adventitious roots on stems are not always of one type. The classification of adventitious root systems of Cannon (1949) emphasizes this point. For example roots may be long, thin and anchoring, or much branched and fibrous (235a), or grow vertically upward or vertically downward (98). Root primordia can thus have a specific fate (topophysis 242) in some plants. Stems of Theobroma (cocoa tree) do not produce adventitious roots unless severed and allowed to form rooted cuttings; adventitious roots on ‘chupon’ stems (which grow vertically upward) themselves develop vertically downward, conversely adventitious roots developing on ‘jorchette’ stems, which grow horizontally, also develop horizontally. An extended account of adventitious roots, and others, is given in Barlow (1986). Root morphology: adventitious root systems | 99 Fig. 99. a) Ficus pumila, end of climbing shoot (cf. 243); b) Senecio mikanoides; c) Acampe sp., roots emerging from between leaf sheaths; d) Jasminium polyanthum, portion of trailing stem. Ar: adventitious root. Ax: axillary shoot. In: internode. St: stipule (52). 100 | Root morphology: tree root architecture The branching systems of tree roots are exceedingly diverse, the architecture of the system of an individual tree changing considerably as it develops. Relatively young trees may have a tap root system based on the development of the radicle. Krasilnikov (1968) describes a range of variations of this theme (101a, d-f);which can be compared with root system descriptions of Cannon (1949) (97). This primary system can then become augmented or completely replaced by a secondary root system. The secondary system (sometimes referred to as adventitious 98) develops by the activity of root primordia on the old primary root system and the production of adventitious roots from stem tissue (100). A further distinction in a tree root system can be made between the skeletal system, i.e. the main framework which will be primary and/or secondary, and additional sub-systems of primary or secondary roots not contributing to the main supporting architecture. Additional distinctive features may be apparent such as buttresses (101c, d), stilt and prop roots (102), and pneumataphores (104). The roots of one individual tree can become naturally grafted to each other and such grafting has been recorded Fig. 100. Pandanus sp. between the roots of neighbouring trees usually An elaborate prop root (102) formation in the manner of Fig. 101g. of the same species but occasionally of different species. The general phenomena of tree root architecture discussed here are those identified by Jenik (1978) in a tentative classification of tropical tree root systems (101) in which the primary root system is always more or less obliterated. Root morphology: tree root architecture | 101 Fig. 101. Adapted from Jenik (1978). Tropical tree root systems. B: buttress. Cr: columnar root. Rs: root spine. Sr: stilt (prop) root. T: tap root. Tr: tabular root. (see 102). 102 | Root morphology: prop roots Fig. 102. Euterpe oleracea Prop roots on a palm. The small outgrowths on the surface of each root are pneumatorhizae (104). Prop or stilt roots are adventitious roots (98) developing on the trunk or branches of a tree or the stem of a vertically growing herb. In a few exceptional cases, horizontal rhizome systems are supported as much as a metre above the ground on stilts roots (Hornstedtia, Geostachys, and Scaphochlamys in the Zingiberaceae and Eugeissonia minor, a palm). Prop roots are also found supporting pneumatophore roots (104). The tentative tropical tree root classification of Jenik (1978) includes a number of permutations of tree stilt root construction (101e-k). Stilt roots may themselves bear stilt roots (101j), Fig. 101i indicates a similar result developing in this case by arching and rooting of shoot systems. Prop roots can take the form initially of spines (101g), which may subsequently elongate to form spine roots. Prop roots may be bilaterally flattened, forming flying buttresses (101c, d); these root buttresses may be positioned at the base of the tree trunk or form flattened tabular root structures running away at soil level from the tree. A prop root usually branches freely once it reaches the ground. It will retain its initial diameter in a monocotyledonous plant; in dicotyledonous plants it may remain very thin until rooted at its distal end and subsequently thicken into a prop or columnar root (101k). In many epiphytic plants long adventitious roots develop and grow into detritus or hang free as aerial roots. Such roots may anastomose around the supporting plant—‘strangling roots’ (1011). : prop roots | 103 Root morphology Fig. 103. Pandanus nobilis. Base of trunk showing prop root production (cf. 100). PP, A 104 | Root morphology: pneumatophores, breathing roots Many woody plants living in swampy or tidal conditions show modifications of that part of the root system which is located above water level or exposed at low tide. These roots are specialized in their anatomy and are generally described as pneumatophores (‘air bearing’) or more precisely pneumorrhizae. They take a range of forms and develop in different ways, and are well endowed with lenticels (114) and internal air spaces which are continuous with those of submerged roots allowing gaseous exchange in the latter. Pneumatophores occur as prop or tabular roots (101) or develop from shallow horizontal roots as laterals that grow vertically upwards (peg roots 104). These may or may not become thickened (105a). In some cases these peg roots are themselves supported by prop roots. Alternatively the shallow horizontal root loops upward above the waterlogged level and back down again. The aerial loop or ‘knee’ then becomes progressively thickened, or can remain relatively thin (105b, d). The horizontal root may remain submerged, the lateral looping once only to produce the knee root (105c). A number of plants, particularly palms, growing in waterlogged conditions develop numerous very Fig. 104. Rhizophora mangle small lateral roots with a mealy appearance often “ ™@ngrove swamp with a tangle of prop roots (102). on the surface of prop roots (102); these are termed pneumatorhizae; individual sites of gaseous exchange visible on the surfaces of pneumatophores are referred to as pneumathodes (Tomlinson 1990). Pneumatophore roots of Avicennia nitida are developing vertically upwards out of the water in the foreground. Root morphology: pneumatophores, breathing roots | 105 Fig. 105. a) Sonneratia sp.?, peg root; b) Mitragyna ciliata, knee root; c) Gonysty/us sp., knee root; d) Symphonia gabonensis, knee root. 106 | Root morphology: modifications Fig. 106. Myrmecodia echinata The swollen root tuber (110) is chambered, and houses ants (cf. domatia 204) The roots of any one plant show a range of morphologies. Some roots may be relatively thick and tough, others very fine and fibrous. In dicotyledons varying degrees of lignification take place. Major roots of dicotyledonous trees can be massive structures, possibly showing annual growth rings in cross-section, and developing a thick bark. Other roots show more specific modifications. They may form prop and aerial roots (102), breathing roots (pneumatophores 104), storage organs (tubers 110), haustoria of parasitic plants (108), or form structures in association with other organisms, i.e. mycorrhizae and nodules (276). Roots can also bear buds (178). Individual roots can shorten considerably in length forming contractile roots (107e) which maintain.a corm or bulb, for example, at a particular soil level. Contraction is brought about either by shortening and widening of cells or total collapse of cells. Adventitious roots (98) of some climbing plants may branch (96), expand into cavities, secrete a slow drying cement (99a) which forms an attachment to the substrate, or actually twine about a support (98). Other aerial adventitious roots particularly of epiphytic orchids are covered with layers of dead cells, the velamen, appearing white when the tissue is full of air. Velamen can become saturated with water up to an inner waterproof layer, except for small areas which remain full of air, and then the root will appear green due to chloroplasts in deeper tissues. However, it appears that the aerial roots do not absorb water from the velamen; water is absorbed only from distal ends in contact with a substrate, the yvelamen acting in a protective capacity. In a ‘imited number of plants selected roots lose the ‘meristematic apex and root cap and develop a woody point. Such root spines occur above or oelow ground in different species (1074). ») Dactylorhiza fuchsii, root tuber at stem base; c) Mirabilis alapa, underground swollen root; d) Dioscorea prehensilis, yranched spiny roots; e) Crocosmia x crocosmiflora, corm oot tuber. ig. 107. a) /ncarvillea delavayi, underground swollen roots; vith contracted roots. Cr: contractile root. Rs: root spine. Rt: Root morphology: modifications | 107 108 | Root morphology: haustoria Fig. 108. Cuscuta chinensis A swollen pad develops at intervals at points of contact of the parasite’s stem with that of the host. Seen as bulges on the two lowest loops Parasitic and hemiparisitic flowering plants (non- photosynthetic and photosynthetic, respectively) obtain the whole or part of their nutritional requirements by the intrusion of haustoria into the host’s tissue. The morphological nature of the haustoria vary considerably and cannot in most cases be unequivocally recognized externally or internally as root modifications. The extreme situation is found in Rafflesia in which the body of the plant consists of delicate branching threads composed of amorphous masses of cells permeating the food and water-conducting system of the host. Only the production of flowers betrays its presence. Other parasitic plants develop adjacent to their hosts; where their roots come into contact, outgrowths of the parasite attach to the surface of the host and connection is developed internally by the formation of an haustorium. The haustoria of one parasitic species can be different in structure on different host roots. Haustoria develop from the stems of climbing parasitic plants which have no contact with the ground after the initial seedling roots. Species in the Loranthaceae are typically hemiparasites, with green leaves. They form mostly woody shrubs although some species reach the proportion of small trees. Their haustoria show a number of distinctive features. The parasite may be attached at one point on the trunk or branch of a host tree (109a, b, e) and the host may respond by developing abnormal swellings, very elaborate ones being termed wood roses. An haustorium can consist of a single structure embedded in the host tissue (a sinker) or a number of these may develop at one point of attachment. Alternatively, structures form which have been called epicautical roots, or runners (cf. 134), developing over the outer surface of the host (109c). At intervals the runner produces attachment discs, or haptera, with haustoria penetrating the host from beneath each hapteron. Runners may grow along a live branch and then turn around and return if a dead broken end is encountered. The host may die distally to the point of attachment of the parasite. Establishment (168) of seedlings of the Loranthaceae is complex. The seed is initially attached to the host branch at the hypocotyl. It is unclear whether there is any root axis present. The base of the hypocotyl swells to form a primary haustorium and is glued to the surface at this stage (in Viscum for example). Distortion of haustorial tissue forces a sinker into the host tissue. The seedling may now be erect with photosynthetic cotyledons. In some species the cotyledons are connate (234) and lie on the host surface. The plumule emerges from the slit between the cotyledons. Details vary considerably from one species to the next. An extensive account of the biology of parasitic plants is given by Kuijt (1969). Root morphology: haustoria | 109 Fig. 109. Parasite/host connections. a) Tapinanthus oleifolius; b) Phoradendron perrottetii (on Protium insigne host); c) Amylotheca brittenii; d) Cuscuta sp. (on Urtica pilulifera host); e) Lysiana exocarpi (on Hakea intermedia host). Er: epicautical root. H: host. Hau: haustorium. P: Parasite. aaa 110 | Root morphology: tuber Expansion of a root laterally by cell division and eae ~ enlargement gives rise in many species to a n ____ swollen root or root tuber (similar underground structures can be formed from swollen stems, 138). Frequently only a proportion of the roots on a plant will form tubers which vary considerably in different plants in their size and shape. In some orchids just one adventitious root (98) swells during each growing season providing storage material for growth after the resting period. A similar development occurs in Ranunculus ficaria. Here, single adventitious roots are produced at the base of buds on the aerial stem. The root swells to produce a detachable ‘tubercule’ which also includes the bud’s apical meristem. Similar tubercules develop from adventitious buds on the stem base. In each case additional adventitious buds can develop on a tubercule itself. Thus organs are produced which are composed of tissue derived from both root and shoot (see dropper 174). In contrast to a stem tuber, a root tuber will have a root cap, at least when very young, and it may bear lateral roots but will not bear a regular sequence of scale leaves subtending buds, although there may be one or more buds present at its proximal end. These buds may be derived from the stem to which the adventitious root is attached or represent adventitious buds (232) arising from the root itself. The primary root of a plant can become swollen to produce a tap root tuber, usually in conjunction with a swelling of the base of the hypocotyl (166). Large woody swellings form on some trees and shrubs and can Fig. 110. Chlorophytum comosum ‘ a Excavated plant showing swollen root tubers. The inflorescences demonstrate false vivipary be partly of root tissue origin. Such woody (176) structures are referred to as lignotubers (138a). Root morphology: tuber | 111 Fig. 111. Swollen storage roots. a) Ch/orophytum comosum, b) Dahlia sp., c) Dioscorea sp., d) Kedrostris africana. Ad: adventitious root (98). Drt: decayed root tuber. Rt: root tuber. \ 112 | Stem morphology: development A stem consists of a series of nodes separated by internodes. Leaves are inserted on the stem at the nodes and commonly have buds in their axils (4). (A forester uses these terms in a different fashion, a node marking the location of a whorl of branches on a trunk, the portions of trunk between whorls constituting internodes.) Internodes may be very short, in which case one node appears to merge into the next. The combined structure of stem and leaves is termed a shoot (4) and thus each bud in a leaf axil represents an additional shoot. The sequences of shoot development give any plant its particular form. Each stem grows in length owing to the activity of an apical meristem situated at its distal end. The dome of cells that forms the apical meristem is constantly changing its size and shape as new leaf primordia (18) are initiated from its flanks (exogenous development) in a regular sequence (218). Older, more proximal, leaves can form some sort of protection over younger leaves (264). The time interval between the formation of two successive leaf primordia on the apical meristem is termed a plastochron(e). The stem can increase in width just behind the apex as well as in length. This is particularly apparent in monocotyledons, especially palms, where later increase in thickness due to the activity of a cambium (16) is not usually possible. The apical meristem of a stem may produce leaf primordia continuously, or rhythmically with intervals of rest (260). Leaf production may be out of phase with stem elongation (283i). The apical meristem of any one shoot is sometimes referred to as the terminal Fig. 112. Linaria sp Abnormal stem development. A ribbon- shaped structure (fasciation 272) instead of cylindrical meristem (or terminal bud), to distinguish it from the axillary meristems (or axillary buds) borne in the axils of its leaves. (Lateral meristem is used in a different context 16.) Each axillary meristem can develop into a shoot in its own right and will have its own apical meristem. The apical | meristem may continue to function more or less _ indefinitely resulting in monopodial growth (250). Alternatively the apical meristem may sooner or later change its activity and terminate with the production of a flower, or whole inflorescence or other organ, or otherwise lose its meristematic capabilities (244). Continued elongation of the axis can then occur by the development of an axillary meristem usually close _ behind the apex. Such growth is termed _ sympodial (250). Stems can develop in a range of shapes (120) and surfaces can become elaborated by bark development (114), emergences (116), adventitious roots (98), and adventitious buds (232). Fig. 113. The ‘marvell of Peru with white floures’ (Mirabilis _ falapa) redrawn from Gerard (1633). The figure illustrates a _ root tuber (cf. 107c) and stem pulvini (cf. 129). Stem morphology: development | 113 WU SANS TS o< “Mj or Lt Mihi <= SS —— Wh My 114 | Stem morphology: bark The surface of a stem (or root and occasionally petiole 40a) can become elaborated by the development of a layer of bark. The bark of any one species is characteristic and is an aid to identification, although it will vary considerably depending upon the age of the trunk or stem. The term bark is often applied to the whole structure that can be pulled away from the wood. However this layer will include at its inner surface the phloem (food conducting tissue) and bast (phloem) fibres. Bark strictly applies only to the outer layer of tissue that develops from a cylinder of meristematic cells within the stem, the cork cambium (or phellogen), and which constitutes a lateral meristem (16). Cells external to the cork cambium are dead, cells internal to this cambium may contain chloroplasts—if the outer layer is thin the bark can appear green. As a stem expands in width, the dead layers of bark are forced apart to be replenished from within. In addition the cork cambium often does not form a simple cylinder in the stem but has an irregular three-dimensional arrangement such that the bark is produced in isolated sections which can become detached independently. These features give bark its variously textured appearance. The bark is punctuated at intervals by small patches of loosely packed cells allowing air to penetrate to underlying live tissues. These cell patches, lenticels, can be conspicuous at the surface particularly in smooth bark (115a). Bark will form characteristic patterns around the scar of a fallen branch or leaf (115e). The natural appearance of the bark of tropical trees is loosely described as belonging to six broad categories by Corner (1940): smooth (115a), fissured (115b), cracked (115c), scaly (115d), dippled-scaly (115e), and peeling (115f). Monocotyledons, with few exceptions, lack a lateral meristem able to produce bark but many, for example palms, develop a hard outer layer of fibres derived from old leaf veins. Fig. 114. Ficus religiosa Part of a young shoot, six internodes visible. Lenticels are conspicuous on the upper three internodes; bark formation commences at each node and is more advanced in the lower, older, internodes. bark | 115 Stem morphology c) Liquidambar styraciflua, cracked; fissured; ’ Castanea sativa ' b) Fig. 115. Bark types. a) Prunus maakii, smooth; f) Acer griseum, peeling. e) Peumus boldus, dippled-scaly; d) Talauma hodgsonii, scaly; 116 | Stem morphology: emergence, prickle Fig. 116a. Chorisia sp Permanent trunk prickles Fig. 116b. Aiphanes acanthophylla Emergences on a palm trunk; root spines (106) occur in similar locations on other species of palm (e.g. Cryosophila and Mauritia spp) In addition to leaves, buds, and roots, a fourth category of structure, an emergence, sometimes develops on a stem, and is usually in the form of a prickle. There is not a particularly clear distinction in the usage of the terms prickle, spine, and thorn (76). Here, prickle is used solely for a sharp structure on a leaf (76) or stem that is woody, at least when mature, and is derived from tissues just beneath the epidermis in contrast to trichomes, i.e. hairs formed from the epidermis (80). Thus, an emergence does not represent a modified stem (124), leaf (70), or root (106). Prickles occur on young stems usually in an irregular arrangement (117) and vary in size. If flattened longitudinally (117a) they may approach in appearance the winged condition of some stems (121d). On older stems the prickle may be shed leaving a scar, or persist and become a relatively massive structure (116a, 117c). Nevertheless prickles are usually relatively easily detached indicating their superficial development, and will not be expected to contain vascular tissue. They are often associated with a climbing or scrambling habit. Stem morphology: emergence, prickle | 117 Fig. 117. a) Rosa sericea var. pteracantha, stem after leaf fall; b) Extatosoma tiaratum; c) Fagara sp., portion of old stem; d) Acacia seya/, stem at point of leaf attachment; e) Rubus fruticosus agg. E: emergence. Efn: extra-floral nectary. Em: emergence mimic. Es: emergence scar. P: petiole. St: stipule (52). 118 | Stem morphology: scars Fig. 118. Philodendron sp Each broad pale scar is that of a detached foliage leaf. The bud that was subtended by each leaf has also abscissed and is represented by a bud scar surrounded by the leaf scar. Adventitious roots (98) also present. Scars on stems either indicate the former position of a structure that has fallen off, or develop in response to injury or grafting. In young tissues the location of injuries may be masked by exudation of latex or resin. In old live tissue the formation of wood and bark will produce various structures growing over the wound. Scars left by the abscission of leaves, roots, shoots, and fruits will be more regular in their shape and location. Leaves often fall due to breakage at precise points of abscission (48) and the scar left on the stem will indicate the former position of vascular strands in the leaf (119a). Many plants shed whole shoot complexes, breakage again occuring at precise locations (268) and the corresponding scars will remain unless subsequently enveloped by further growth of the stem (115e). Increase in girth will lead to the separation of scars that are initially close together, those of a leaf and its pair of stipules for example (119f). Stipules in many plants abscise at an early stage in leaf development; their existence is only detectable by identifying the persistent stipular scars (78). The relative position of scars on a stem can aid the interpretation of the remaining structures (4) and indicate for example if a shoot system is monopodial or sympodial (250). The scale leaves separated by very short internodes of a terminal bud fall to leave a ring of scars indicating the location of the bud when it was dormant. If dormancy is a response to annual drought or cold, the shoot system can be aged by counting the number of rings of scars (269b). SSS SS a ee ee ee al Stem morphology: scars | 119 Fig. 119. a) Ara/ia spinosa, end of shoot in winter; b) Hedychium sp., portion of rhizome (cf. 131e); c) Pterocarya fraxinifola, end of shoot in winter; d) Liriodendron tulipifera, winter shoot with remains of terminal flower; e) Magnolia grandiflora, flower after shedding of petals and stamens; f) Robinia pseudacacia, bark with remains of node features; g) Fagus sylvatica, end of shoot in winter. C: carpel. Csc: carpel scar. F: fruit. Is: inflorescence scar. Ls: leaf scar. Ps: perianth scar. Sls: scale leaf scar. Ssc: site of shed shoot. St: stipule. Stas: stamen scar. Sts: stipule scar. Vs: vein scar. 120 | Stem morphology: shape Fig. 120a, b. Miconia alata Two stages in the maturation of a stem internode which is fluted The young ‘wing’ tissue (a) is shed following the development of woody ridges and bark (b) The majority of aerial stems are more or less cylindrical in shape. The herbaceous and young shoots of shrubby species of some families, typically the Labiatae, are square in cross-section becoming round if woody. Underground stems have a variety of shapes (130, 136, 138). The stems of succulent plants are typically swollen (202) and in others the stem is flattened and mimics a leaf (126). The bases of leaves in some cases are extended some distance down the stem forming ridges (24). If particularly extended the stem becomes winged or pterocaul (12 1a, d, e). In such cases leaves may fall off very soon or be represented by scales, the photosynthetic activity being confined to the green stem and its flanges. A simple cylindrical shape may become elaborated by the formation of bark (114), or in the case of climbing plants develop a range of contortions and twistings (121c) due to differential growth rates of different tissues and the production of areas of short-lived and easily ruptured cells. The old but living stems of some desert plants are similarly disrupted and split following the formation of longitudinal sections of cork within the wood. The trunks of some tropical trees become so deeply fluted that holes develop through from one side to the other, a condition known as fenestration. (a) Stem morphology: shape | 121 Fig. 121. a) Cissus sp., portion of old stem; b) Cissus quadrangularis; c) Bauhinia sp., old liane (308) stem; d) Genista sagittalis; e) Baccharis crispa. Ls: \eaf scar. Pt: pterocaul stem. St: stipule (52). Ste: stem tendril (122). 122 | Stem morphology: tendril and hook Fig. 122a. Gouania sp. A stem tendril bearing leaves. Fig. 122b. IMligera sp Axillary shoots take the form of recurved hooks. Numerous climbing plants possess tendrils, or hooks acting in the manner of grappling irons. These structures may represent modified leaves (68), parts of leaves (petiole 40, stipule 56) or be derived from stems. Prehensile stem tendrils can become secondarily thickened to form permanent woody clasping hooks (122a, 123a). Alternatively a tendril will twine around the support and subsequently shorten in length by coiling up, the proximal and distal ends of the tendril often twisting in opposite directions. Stem tendrils may be branched; some have adhesive discs at their distal ends. Stem tendrils and hooks represent either modifications of axillary shoots or are terminations of a shoot, continued growth of that axis being sympodial (250). Frequently tendrils or hooks are produced as an apparent alternative to an inflorescence and in such cases have been referred to as modified inflorescences (145b, d). Tendrils may or may not bear leaves and buds and their true identity is often difficult to interpret giving rise to different published opinions. This is particularly so for the families Vitidaceae, Passifloraceae, and Cucurbitaceae. In the latter family, the single (123e) or sometimes pair of tendrils at a node is usually taken to represent a modified leaf, a prophyll (66) although this is not confirmed for Bryonia by Guédés (1966). Such decisions should be arrived at after careful study of the development of the shoot at the apical meristem and in particular the location of new tendril primordia in relation to other structures, i.e. leaves and buds, being produced (4, 6). Developmental studies of the formation of the tendril in Passiflora species of Passifloraceae indicate that each leaf subtends a collection of accessory buds (Shah and Dave 1971; 237c). A central bud forms the tendril which is therefore a modified stem, one or more lateral buds will develop into flowers or inflorescences, and yet another bud above (distal to) the tendril may develop into a vegetative shoot. An older interpretation (Troll 1935; his Fig. 659) based on the mature morphology of members of other genera in the family is that the tendril represents an axillary shoot and the flower or inflorescence is a lateral shoot borne on it but usually without subtending leaves (145b, 238). Similar alternative interpretations are promoted to describe the tendril in the Vitidaceae. This stem tendril, frequently branched and bearing small leaves (121b), is located on the opposite side of the stem to that of the foliage leaf at the same node (121b, 123d). These plants often show a very precise sequence of nodes with and without tendrils (229b). Accounts of the Vitis shoot usually take the tendril to be the terminal end of a shoot and the whole axis to be sympodial (250), a precocious lateral bud extending the growth. Studies of the development of the tendril at the shoot apex indicate that it arises on the side of the apical meristem, i.e. it is not a terminal structure (Tucker and Hoefert 1968). If the axis is considered to be monopodial then three accounts are available. Either the bud that forms the tendril is initiated 180° around the stem away from the leaf that should subtend it (Shah and Dave 1970), or the tendril bud, whilst probably subtended by a leaf becomes displaced from it during stem growth and appears 1 | at the node above, a form of adnation (234) (Millington 1966; Gerrath and Posluszny 1988). Finally, the tendril is explained as an organ ‘sui generis’-—a thing apart, and therefore not in need of interpretation (206)! Additional developmental | studies may help; nevertheless the mature plant has a leaf opposed tendril, and the plant is _ always right. _ Fig. 123. a) Artabotrys sp., single fruit on hooked _ inflorescence axis (144); b) Antigonon /eptopus, | inflorescence tendril (144); c) Hippocratea paniculata; d) Vitis cantoniensis; e) Gerrardanthus macrorhizus | (Cucurbitaceae 122). Acb: accessory bud (236). Ih: ' inflorescence hook. Ite: inflorescence tendril. Ls: leaf scar. Ste: stem tendril. TI: leaf opposed stem tendril. Stem morphology: tendril and hook | 123 124 | Stem morphology: spine A spine (6) may represent a modified leaf (70), stipule (56), leaf stalk (40), root (106), or flower stalk left after the fruit has dropped (144), or may represent an emergence (76, 116), or it may represent a modified stem. There is an inconsistency in the use of the terms spine, prickle, and thorn (76). A stem spine is formed if the apical meristem of a shoot ceases to be meristematic and its cells become woody and fibrous. Such a spine may bear leaves and therefore buds which may also develop as spines (125c, 242), or no trace of such lateral appendages may be visible (125a). In the latter case the stem origin of the spine is detectable because it will be subtended by a leaf or leaf scar (6). Frequently the spine represents one of a number of accessory buds (124a, 236b) in the leaf axil. This is not always apparent from the mature specimen. Spines are either lateral on longer usually indeterminate (125b) shoots, or terminal forming a determinate shoot (125e). Ifa relatively long vegetative shoot eventually ends in a spine, only the most distal portion is referred to as a spine. Fig. 124a. Gleditsia triacanthos Two shoots (236) developing in the axil of a leaf (lost). The upper shoot is represented by a spine. % Fig. 124b. Pachypodium lameri A condensed branch system (238) of spines developing in the axil of each leaf. Stem morphology: spine | 125 Fig. 125. Stem spines in axils of leaf or leaf scars. a) Balanites aegyptiaca, b) Aegle marmel/os, c) Prunus spinosa, d) Carissa bispinosa, e) Colletia infausta, f) Genista horrida, g) Crataegus monogyna. Ap: apex parenchymatization (244). L: leaf. Lb: leaf base. Ls: leaf scar. Ss: stem spine. St: stipule (52). 126 | Stem morphology: cladode, phylloclade, flattened green stem The stems of some plants are flattened structures which are green and photosynthetic and bear small scale leaves. Such flattened stems are referred to as phylloclades or cladodes. A plant Fig. 126a. . RiidiilenBeckia may be composed entirely of these structures, or platyclados they may be borne on more familiar cylindrical Flattened stems of many internodes—phylloclades which arise in the axils of leaves. stems (247a). A phylloclade (cf. phyllode which is a flattened leaf petiole 42) consists of a stem representing a number of internodes (126a, b, 127b). Phylloclades can be recognized by the presence of scale leaves or scars where temporary leaves have fallen off. Buds in these leaf axils will give rise to additional phylloclades or to inflorescences (126b). In the case of phylloclade-bearing cacti (127b, 203a), the leaf/bud site is marked by an areole (202). A cladode is a flattened stem of limited growth, the apical meristem aborting, and the stem usually consists of only one or two internodes (127a, c,d). A cladode or a phylloclade is subtended by a leaf, which is often a scale leaf or a scar where this has dropped (127d). A cladode may bear a scale leaf plus subtended axillary bud on its surface (127d’) and may then at first sight resemble an epiphyllous leaf (74). A group of cladodes may appear to rise in the axil of a single scale leaf (127a) and then represent proliferation shoots (239g). Pterocauly (121e) describes the condition in which a cylindrical stem has extended flattened wing-like edges. The existence on the one hand of leaves bearing buds (74) and on the other hand flattened stems, usually described as phylloclades or cladodes, allows scope for considerable discussion concerning the nature of these organs. Conventional morphology Fig. 126b. Phyllanthus angustifolius Flower clusters along the edge of the flattened stems. Stem morphology: cladode, phylloclade, flattened green stem | 127 _ vill wish to fit each example into a discrete (a) (b) (c) _ategory whilst recent developmental studies ‘dvocate a continuum of expression of leaf/stem “eatures in such organs, i.e. there is a _ransference of features between organ types, a shenomenon referred to as homeosis (Cooney- ovetts and Sattler 1986; Sattler 1988). | | | | { (Fig. 127. Flattened green stems in the axils of scale leaves. a) Asparagus densiflorus, b) Rhipsalidopsis rosea, c, c') Semele androgyna, d, d') Ruscus hypogl/ossum. Cl: cladode. _Cicb: cladode condensed branching (cf. 239g). Fp: flower pedicel. Ls: leaf spine (202). Pc: phylloclade. SI: scale leaf. St: stem. ! 10 mm } 128 | Stem morphology: pulvinus, swollen joint A pulvinus is a swollen joint on a stem or leaf. In the latter case a distinction can be made (46) between a pulvinus which allows reversible changes in orientation, a pulvinoid which allows irreversible movement, and an articulation joint marking a point of future breakage. Articulation joints occur on stems resulting in stem shedding (268), identified by the presence of scars plus fallen stems, but swollen stem joints allowing movement are mostly of the pulvinoid type, i.e. bending at the joint is likely to be due to cell division in a meristematic region (112) and therefore to be non-adjustable. Nevertheless many stems bend at a pulvinus if wilting and then recover the original position if watered. In these cases rigidity is maintained by turgidity, mechanical tissue being largely absent whilst the joint remains meristematic. It is not clear if any species possess a pulvinus that does allow repeated bending one way and the other in the manner of a leaf pulvinus. A stem pulvinus can become considerably enlarged with growth (128a) and eventually become woody (lignified). Fig. 128a. Rhoicissus rhomboidea An old stem pulvinus that has become enlarged and lignified (woody). Fig. 128b. Piper dilatatum Stem pulvini, swellings at every node Stem morphology: pulvinus, swollen joint | 129 E x< fo) = ® & ra] ‘3 o n ” =! oO oO ° 2) =} = = = a eu . . Lert ot ae, " 7 . Fig. 162. Ocimum basilicum Seedlings from above. Epigeal germination (cf. Fig. 165a) with pairs of relatively large cotyledons flanking pairs of developing foliage leaves . > ~ 3 ay +; A seed (158) usually contains just one embryo in which the first stages of differentiation of tissues and organs have located the potential shoot system and root system. The seed also contains a limited quantity of stored food which will allow the embyro to grow out of the seed coat and develop into an independent photosynthesizing, i.e. food producing, structure. This is the process of germination (164), and the young plant is termed a seedling up to an indeterminate arbitrary age (establishment 168, 314). The morphological details of the embryo within the seed and of the seedling as it emerges, vary depending upon the type of germination and the nature of the plant, dicotyledon or monocotyledon. The embryo and therefore the seedling of a dicotyledon possesses two leaves (cotyledons) attached at the cotyledonary node of an axis that has a primary root (or radicle) at one end and a shoot apical meristem (16) or plumule at the other (163). The junction of the root end and the shoot end (called the transition zone) can be more or less abrupt and not necessarily easily identifiable without anatomical investigation of the vascular system, although a prominent ‘root collar’ or ‘peg’ (163c) may be present at this point. The portion of axis between the cotyledonary node and the transition zone is called the hypocotyl (166), that immediately above the cotyledons, the epicotyl. The cotyledons themselves may be variously shaped, lobed or elongated, and fold together within the seed in numerous ways similar to the folding of the leaves in a bud (38). Usually they are of similar size and shape but in some dicotyledons ee tat one is very much larger than the other (163f, 209). They are usually opposite each other at the node; the location of subsequent leaves on the stem progressively conforms to the phyllotaxis of the mature shoot (218). Axillary buds, sometimes more than one (236), occur in the axil of each cotyledon, which often has a very short petiole. Cotyledons play a crucial role during the process of germination (164) and may contain stored food, or become photosynthetic, or both. The single cotyledon of the monocotyledon does not contain stored food, this being present in the form of endosperm adjacent to the embryo inside the seed. The cotyledon absorbs the endosperm when the seed germinates. It may also perform a photosynthetic function (164). Fig. 163. a) Vicia faba, hypogeal dicotyledon; b) Triticum aestivum, hypogeal monocotyledon; c) Cucumis sativus, epigeal dicotyledon; d) Phoenix dactylifera, hypogeal monocotyledon; e) A//ium cepa, epigeal monocotyledon; f) Cyclamen persicum, epigeal dicotyledon (anisocotyly 32). Co: cotyledon. Col: coleoptile (164). Ep: epicotyl. Fl: foliage leaf. H: hole. Hp: hypocotyl. P: peg. SI: scale leaf. T: testa. > a 164 | Seedling morphology: germination During germination, morphological development transforms a seedling dependent upon food stored in the seed, into a seedling able to photosynthesize its own food. At first water is taken into the seed by imbibition, but the first act of establishment (168) for the seedling is the production of a root or roots that will absorb additional water and anchor the plant. The food stored in the seed is either present in the cotyledons (dicotyledons only), and/or as endosperm, a product of fertilization in addition to the embryo and situated within the testa alongside the embryo. In some species part of the ovule tissue (154) also acts as a food source, the perisperm. A number of different sequences of development at germination can be recognized (165a-g) depending upon the food source, the role played by the cotyledon(s) and the manner in which the seedling axis elongates producing a photosynthetic array of leaves. Two principle modes of germination are given the names epigeal and hypogeal with reference to the location of the cotyledon(s) during this process; ‘geal’ indicates soil surface (‘earth’), ‘epi’ above, and ‘hypo’ below. Thus an epigeal seedling (165a, c, e) develops such that its cotyledons (163c) or cotyledon (163e) is above ground. During hypogeal germination, axis elongation is such that the cotyledons (165b) or cotyledon (165f, g) remain below ground or at least at ground level. For the cotyledons to be carried above ground, the portion of axis beneath the cotyledons (the hypocotyl 166) must elongate. For the cotyledons to remain below ground, the portion of axis above the cotyledons (epicotyl) must elongate, i.e. hypogeal germination, epicotyl elongates; epigeal germination, hypocotyl elongates. The permutations of functions performed by the cotyledon(s) in germination is illustrated in Fig. 165. An unusual form of germination in which elongation of the cotyledonary petioles plays a role is found in Vitellaria paradoxa (41g). In monocotyledons the cotyledon may elongate at germination, its distal tip remaining within the seed coat with the endosperm and its proximal end pushing the rest of embryo out of the seed coat (163d, 165f, g). The cotyledon is attached to the stem axis around most or all of its circumference as is typical of monocotyledon leaves (14), and can thus form a tube or solid structure with hollow base with the shoot apex at first hidden inside. The second leaf then emerges through a hole or slit in the side of the cotyledon (163e). In other monocotyledons the cotyledon does not elongate (grasses) but remains within the seed absorbing endosperm. However, different interpretations are applied to the exact extent of the cotyledon in these plants; the second structure produced, which forms a photosynthetic sheath, the coleoptile, is regarded either as the second leaf or as part of the cotyledon itself (180). The primary root (radicle) of a dicotyledon seedling will bear lateral roots as it enlarges in circumference. A number of root primordia (94) are usually present in a monocotyledon embryo in addition to the primary root. These are adventitious roots (98) associated with leaf nodes. Roots present as primordia in the embryo before germination are referred to as seminal roots. A distinction is made, at least in grasses, between the primary seminal root (radicle) and the lateral seminal roots (180). Fig. 164. Cucurbita pepo Seedling with epigeal germination. The photosynthetic cotyledons have expanded considerably in size and are now much larger than the seed coat (testa) (still visible) that contained them. photosynthetic storage endosperm transition zone LEZN Fig. 165. a, c) epigeal germination; b) hypogeal germination; d) a mixed form in the dicotyledon Peperomia peruviana; e) epigeal germination in a monocotyledon (e.g. Allium cepa 163e); f, g) alternative forms of monocotyledon germination as in Phoenix dactylifera (163d) and Triticum aestivum (163b). Co: cotyledon. Ep: epicotyl. Hp: hypocotyl. Seedling morphology: germination | 165 (d) (g) 166 | Seedling morphology: hypocotyl Fig. 166. Rhizophora mangle The embryo germinates whilst the fruit and seed are still attached to the plant. Germination results in the elongation of the hypocotyl which constitutes most of the seedling at this stage. The epicotyl is still inside the seed, the radicle (seedling root) is at the lower pointed end of the seedling. The hypocotyl is the length of stem linking the node at which cotyledons (in dicotyledons) are attached to the proximal end of the primary root. It is an elongated structure in plants with epigeal germination (163c). The junction of hypocotyl and root, the transition zone, is often ill-defined externally and is anatomically distinctive internally. The vascular anatomy of the hypocotyl is dominated by the veins serving the cotyledons. The hypocotyl shows some root-like features, and may bear ‘root’ hairs and often adventitious roots (98). Having no leaves by definition, it can therefore superficially resemble the primary root with which it is directly connected. It also frequently bears buds, these being termed adventitious (167e) as they are not in leaf axils. Thus an extensive shoot system can arise below the cotyledons, and the plumule (162) may fail to develop. The hypocotyl can be contractile in exactly the same manner as a contractile root (106). It can also form a storage tuber and become considerably swollen. In different species the stem internodes above the cotyledons and the root below the transition zone may or may not be swollen also. Without a developmental study of the anatomy of these tubers it is difficult to judge how much represents stem or hypocotyl or root (167a-d). The stem portion should bear leaves or leaf scars and the root portion lateral roots, possibly in orderly vertical rows. Elongation of the embryo hypocotyl before seed and fruit are shed gives rise to vivipary, in some mangroves for example (166), the whole seedling eventually dropping from the tree. True vivipary, the germination of seed before dispersal, is relatively rare, in contrast to false vivipary (prolification 176) which represents a development of rooting vegetative - buds instead of flowers. The hypocotyl of very young seedlings of some species of parasitic plants plays a part in the attachment of the parasite to the host (108). Seedling morphology: hypocotyl | 167 Fig. 167. a—d) root tubers (110) incorporating an upper (proximal) portion of swollen hypocotyl. a) Pastinaca sativa, b) Beta vulgaris, c) Cyclamen hederifolium, d) Centranthus ruber, e) Antirrhinum majus, Ab: shoot formed from adventitious bud (232). Co: cotyledon. Hp: hypocotyl. R: radicle. i... 168 | Seedling morphology: establishment growth The process of germination (164) establishes a young seedling such that it is anchored in the ground, can take up water, and can photosynthesize. The process of establishment continues however, and is marked by a sequence of morphological events elaborating the root system and extending the shoot system. The morphological status of the plant (314) may bear no relation to its actual age; very small tree ‘seedlings’ in a forest may grow millimetres a year and be of considerable age before conditions allow substantial increase in size. Establishment is a particularly notable aspect of monocotyledonous seedling development. Stems and roots of these plants mostly lack the ability to grow in girth, all roots are relatively thin and borne on the stem (adventitious roots 98), and an increase in stem surface at ground level thus precedes extra root production (Holttum 1954). A number of modes of establishment growth in monocotyledons are noted in Tomlinson and Esler (1973) as follows: A. (169c) Each successively produced internode of a palm seedling axis is slightly wider than the previous one. The internodes themselves are very short and the result is that the seedling develops in the form of an inverted cone which is kept buried in the soil by contractile roots (106). Once the cone is established, a trunk can develop by the production of longer internodes, and the large cone surface has room for many adventitious roots. B. (169i) Many monocotyledons are rhizomatous (130) and with very few exceptions the rhizome develops sympodially (250). This is also true of the seedling establishment—buds at the base of the plumule grow out as small stout sympodial units, the distal ends of which grow erect; these in turn bear buds giving rise to slightly larger units, and so on. The seedling becomes established in terms of size, spread, and stem surface next to the ground allowing adventitious root development. . (169h) A variety of the sympodial sequence of B involving change of growth direction with respect to gravity leads to even greater rooting surface and firm planting of the established seedling. . (169f) This change of orientation may be confined to the seedling plumule alone, with no lateral branches but with a pronounced increase in stem width from internode to internode as in A. . In some Cordyline species (Agavaceae) the plumule grows in the manner of A above but not necessarily with such compact internodes, and can increase in girth due to secondary thickening (16). In addition, one bud near the base of the seedling develops into a rhizome that grows vertically downwards, anchoring the plant on production of adventitious roots (169k). A sequence of such orientation changes is exhibited by Costus spectabilis (169d). F. (169g) Seedling growth may be rapid and orthotropic (246), stability being maintained by a climbing or scrambling habit accompanied by the production of prop roots (102). These forms of establishment are not necessarily confined to the seedlings; dormant buds developing much later (reiteration 298) may produce shoot sequences similar to that of the original seedling axis. The seedling axis of a dicotyledonous plant is able to increase in size indefinitely owing to cambial activity (16, 169e) in most instances and the process of establishment does not usually take the same form as that of monocotyledons, the root system being largely derived from branching of the radicle rather than by adventitious root production. In dicotyledondéus plants with rhizomes or stolons (169a), the role of the radicle is not so pronounced, lateral spread of these stems giving added shoot to ground contact and potential for adventitious root production as in monocotyledons. Radial production of branches from a seedling establishes a pattern that is not necessarily maintained by later branching sequences (306, 312). Nevertheless, seedling dicotyledons can possess contractile roots and contractile hypocotyls, and elaborate establishment mechanisms exist such as that of Oxalis hirta (169j). Precocious development of buds in the axil of a cotyledon, or buds on the hypocotyl (167e), also act as establishment mechanisms and bending of axes can play a part as in Salix repens (267b). An extreme form of Seedling morphology: establishment growth | 169 precocious development is found in plants in ' which the seedling germinates while still te) ’ i Y mA Si x ’ ras (b) contained in the fruit, and whilst the fruit is still attached to the parent plant. Such precocious deveiopment is termed vivipary, an example of which occurs in the mangrove Rhizophora (166). An account of the range of establishment types of forest trees in relation to their mode of (d) germination is given by Miquel (1987), and of plants with underground storage organs (170) by Pate and Dixon (1982). (f) (h) Fig. 169. Examples of establishment growth. a) production of stolons from parent plant; b) production of successively A larger, but short-lived sympodial units (compare h); c) (j) increase in width of successive internodes; d) increase in size of successive sympodial units with alternating growth direction; e) increase in width due to cambial activity; f) as ‘ce’ with initial downward growth; g) initial vertical growth supported by prop roots; h) increase in size and depth of successive long-lived sympodial units; i) increase in size of successive sympodial units; j) Oxa/is hirta, contraction of radicle with elongation of foliage leaf petiole resulting in descent of its subtended bud (Bu) (Davey 1946); k) production of single downward growing side shoot. 170 | Vegetative multiplication: rhizome, corm, tuber, bulb, stolon, runner Vegetative multiplication (also referred to as vegetative reproduction if contrasted with sexual reproduction) is a process involving the death of tissues located such that part of an existing plant becomes detached and independently rooted. Precise abscission zones may form as in the detachment of bulbils (172) or a plant may fragment due to decay between living components as in the case of root buds (178). Each of the morphological structures known as rhizome (130), stolon (132), runner (134), corm (136), bulb (84), and tuber (stem 138, root 110) undergo vegetative multiplication by death and decay of old tissue. Stolons and runners consist of relatively long and thin sections of stem having long internodes, alternating with sections with very short internodes, and producing adventitious roots (98). Death of the stolon or runner separates these rooted and now independent daughter plants (each of which is termed a ramet, collectively a genet or clone) (171a, b). A rhizome is typically a stouter stem than a stolon and usually fragments only if it is branched, the old proximal portion decaying and separating the ramet into two new ramets each time the rotting reaches a branch junction (17 1c, d). Definitions of ‘rhizome’ usually emphasize horizontal growth below ground level. A number of epiphytic plants have stems with a rhizome morphology growing more or less vertically on tree trunks (294a). Some species of woody monocotyledon (e.g. Cordyline) produce ‘aerial’ rhizomes developing in a downward direction similar to the seedling establishment rhizome of the same plant (169k), and able to root as independent plants in some circumstances. A corm consists of a squat swollen stem orientated vertically in the soil and bearing daughter corms, sometimes called cormels, at its distal or proximal end. The daughter corms represent buds developing in the axils of leaves on the parent corm. Eventual death of the parent corm separates the daughters (171f, h). The same procedure of vegetative multiplication occurs in bulbs (171m). Again the stem axis is vertical but food is stored in leaf bases or scale leaves rather than in the swollen stem as in the case of a corm. Buds in the leaf axils may develop into daughter bulbs which will be located in a radial manner around the parent, the latter eventually rotting away. A tuber may be formed from a swollen stem bearing buds in leaf axils or a swollen root bearing adventitious buds not associated with leaves. In each case the tuber often has a narrow, possibly elongated, connection to the parent plant and breakage or rotting of this connection results in vegetative multiplication (171k). Long thin underground stems connecting stem tubers to the parent plant are sometimes referred to as ‘stolons’. Root tubers are connected to the parent by thin roots. Some stem tubers are sympodially branched to a limited extent and disintegrate into a corresponding number of parts. They may be regarded as rhizomes or tubers depending upon the definition employed. Intermediate types will always be found in any attempt to categorize sharply morphological structures (e.g. 296). Aerial shoots of some herbs and shrubs bend under their own weight, touching the soil. Adventitious root production (98) then results in natural layering, the rooted portion of the stem becoming an independent plant if connections with the parent decay. An extensive account of underground storage organs is given in Pate and Dixon (1982). Fig. 170. Cylindropuntia leptocaulis The large round fruits bear detachable propagules with tenacious barbed spines. = oo Vegetative multiplication: rhizome, corm, tuber, bulb, stolon, runner | 171 (b) Fig. 171. Examples of natural cloning. a, b) separation of individual ramets in stoloniferous plant by death of intervening connections; c, d) disintegration of rhizomatous plant, plan view, by death of proximal components; e-h) production of daughter corms on parent corm which ------ subsequently rots; i-k) persistence of stem tuber after death of remaining plant; |, m) production of daughter bulbs in parent bulb which subsequently rots. (c) (d) (e) 1 Sr (g) (h) qm Se —— rr ies / } as ‘ ' ! — MS £m verre Le 7 eS eS atone aft I ne (k) (1) (m) i Pa GF PORES At giro. Ay? JO ERS 4, / bap ao \ \ Lh RTL Key 7 LP SE Se ae 4 i is / /\ . ' \ Ay | i Nin } a at I sifirg\ ey % 6 ant aL A aN (Aaa hati, oa ? | ql ‘! TE NS { Me v/s \ ’ 172 | Vegetative multiplication: bulbil, detachable bud with roots Fig. 172. Lilium cv. minos A detachable bulbil with adventitious roots has formed in place of a flower. A bulbil is merely a small bulb (84), that is a short stem axis bearing fleshy scale leaves or leaf bases and readily producing adventitious roots. However, the term is also sometimes inaccurately applied to any small organs of vegetative multiplication such as axillary stem tubers found on the aerial stems of some climbers (139a). Also there are alternative terms, bulblet, bulbet, bulbel, which are variously given precise definitions or used indiscriminately as synonyms. Small bulbs are mostly found in one of two locations, either on an aerial stem, representing axillary buds and especially replacing flowers in an inflorescence (176), or developing in the axils of the leaves of a fully sized bulb. The former are the type most consistently termed bulbils. Small bulbs developing within an existing bulb are of two types: one or more larger bulbs which will replace the parent bulb (renewal bulb in the terminology Mann 1960) and a number of smaller bulbs in scale leaf axils which will be liberated on the death of the parent bulb (increase bulbs). Often more than one increase bulb is produced in the axil of each leaf (84, 236) and sometimes they are adnate (234) to the underside (abaxial surface) of the next youngest leaf, or adnate to the adaxial surface of the leaf some distance out from the node. Increase bulbs can be formed on the end of thin stems (dropper 174) and are then dispersed away from the parent bulb. More elaborate mechanisms occur in some plants, e.g. Oxalis cernua (169j). Very many water plants undergo vegetative multiplication surviving periods of cold, dryness, or nutrient depletion by the production of detachable buds. These are called turions (‘winter buds’) regardless of the variety of their form. In the Lemnaceae (212) they simply represent particularly small fronds. In Utricularia species the apical meristem produces scale leaves around a compact bud covered with hairs and mucilage. In other plants the turions may more closely resemble other vegetative buds on the plant but are more compact and a darker green. Either lateral buds or the apical bud of a shoot or both may form turions which may be easily detachable owing to the formation of an abscission layer of cells at their base, or may merely persist when the rest of the plant rots. The leaves of a turion contain stored food and adventitious roots are produced when favourable conditions return. This form of ‘over-wintering’ typifies the category of plant form recorded as hydrophyte by Raunkiaer (1934) (315g). Vegetative multiplication: bulbil, detachable bud with roots | 173 Fig. 173. a) Cyperus al/ternifolius, top of aerial shoot viewed from above; b-d) distal ends of inflorescence axes: b) Globba propinqua, c) Costus spiralis, d) Allium cepa var. viviparum. Ad: adventitious root. Bu: bulbil. Bu(d): developing bulbil. F: flower. la: inflorescence axis. 174 | Vegetative multiplication: dropper, underground bulb on extended axis Fig. 174. Erythronium dens-canis The distal end of a dropper. Longitudinal section on right. Two potential shoots are present. The one in the centre represents the original bud associated with the dropper. The lateral bud at the left is in the axil of a scale leaf, now detached Buds produced in the axils of leaves at the base of a bulb (84, 171m) or corm (136, 171f, h) usually develop into independent plants with adventitious roots (98) and thus will be located close to the base of the parent. In a number of species, however, the bud may be carried away horizontally or vertically from the parent on the end of a slender root-like structure (175h). These are referred to as droppers or sinkers. The sequence of development of a dropper in different plants varies in detail and can only be deduced by careful morphological dissection at all stages of growth. The elongated portion may represent one very long internode or more accurately hypopodium (262), which is the portion of axis between the first leaf (prophyll 66) of the axillary bud and the parent axis. In some cases an adventitious root primordium forms adjacent to the bud and the two grow out as one combined structure (i.e. they are adnate 175a-e). The base of the subtending leaf can also grow out to keep pace with the developing dropper and form an outer layer of the whole structure to which it may be fused (174). Enlarged structures formed in this way are sometimes referred to as stem tubers (138). Finally the root portion may be very much larger than the shoot, the swollen organ then being referred to as a form of root tuber (110). Vegetative multiplication: dropper, underground bulb on extended axis | i75 Fig. 175. a-g) Development of a dropper in Herminium monorchis; h) Ixia conica, bulb with emerging droppers. Ad: adventitious root. B: bud. D: dropper. Ls: Leaf sheath. T: tuber. (a—e, redrawn from Troll 1943; f-h, redrawn from Raunkiaer 1934). 176 | Vegetative multiplication: prolification, false vivipary Fig. 176. Deschampsia alpina Small tillers (182) have developed in place of spikelets in the inflorescence. This is false vivipary; true vivipary being the condition in which seeds germinate before being shed from the parent plant (168) An apical meristem in an inflorescence will develop to give rise to either a flower, or another unit of the branching structure (142). Each such meristem is in the axil of a modified leaf, a bract (62), although these may be absent. Bracts may be present that do not subtend an active axillary meristem. i.e. the bract is ‘sterile’ (e.g. grasses 186). In some plants, meristems that would normally develop into flowers develop instead into vegetative buds usually associated with adventitious roots (177a). These vegetatively produced plants will grow independently if shed or deposited on the ground by the collapse of the inflorescence axis. The leaf bases of the buds may be swollen, the structure resembling a bulb and such a structure is then called a bulbil (172). The production of vegetative buds instead of flowers, or as happens in some grasses, the production of tillers in normally sterile spikelets (177b, c, d), is termed prolification or false vivipary (proliferation is also used in this context but cf. 238). True vivipary occurs when a seed germinates without being shed from the parent plant (168). An inflorescence axis will revert to vegetative extension in some plants (261b), or axillary shoots may develop that are vegetative rather than reproductive (199a and 253a). Vegetative multiplication: prolification, false vivipary | 177 Fig. 177. a) Chlorophytum comosum, trailing inflorescence; b) Festuca ovina var. vivipara, inflorescences with tillers (182) instead of spikelets; c) Dacty/lis glomerata, ditto, cf. 185g; d) Poa x jemtlandica, a single spikelet containing both flowers and vegetative bud. Ad: adventitious root. B: vegetative bud. F: flower. 178 | Vegetative multiplication: root buds Buds capable of developing into shoot systems occur on the roots of a number of plant species, both monocotyledons and dicotyledons. In many plants such buds, which are termed root buds, only form if the root is damaged, buds then differentiating in the callus tissue that forms. In a few plants a root bud can form so close to the root apex that without anatomical study of its development it appears as if the root has turned into a shoot. This is particularly so if the root apex itself becomes parenchymatized (244). Root bud primordia arise endogenously (94, 178), that is within the root tissue as do lateral root primordia, and not exogenously at the surface as is typical of buds arising on stems. The precise location within the root is variable. It may be in exactly the same position as would normally be occupied by a lateral root primordium, typically in the pericycle, and then root buds will appear in a number of rows depending upon the detailed root anatomy (97k). Frequently a root bud primordium develops in very close proximity to a lateral root before the latter emerges from the main root. Alternatively the root buds differentiate in the cortex and are not associated in any way with lateral root positions. In some trees, root buds develop within the living part of the root bark and may remain dormant for extended periods. Nevertheless extensive clones of trees develop from such root buds and a stand of trees (e.g. Populus spp., Liquidambar sp.) may be actually connected via the root system as a consequence. Root grafting between initially separate individuals can produce the same effect. In herbaceous plants, however, the root connections between developing root buds are likely to decay and individual plants will lose contact with each other. Fig. 178. Rubus idaeus Shoot apical meristems emerging from within root tissue (endogenous development). SSC ay Q Vs Fig. 179. Rubus idaeus. Various stages of shoot oY ‘ development from root buds. 180 | Grass morphology: vegetative growth A young grass seedling consists of a very short stem axis virtually lacking internodes such that the nodes bearing leaves are very close together. Each leaf is attached at its node around all or most of the stem circumference and thus forms a tube, the leaf sheath, which may be open on one side, and which surrounds the next youngest leaf. The first leaf of the seedling is represented by a reduced absorptive organ contained within the grass fruit termed the scutellum. The second leaf emerges above ground on germination and forms a simple green tube, the coleoptile with a hole in its apex (163b cf. 164). Subsequent leaves each develop to a greater or lesser extent in two stages. The part of the leaf which is formed first and therefore becomes the distal end is usually flat and termed the lamina. It increases in length due to cell division at its proximal (4) end (an intercalary meristem 18). The lower proximal end of the leaf forms the leaf sheath, is the more or less tubular part, and is distally flattened in some species; it also has an intercalary meristem at the lowest proximal point. The lamina can bend backwards relative to the sheath at a sometimes distinctive zone of cells, the lamina joint. Also found at the junction of lamina (or blade) and sheath is a flange of tissue, the ligule (181h), which sometimes may be replaced by a fringe of hairs (181b, c) or is absent. Outgrowths at the side of the lamina joint region are termed auricles (181h). Some grass leaves (and especially bamboos 192) have rather wide short laminas with a distinct petiole connecting sheath to lamina. This petiole is not homologous with the petiole of dicotyledon leaves (20). Leaves on a grass stem are located in two rows on opposite sides (distichous phyllotaxis 219c) and the young leaves will be folded together in an imbricate and usually equitant manner (vernation 39g) depending upon the degree of folding along the mid line. As a young grass plant develops, longer internodes will be formed either during the Fig. 180. Dactylis glomerata var. hispanica A prostrate form of a tussock forming grass. A tiller (side shoot) is developing from the axil of every leaf on the main axis. production of a vertical inflorescence axis or between successive leaves of a rhizome (131f) or stolon (133d). The nodes on the vertical axis, the culm, often appear swollen and form adjustable joints. It is actually the very proximal end of the leaf sheath which forms the swollen portion, i.e. a leaf pulvinus (46). Swollen stem tubers (181e, f) are formed in some grasses. The leaf sheath attached at a node will often be tightly folded or rolled around the next internode and also encircled by sheaths attached at lower nodes. To see which leaf belongs to which node it is thus necessary to pull these structures away from each other. This will also allow the identification of the buds in the leaf sheath axils, and the point of insertion of side shoots (tillers 182) to be determined. All the roots on a grass plant (except the very first, primary, seminal root which is protected by a dome of tissue termed the coleorhiza) are adventitious (98), that is they are formed from root primordia developing in the stem usually at leaf nodes. Adventitious root primordia present in the embryo before germination, usually located at the nodes of the coleoptile and first foliage leaf, are termed lateral seminal roots. Subsequent roots are referred to as nodal. The majority of grasses branch repeatedly, lateral daughter branches (tillers) usually having the same general morphology as their parent (182). Fig. 181. a) Stenotaphrum secundatum, vegetative shoot with alternating long and short internodes; b) Phragmites communis, leaf blade/sheath junction; c) Cortaderia argentea, leaf blade/sheath junction; d) Stenotaphrum secundatum, rhizome (130); e) Panicum bu/bosum, base of swollen tiller; f) Arrhenatherum elatius var. bu/bosum, series of swollen internodes (cf. pseudobulb 198); g) Arundo donax, single leaf; h) Lo/ium perenne, \eaf blade/sheath junction; i) Poa annua, leaf blade/sheath junction. A: auricle. Il: long internode. Is: short internode. Isw: swollen internode. Lb: leaf blade. Lh: hairy ligule. Lm: membranous ligule. Ls: leaf sheath. Rh: rhizome. 182 | Grass morphology: tillering It is customary to refer to the first shoot of a grass plant, i.e. the axis developing from the epicotyl (162) as the parent shoot. All subsequent shoots must develop from axillary buds and are termed tillers. The first leaf (prophyll 66) on a tiller is usually much smaller than later formed leaves and may not be constructed of distinct lamina and blade. It is in an adaxial position (4). A tiller may develop in such a manner that it closely resembles the parent shoot, growing vertically and bearing a terminal inflorescence. Its vertical growth will cause it to extend up trapped between the parent shoot axis and the sheath of its subtending leaf (183a). This type of development is termed intravaginal. Each leaf at the base of the parent shoot, including the coleoptile, can subtend such a tiller. Subsequently, buds in the axils of leaves at the base of each tiller may themselves develop into tillers. A compact grass plant will thus be formed. Adventitious roots will develop from lower nodes of both parent shoot and tillers. Each tiller will thus have its own set of leaves, roots, and daughter tillers. Compact intravaginal growth gives a ‘caespitose’ (‘clumping’, ‘bunch’, ‘tussock’, ‘tufted’) habit. Conversely a tiller may grow out sideways away from the parent shoot and therefore be more or less at right angles to it. This results in the tiller breaking through the base of its subtending leaf sheath and is referred to as extravaginal tillering (183b). The tiller so formed will usually be a procumbent shoot lying on the ground or on surrounding vegetation (cf. 182), or grow strongly and horizontally away from the parent above ground (a stolon 133d) or beneath ground (a rhizome 131f, as is typical for the bamboos 195). The leaves on such horizontal axes are often cataphylls (scale leaves 64) especially if the tiller is underground. Eventually the tip of the extravaginal tiller will turn to grow erect, horizontal growth being continued by a daughter tiller (i.e. sympodial growth 250). Nodal roots are likely to form on horizontal tillers. Leaf arrangement in a grass plant is distichous (219c). A plan diagram of a tiller system should therefore appear as in Fig. 183d, with all leaves formed in one plane and a fan-shaped tussock developed. This does not always occur, however, as each tiller bud is displaced (230) somewhat round the node at which it is attached, i.e. it is not in line with the mid-point of its subtending leaf (183e). This has a corresponding effect on the shape of the grass tussock. Developmental studies of tillering in grasses (including cereals) are often aided by some system of ordering (284) for the tillering sequences. The leaves on the parent shoot are recorded as C (for ‘coleoptile’), L1, L2, etc. The tiller in the axil of the coleoptile is designated TC, that in the axil of the first foliage leaf T1, and so on. If T1 itself bears tillers, the first will be in the axil of the prophyll of tiller 1 and can be identified as T1.PT. The next tiller will be in the axil of the first foliage leaf of tiller 1, and is referred to as T1.L1T. Figure 183c illustrates how such a labelling system can be built up should it be necessary to refer to any individual tiller or leaf accurately. (A similar system can of course be designed for any type of plant.) Detailed aspects of grass (specifically cereal) morphology are given in Kirby (1986). Fig. 182. Arundo donax A stout grass which is unusual in that vegetative shoots develop from the aerial stem. Each tiller is breaking through its subtending leaf sheath, a situation normally found in rhizomatous or stoloniferous grasses (cf. Fig. 183b). This specimen is variegated. _——— Grass morphology: tillering | 183 (a) ; (b) 65 WW 2 hy i ® ) p19 96 9» QY D §69 (c) B® ; oi oh eee. Nias Fig. 183. a) Intravaginal tillering; b) extravaginal tillering; c) labelling system; d) plan diagram hypothetical distichous A? arrangement of leaves with buds in line with mid-veins; e) A » Y typical plan diagram showing displacement of buds around T Oo : axis away from mid-line. Sl: subtending leaf. T: tiller. T4P} OY Q® T4 S| H I T3P L BS EA L4 H LI | a T3L1 = = y, Sl T2L2 wi y. T2PT T T2L1T | 13 £-L3 T2L1 T2P T L2 TIP T1 184 | Grass morphology: inflorescence structure Fig. 184. Aegilops ovata The rounded outer glume of each spikelet bears long terminal awns. Inflorescences are categorized on the basis of the arrangement of flowers and the pattern of branching (140). Flowers in grasses and bamboos, however, are aggregated into groups enclosed between a pair of scale leaves (glumes). Each such package is termed a spikelet (186), and it is the arrangement of spikelets that is used to describe the grass inflorescence rather than the arrangement of individual flowers. Similarly if a spikelet is borne on a stalk, the stalk is referred to as a pedicel although this term is normally applied to the stalk of an individual flower. The stem with elongated internodes supporting an inflorescence is termed a culm. The culm also bears vegetative branches in bamboos and other stout grasses (182). The main axis of the inflorescence is termed the r(h)achis. This may be variously branched and the branches relatively long or short or both, the general form being a raceme (panicle if repeatedly branched 141g, 185d, e, g, h, i). The nodes at which individual branches occur can be grouped very close together at intervals along the rachis, branches appearing to be attached at one point. The spikelets themselves may be carried on long or short pedicels, or be sessile forming a spike (185a, b, c). If a number of spikes are all apparently attached at one point, a digitate arrangement results. The inflorescence of barley (Hordeum spp.) consists of an axis bearing two rows (one on each side) of spikelets grouped in threes on extremely short pedicels (189j). In wheat (Triticum spp.) two rows of solitary sessile spikelets are present (188c). In such a spike, the sessile spikelet may be sunken into the rachis or the two rows may appear side by side rather than front to back due to displacement during development.-Each spikelet forms from one bud but its axillant leaf (bract 62) is usually missing although some evidence of a ridge of tissue may be visible. The ‘collar’ at the base of a wheat inflorescence represents such a subtending bract. The extreme distal end of the rachis or side branch may terminate in a fully formed spikelet, or a partly complete sterile spikelet, or may terminate blindly in a non-meristematic point. When the fruits are mature a grass inflorescence usually falls apart. Points of articulation (breakage) vary amongst species. Single spikelets with or without their glumes (186) may be shed, or groups of spikelets or whole spikes or whole inflorescences may fall. Some spikes break into segments each bearing just one spikelet. (a) 10 mm Fig. 185. Example of variation in grass inflorescence structure. a—c) spikes, d—i) panicles: a) Nardus stricta, b) Lolium perenne, c) Agropyron (Elymus) repens, d) Agrostis tenuis, e) Holcus lanatus, f) Briza maxima, g) Dactylis glomerata, h) Arrhenatherum elatius, i) Oryza sativa. C: culm. G: glume. P: pedicel. R: rhachis. S: spikelet. 186 | Grass morphology: spikelet and floret structure An individual grass flower contains a superior ovary (146) with three (two or one) styles, three (two or one) stamens (six in bamboos and a few other grasses), and two (occasionally three or none) small structures, lodicules, representing perianth segments (146). Unisexual flowers also occur in some species. Flowers are borne in groups in numbers characteristic for a species (from one to many), along a short axis, the rachilla. Each flower is in the axil of a bract, termed the lemma, and the stalk of each flower bears a bracteole, the palea (187j). The lemma usually envelops the palea, the two protecting the flower inside which is only visible when the lodicules swell up forcing the lemma and palea apart and exposing the anthers and the style (186). Grasses are almost all wind pollinated (cf. 192b). The whole structure, lemma, palea, and flower, is referred to as a floret (187j). The rachilla has at its base two extra bracts which are sterile, i.e. they do not subtend florets. The most proximal bract, which is the prophyll of the rachilla but which is not necessarily in an adaxial position, is called the lower glume. The second sterile bract is the upper glume. This pair of glumes will to a varying extent envelop a characteristic number of florets. The whole unit, glumes plus florets, is termed the spikelet (187k) and is a consistent feature of the grasses. Grass inflorescences are described according to the arrangement of spikelets rather than that of individual flowers (184). When studying a grass inflorescence the first step will always be the identification of spikelet units by locating pairs of glumes at their bases. An individual spikelet may contain different types of floret (Sorghum 191a) or the inflorescence may contain different types of spikelet, for example fertile and sterile (Cynosurus 187e, f). In Setaria and Pennisetum (191b) the terminal spikelets are missing and only their stalks are present as bristles (referred to incorrectly as an involucre 146). The inflorescence of Coix is represented by a limited number of branches each terminating in a hard glossy bead-like structure. The ‘bead’ represents the strengthened base of the leaf subtending a short rachis. The rachis bears one female spikelet, which remains encased by the bead, and a series of several male spikelets borne on an axis that protrudes out of the bead. Glumes, lemmas, and paleas are typically membraneous cataphylls (64) and vary in shape and complexity; one or more of these structures may be absent from a spikelet (185b). They may pes a Pree: —sC:ststtstCtittCCiCtCitiaatét#é#é#é#é44..#He_ www be rounded or keeled, that is folded along the mid line, i.e. conduplicate (37j), and may bear an awn which is an extension of the mid vein. The awn is located at the tip of the lemma (189h) or glume (189d) or the vein departs from some point on the dorsal side. The base of a dorsal awn is often much twisted and the awn itself kinked above the twisted portion. Such a geniculate awn (187d) responds to drying or wetting by rotating and levering the associated fruit amongst vegetation or soil particles. If several veins end in awns they may be spread apart (184, 187h) or be twisted together for part of their length. Hairs are often present in and amongst spikelets and can appear to be the most conspicuous part of the inflorescence. Florets are occasionally replaced by small tillers complete with adventitious roots (false vivipary 176). Fig. 186. Arrhenatherum elatius Anthers and stigmas protruding from the fertile hermaphrodite spikelets. One floret in each such spikelet has a lemma bearing an awn, several of which are visible here. (a) (e) (h) Grass morphology: spikelet and floret structure | 187 Fig. 187. a) Phalaris canariensis, single spikelet; b) Pha/aris canariensis, single floret; c) Stipa pennata, single floret (awned lemma); d) Avena sp., single floret (geniculate awn on back of lemma); e) Cynosurus cristatus, sterile spikelet; f) Cynosurus cristatus, fertile spikelet; g) Miscanthus, sp., single spikelet; h) Aegi/ops ovata, group of spikelets; i) Poa annua, single spikelet; j) diagram of single floret; k) diagram of single spikelet. Aw: awn. Awg: geniculate awn. C: cicitrix. Fl: floret. Fls: sterile floret. Gl: lower glume. Gu: upper glume. H: hair. L: lemma, Lo: lodicule. O: ovary. P: palea. Ra: rachilla. St: stamen. Sy: style. 188 | Grass morphology: cereal inflorescence Fig. 188. a) Hordeum vulgare var. distichum, part of an inflorescence spike; b) Hordeum vulgare, abaxial view cluster of three spikelets (one fertile, two sterile); c) Triticum aestivum, single spikelet; d) Triticum aestivum, inflorescence spike. Aw: awn. G: glume. L: lemma. O: ovary. P: palea. A cereal is a cultivated species of grass, its morphology invariably modified by selection. Nevertheless all the features of grass vegetative growth (180, 182), inflorescence (184), and spikelet construction (186) apply to the cereal plant. The inflorescence of wheat (Triticum spp.) is a spike (188d) in which one spikelet only (188c) is present at each node as can be detected by the presence of two glumes at each node. Between each pair of glumes is a short axis (rachilla) bearing a limited number of hermaphrodite florets. Two, three, or occasionally four of the lower florets will be fertile. Spikelets alternate on either side of the inflorescence axis (rachis) and there is usually a terminal spikelet present. Differences in appearance between species and varieties of wheat depend upon the number of fertile florets per spikelet, the compactness of the inflorescence spike, and the extent of awn development on the lemmas, and occasionally on the glumes also. The wheat grain (a caryopsis fruit 157b) falls out from between its lemma and palea when threshed. The inflorescence of rye (Secale sp.) is very similar to that of wheat but habitually each spikelet contains two fertile florets and the remains of a sterile floret (189a). An inflorescence of barley (Hordeum sp.) may resemble that of wheat. It is a spike in which spikelets occur in groups of three on a very short axis at each node (188a). This axis itself ends without a terminal spikelet, all spikelets being lateral. Nevertheless the three spikelets are arranged such that one appears to be central, flanked by the other two. The two lateral spikelets may be sterile (188b). Each spikelet contains only one floret. Thus at each node, on alternating sides of the rachis, there are to be found three pairs of glumes, often very small, particularly if associated with lateral sterile spikelets, and three sets of lemma plus palea. The lemmas invariably bear long awns or occasionally elaborate hooded distal ends (189e'). The hood of a hooded barley may incorporate an extra epiphyllous (74) spikelet. If all three spikelets at a node are fertile the inflorescence spike appears to consist of six (three each side) rows of spikelets (189f, f'). If the lateral spikelets are sterile, the inflorescence appears to be composed of two rows of spikelets only (a central one on each side) (189h, h’), although the glumes of the lateral spikelets will be found in place. An apparently four-rowed barley results if the lateral fertile spikelets of one side interdigitate with the lateral spikelets of the other (189g, g'). Other commonly cultivated cereals are described in the following section. (Continued on page 190.) Fig. 189. a) Secale cereale, part of inflorescence spike; a’) diagram of spikelet layout; b) 7riticum durum, inflorescence spike, b') spikelet layout; c) Triticum sp. (Nepalese); d) Aegilops speltoides, inflorescence spike; e) Hordeum sp. (hooded), e') Hordeum sp. (hooded) single floret; f) Hordeum vulgare var. hexastichum, f') spikelet layout; g) Hordeum vulgare var. tetrastichum, g') spikelet layout; h) Hordeum vulgare var. distichum, h') spikelet layout; i) as f), cluster of three spikelets; j) as g), cluster of three spikelets; k) as h), cluster of three spikelets. F: floret. G: glume. L: lemma. Lh: hooded lemma. P: palea. R: end of axis bearing spikelets. Sf: sterile spikelet. (a) 7 Sa hk oe iF - Za = a S 5 ta: ——<<— SS 190 | Grass morphology: cereal inflorescence continued (a) 10 mm Fig. 190. a) Avena sp., single spikelet (lemmas awnless); b) Avena Sativa, single spikelet. Awg: geniculate awn. Gl: lower glume. Gu: upper glume. L: lemma. P: palea. R: rachilla Avena sativa (Oat) The inflorescence of oat forms a loose panicle (141g, 185h) with crowded nodes such that pseudowhorls of branches may be present. Each ultimate branch ends in a conspicuous spikelet (186) which demonstrates spikelet construction very clearly. The glumes are especially large and protect a rachilla bearing up to 7 florets of which the lower 1, 2, or 3 may be fertile. Paleas are relatively inconspicuous, but the lemma may bear an awn, often geniculate (190b). The base of the lemma is variously swollen (the callus) and bears hairs. The floret often disarticulates at this point possibly leaving a prominent scar, the cicatrix (187d). Oryza sativa (Rice) The inflorescence is a panicle partly enclosed by the most distal leaf of the culm (185i). Each branch of the panicle terminates with a spikelet containing one fertile (rarely more) floret. Six stamens are present (most grasses have 1, 2, or 3; bamboos also have 6). The glumes are small (191c), the lemmas variably awned. The spikelet stalks (pedicels 184) break below the spikelet. Zea mays (Maize, Corn) Maize has two distinct inflorescence forms: totally female inflorescences (‘ear’, ‘cob’) borne in the axils of leaves on the culm, and a terminal male inflorescence (‘tassel’). The latter is a panicle with pairs of similar spikelets, one sessile and one on a short pedicel. Each spikelet consists of a pair of glumes surrounding two male florets. The female inflorescence is a spike contained within a number of large scale leaves (the husk) borne on the proximal end of its rachis. Female spikelets occur in pairs and each spikelet contains one sterile and one fertile floret. Glumes, lemmas, and paleas are all shorter than the large ovary. Styles (silk) are very long and emerge from the distal ends of the ensheathing husk. Sorghum bicolor (Sorghum, Great Millet) (‘Millet’ is a general term covering many distinct cereal grain species with numerous common names) The inflorescence is a panicle with spikelets borne in pairs, one hermaphrodite and sessile, one male or sterile and borne on a short pedicel (191a). The hermaphrodite spikelet is considerably larger than the male spikelet and contains two florets. The lower floret is sterile and lacks a palea, the upper one has a lemma but again the palea can be absent. The stalked male spikelet likewise contains a lower sterile floret represented by a lemma only, and the upper male floret also without palea. In some forms this spikelet consists of a pair of glumes only. Panicum miliaceum (Common Millet, Proso Millet) The inflorescence is a panicle, with solitary spikelets. The upper glume of each spikelet is longer than the lower and envelops a lower sterile floret and an upper fertile floret (191f). Pennisetum typhoides (Bulrush Millet, Pearl Millet) The inflorescence is a dense panicle or loose branched spike. The spikelets occur in pairs aggregated into dense groups. Proximal to each Grass morphology: cereal inflorescence continued | 191 pair are borne numerous pedicels lacking terminal spikelets and forming a mass of bristles (termed an ‘involucre’, see comment for Setaria 186) varying greatly in length in different varieties (191b). Each spikelet contains one male floret and one hermaphrodite floret. Eleusine coracana (Finger Millet, African Millet) The inflorescence consists of a radiating cluster of spikes located on the top of the culm. Each spike bears two rows of spikelets on the outer side of its rachis. The rows overlap each other to some extent. Each spikelet, identified by its basal pair of glumes, contains up to a dozen florets which are borne left and right on the spikelet rachilla (191d). The florets are hermaphrodite and lemmas and paleas conspicuous. Fig. 191. a) Sorghum bicolor, spikelet pair; b) Pennisetum typhoides, spikelet pair; c) Oryza sativa, single spikelet; d) Eleusine coracana, single spikelet; e) Pennisetum typhoides, single spikelet anthers removed; f) Panicum miliaceum, 1mm ‘4 single spikelet. Aw: awn. F: floret. Gl: lower glume. Gu: 1mm Gu upper glume. |: involucre. L: lemma. Lf: lemma of fertile Gl floret. Ls: lemma of sterile floret. O: ovary. P: palea. R: rachilla. Ss: sterile spikelet. St: stamen. Sy: style. 192 | Grass morphology: bamboo aerial shoot rat Ne oY, | WE, arundinacea > The branching inflorescence of a dying plant (cf. Fig. 194) (the leaves in the foreground are of a palm) Fig. 192a. Bambusa li i Fig. 192b. Piresia sp. An entire plant: one of the smallest bamboos. Leaf litter has been removed to expose a plagiotropic (246) underground inflorescence which bears scale leaves and, in this specimen, two distal spikelets. Pollination is probably performed by ants Bamboos are grasses (family Gramineae, tribe Bambuseae) and can usually be recognized by a combination of woodiness and persistence of both culm (180) and rhizome (194), by the vegetative branching of the culm, by a short petiole between sheath and blade of vegetative leaves (193b), and by spikelets containing more component parts than other grasses (i.e. possibly >2 glumes, or sterile lemmas, > 2 lodicules, > 3 stamens, > 2 styles 193f, g). The bamboo culm consists of a series of more or less elongated internodes separating distichously (219c) arranged scale leaves at the nodes. The scale leaf represents the sheath only of a complete leaf and may have a small portion of lamina at its distal end (193d) together with a ligule and auricles depending upon the species and its position in the heteroblastic sequence (28). Each scale leaf subtends a bud (193e). Such vegetative buds usually bear their own buds and develop into an elaborate condensed branching system (193c, 239c). Some species also possess true accessory buds (236). Branch complexes are persistent and continue to branch on a seasonal basis. Scale leaves on the culm and prophylls within the lateral branching complexes fall easily but are then represented by prominent scars (193c). Dormant buds are frequently sunken into a more or less prominent groove. The lateral branches may take the form of stem spines (124), or slender vegetative branches bearing foliage leaves or inflorescences in flowering individuals (192a, 193a). Bamboo infloresences, borne laterally on the culm branches, are mostly panicles (141g) but many incorporate sub-units Grass morphology: bamboo aerial shoot | 193 of sessile spikelets. Lower spikelets in a group may be replaced by a reserve bud (pseudospikelet; McClure 1966) allowing additional inflorescence branches to occur (indeterminate inflorescence; McClure 1966); in the absence of these extra buds the inflorescence will be determinate (McClure 1966). Bamboos are wind pollinated; an exception is Piresia (192b). Fig. 193. a) Arundinaria sp., flowering tiller; b) Sasa palmata, single foliage leaf; c) Sinarundinaria sp. | (f) (g) condensed branching on aerial shoot (see 239c); d) Sasa palmata, single scale leaf; e) Bambusa arundinacea, section | SO) Lo ( oP of aerial shoot; f) floral diagram bamboo spikelet; g) floral ‘£ a L \ diagram grass spikelet. Axb: axillary bud. L: lemma. LI: leaf . lamina. Lo: lodicule. Lp: leaf petiole. Ls: leaf sheath. P: CS 2 St v f palea. RI: rudimentary lamina. S: stem. Sl: scale leaf. Sls: & P scale leaf scar. Sp: spikelet. St: stamen. 10 mm 10 mm Se 194 | Grass morphology: bamboo rhizome Fig. 194a, b. Bambusa arundinacea a) Excavated rhizome system from dying clump. One aerial culm is present at the top of the picture. Model of McClure (295c) SSN Ps Se b) Close-up of upper surface of rhizome segment showing rows of dead adventitious roots (98) alternating with scale leaf scars in which vein endings can be seen. Bamboos develop extensive and persistent woody underground branching rhizome systems. The rhizome branch bears scale leaves (64) only, and adventitious roots (98) are produced extensively at the nodes (194b). Two basic rhizome types are recognized by McClure (1966). (i) Pachymorph (cf. pachycaul, 130)—short and fat rhizome branches, usually solid and terminating distally in a vertical culm (194a). Buds on these rhizomes always give rise to other rhizome branches (195a). (ii) Leptomorph—long and thin, usually hollow and extending more or less indefinitely underground, i.e. rarely turning erect to form a terminal culm. Buds on these rhizome branches usually become aerial culms or occasionally additional underground leptomorph rhizome branches (195d). The proximal end of any new rhizome branch or lateral culm is always relatively thin and referred to as the rhizome, or culm, neck (195b). The neck is often orientated somewhat downwards (195a) especially in seedlings (see establishment growth 168) and usually has no buds in the axils of its scale leaves and no adventitious roots. The neck of a pachymorph rhizome may be short (195a) (or long 195b); that of a leptomorph rhizome is always short (195d). The junction of culm neck and culm may be extended by a series of short internodes termed by McClure (1966) a metamorph axis type 1 (195c). These only occur on laterally borne culms. The distal ends of pachymorph rhizomes and of leptomorph rhizomes that terminate in culms, may be extended by a series of long internodes termed by McClure (1966) metamorph axis type 2 (195f). Grass morphology: bamboo rhizome | 195 Combinations of these distinctive features are to be found in different bamboo species (195e, g). The non-woody and non-persistent underground parts of other members of the grass family often show similar morphologies to those found in the bamboos (181d), and both form comparable branching patterns to the rhizome systems of gingers (311). Fig. 195. Bamboo rhizome types. Redrawn from McClure (1966). a) pachymorph; b) pachymorph, long neck; c) metamorph axis type 1; d) leptomorph; e) pachymorph, long neck and metamorph type 1; f) metamorph axis type 2; g) leptomorph and pachymorph, short neck. Axb: axillary bud. Ln: long neck. M: metamorph type 1. Mm: metamorph type 2. Sls: scale leaf scar. Sn: short neck. 196 | Sedge morphology Members of the sedge family (Cyperaceae) show a considerable range of vegetative and reproductive morphology and many superficially resemble grasses (180). Vegetative leaves have a sheath (forming an entire cylinder around the stem) and a narrow blade with a ligule at the junction of the two. The stem is usually solid and the leaves are borne on it in three rows (tristichous 219e, grasses are distichous 219c). The aerial shoots of sedges invariably represent the distal ends of the underground sympodial rhizome segments (269d). These rhizomes are of many types, pachymorph and leptomorph (see bamboo terminology 194) or may incorporate stem tubers (138) at the base of the aerial shoot. The tristichous leaf arrangement frequently governs the directional spread of successive rhizome branches in the sympodial sequence (197c, c'). Also, successive sympodial units are frequently adnate for part of their length giving a misleading monopodial appearance particularly in leptomorph species (235a). The inflorescence is of variable construction again showing much the same overall range of types as those of the grasses (184). Individual flowers, hermaphrodite, male, or female, are found in characteristic positions within the ultimate units of the inflorescence (termed spikelets as in grasses 186) and lack perianth segments or else these are represented by bristles or scales. Flowers themselves are subtended by scale leaves (‘glumes’). Interpretation of the detailed arrangement of parts is usually facilitated by the presence of prophylls in the usual adaxial position (66), but arrangements are not as consistent as in that of the grass spikelet (186). In the genus Carex and others the prophyll subtending the female flower is a large flask- shaped structure, the utricle which surrounds the flower. Such a female flower is lateral on a rachilla which may be a conspicuous feature within the utricle. The units of the inflorescence within the Cyperaceae are discussed at length by Eiten (1976); a grossly simplified selection of the range to be found is shown in Fig. 197d-i. Fig. 196. Bulbostylis vestita The vertical stem is protected from natural fire in its savanna habitat by the mass of persistent leaf sheaths. Model of Corner (291d). : (a) (a’) (d) (f) (g) (i) Gh Sedge morphology | 197 Fig. 197. a) Cyperus a/ternifolius, single spikelet; a‘) ditto, with glumes removed; b) Cyperus a/ternifolius, inflorescence; c, c') Eriophorum, alternative rhizome bud locations at base of aerial shoot; d—-i) redrawn from Eiten (1976), selection of sedge floral types. F: female flower. Gf: fertile glume. Gh: hooked glume. Gs: sterile glume. H: hermaphrodite flower. M: male flower. 198 | Orchid morphology: vegetative organization Orchid species (Orchidaceae) have distinctive and usually elaborate flowers (200). Vegetatively they show a range of forms outlined here as an example of constructional variation in one distinctive taxonomic group (see also 253). The majority of orchids have either a sympodial or less frequently monopodial rhizome although as the plant may be epiphytic this stem system will not be below ground (170). Monopodial orchids have lateral inflorescences (253b, 199b), sympodial orchids have lateral (253a, d, 199d) or terminal (253c, 199e) inflorescences. A distinctive feature of many orchids is the pseudobulb (199d-f). This represents a swollen segment of stem of one or more internodes and is thus morphologically equivalent to a corm (137d). The location of a pseudobulb within the rhizome system of a particular orchid is usually very precise as may be the number of leaves it bears, and shows a range of permutations (199, 253). Orchid leaves are variable in shape in different species and scale leaves are often present. In addition to storage pseudobulbs, orchids may possess variously swollen roots (root tuber 100). In some instances these incorporate stem tissue with a shoot apex and are equivalent to the droppers (175f, g) of other plants. A second feature of many orchid roots is an extensive water absorbing covering, the velamen (106). Such roots may also be photosynthetic; an extreme case, Campylocentrum, is shown in Fig. 198. Fig. 198. Campylocentrum pachyrhizum The shoot system of this orchid is at the centre of the picture together with dead inflorescence axes. The bulk of the plant consists of green flattened adventitious roots (98). Orchid morphology: vegetative organization | 199 Fig. 199. Examples of growth form variations of Orchids. a-c) without pseudobulbs, d—f) with pseudobulbs. See also 253. a) Restrepia ciliata, b) Acampe sp., c) Epidendrum sp., d) Bulbophyllum sp., e) Pholidota sp., f) Coelogyne fimbriata. Ai: axillary inflorescence. P: pseudobulb. Ti: terminal inflorescence. Vs: vegetative shoot. 6 200 | Orchid morphology: aerial shoot and inflorescence The orchid flower exhibits a number of features that collectively define the family, although they are also found in part in other groups. Dressler (1981) lists seven characteristics: (1) Stamens on one side of flower (usually one active); (2) Stamens adnate to pistil (=column) (201d’) (3) Petal opposite stamen elaborate (cf. 201b) (=labellum or lip); (4) Part of stigma represents pollination apparatus (=rostellum) (201d'). (5) Pollen massed into pollinia (201a'); (6) Flower stalk (pedicel) often twists (resupination) (201e); (7) Extremely small seeds (159h). Fig. 200. Paphiopedilum venustum An orchid flower is composed of six perianth segments in two whorls of three. The adaxial petal of the inner whorl is the elaborated labellum. Twisting of the pedicel (resupination) turns the flower through 180° in most cases bringing the labellum into an apparently abaxial position. One (sometimes two) stamen unites with the style to form a column. The upper part of the column bears the elaborated anther (clinandrium) and elaborated stigma (rostellum). Flowers may be solitary, or may be aggregated into inflorescences. Arrangement is usually racemose (141b), occasionally cymose (1410), and rarely leaf opposed (230). —— = = — Orchid morphology: aerial shoot and inflorescence | 201 Fig. 201. a) Dacty/orhiza fuchsii, flower; a!) Dactylorhiza fuchsii, pollinia; b) Disa ‘Diores’; c) Coe/ogyne sp., flower; d) Doritis pulcherrima, flower; d') Doritis pulcherrima, column; d'') Doritis pulcherrima, pollinia; e) Rossioglossum grande, inflorescence showing resupination. A: anther (clinandrium) associated with rostellum. C: column. L: labellum (lip). P: lateral petal. Po: pollinium. S: lateral sepal. Sd: dorsal sepal. St: stigmatic surface. ee 202 | Cacti and cacti lookalikes _ Fig. 202a. Alluaudia adscendens (Didiereaceae) Each pair of leaves with associated stem spine (124) has Each pair of spines represents a pair of stipules (52). A developed from the bud in the axil of a leaf now shed Fig. 202b. Euphorbia ammati (Euphorbiaceae) minute bud is present in the axil of each leaf scar. The spines of cacti (Cactaceae) represent modified leaves. In Pereskia species, normal bifacial leaves are present with the leaves of their axillary buds developing as spines. Two spines only per bud in some species (203b) represent modified prophylls (66). In the majority of cacti, the green stem is either flattened (and thus can be termed a phylloclade 126, 203a, 294a) or is swollen with conspicuous protuberances (‘tubercles’, ‘mammillae’, leaf cushions 203g). These tubercles may merge into vertical ridges (203c). In flattened Opuntia spp., small and temporary leaves can be seen on newly developing stems (203a). Each leaf subtends an axillary bud, the leaves of which are again represented by groups of spines. Each such group of spines is termed an areole (cf. 34). Some spines (glochids) are barbed and are easily detached. In species with tubercles (mammillae) an areole is usually found on the distal end of this structure having originated in the axil of a leaf whose tissue is now incorporated in the tubercle. Such meristematic activity (leading to the development of combined tissue) is an example of adnation (234) and in this case results in an axillary bud situated on its subtending leaf (epiphylly 74). Some species of the genus Mammillaria exhibit another form of meristem reorganization leading to the symmetrical division of the apex, i.e. true dichotomy (258). The apical meristem of the areole may die, remain dormant, continue to produce more leaves as spines, or develop into another vegetative shoot or a flower. In some species two buds (i.e. accessory buds 236) are associated with each leaf site, one developing into the areole on the tubercle, the other having the potential to become either a vegetative shoot or a flower or occasionally a second areole (203d). This second bud will be found somewhere on the adaxial side of the tubercle. All the leaves of a bud forming an areole do not necessarily develop into similarly sized spines. The spines on the abaxial side of the areole are usually the largest. Hairs (trichomes 80) may be found amongst the spines. Members of the related family Didiereaceae similarly have the leaves of axillary buds modified in the form of spines (202a). Some members of the Euphorbiaceae (203f, h) and Asclepiadaceae (Stapelia and Ceropegia 203e) resemble cacti with swollen, flattened, ridged, or tubercled stems. Spines present in the Euphorbiaceae are either present in pairs and then represent modified stipules (202b), or are solitary in the axil of a leaf or leaf scar and then represent modified inflorescence axes (144) or persistent leaf bases (40). Cactus-like members of the Asclepiadaceae bear dormant vegetative or reproductive buds in the axils of leaf spines. Fig. 203. a) Opuntia sp.; b) Pereskia aculeata, single node; c) Discocactus horstii, whole plant; d) Mammillaria microhelia, single mammilla; e) Ceropegia stapeliiformis, juvenile whole plant; f) Euphorbia caput-medusae, distal end of lateral shoot; g) Gymnocalycium baldianum, whole plant from above; h) Euphorbia obesa, whole plant from above; i) Lophophora williamsii, whole plant from above. A: areole. Af: flowering areoles (adult plant body). As: sterile areoles (juvenile plant body). Fs: flower scar. L: leaf. Ls: leaf spine. Lsc: leaf scar. Ph: phylloclade (126). Ps: prophyil spine (66). Cacti and cacti lookalikes | 203 204 | Domatia: cavities inhabited by animals Fig. 204a. Ardisia crispa Bacteria inhabit cavities present in bulges along the edge of the leaf Fig. 204b. Psychotria bacteriophila The bacteria inhabit small Cavities to be seen scattered in the underside of the leaf. A domatium (plural domatia), literally a small house, is a cavity within the structure of a plant (stem or leaf or root) (106, 205b) which is inhabited by ants, or possibly mites. The morphology of domatia vary considerably. They are formed by the plant even in the absence of the animal (unlike galls 278) and may be coupled with the production by the plant of some sort of food body or nectary (78, 80). The form of the domatium may be simple such as the hollow groove in the adaxial base of the leaf of Fraxinus (mites), or the cavity formed by the over-arching of tissue at the junction of two major veins (mites 205c). Elaborate examples take the form of hollow internodes (78) or petioles (205d) with entrance holes (ants) or hollow swellings on the under surface of leaves (ants). Ants inhabit the hollowed out woody stipular spines of Acacia species (205a, a'). Quite distinct from domatia are cavities in leaves inhabited by bacteria (‘leaf nodules’) which are found typically in members of the Rubiaceae. Bacteria either accumulate in hydathodes (water excreting glands) on leaf margins (204a) or in enlarged substomatic cavities on either leaf surface (204b). The bacteria probably invade these cavities during leaf development in the bud where they are associated with mucilage secreted by colleters (80). Domatia: cavities inhabited by animals | 205 Fig. 205. a) Acacia sphaerocephala, series of stipule spine pairs (56); a') ditto, section of spine; b) Myrmecodia echinata, whole plant; c) Coffea arabica, leaf from below; c') ditto, single domatium; d) Tococa guyanensis, petioles Sinarundinaria sp., condensed branching, see 193c; d) Crataegus monogyna, shoot cluster at node; e) Stachys sylvatica, flower cluster at node; f) Forsythia sp., flower cluster at node; g) Asparagus p/umosus, condensed shoot system of cladodes. B: bract. Cl: cladode (126). Css: condensed shoot system. F: flower. Fb: flower bud. Fr: fruit. SI: scale leaf. Ss: shoot spine (124). Vs: vegetative shoot. of pair of leaves. H: entrance hole. Ls: leaf scar. Sp: swollen hollow petiole, entrance holes on abaxial side. Sts: hollow stipular spine. Swo: swollen root containing cavities. 206 | Misfits: theoretical background Morphology is the study of shape. The study of plant shape has often been associated with a philosophical attitude. One of the first plant morphologists, Theophrastus (circa 400 Bc) was a philosopher and this linkage has persisted throughout time. The history of the subject is detailed by Arber (The Natural Philosophy of Plant Form, 1950) and the philosophical attitude by Sattler (1982, 1986). The approach of the German poet and philosopher Goethe (b. 1749) exemplified the recurrent desire of botanists to find a structural identity for plants, something as obvious as the head, tail, and heart of an animal. What is the meaning of a plant, what is its gestalt? Goethe recognized the change in form (‘metamorphosis’) of leaves in a plant; a study of development will show that the foliage leaf, the sepal, and the petal will each originate from an initially equivalent leaf primordium at the shoot apex. Different structures having the same origin in this manner are said to be homologous (1). Thus the classical interpretation of plant parts, as described by Sachs (1874), recognized four categories of organ each with many homologous variations. These were stem (caulome), leaf (phyllome), root (rhizome—a term now applied only to underground stems, not roots), and hair (trichome). Stem and leaf together constitute a shoot. Plants have been described in terms of alternative structural units more recently (282) but these four basic and usually recognizable morphological categories are universally employed. Nevertheless there are many instances where an attempt to identify the parts of a plant in accordance with this classical framework fail or become a matter of opinion (e.g. Sattler et al., 1988). The final appearance of a structure does not indicate how it develops and a developmental study will often be helpful in interpreting a particular morphology (e.g. 20, 44). This is especially so when meristematic activity leads to two organs developing as one or remaining connected. This phenomenon may be ‘normal’ for the plant in question (cacti tubercule 202, epiphylly 74, adnation and connation 234) or result from an abnormal disruption of meristem activity, e.g. fasciation (272)—one form of disruptive development or teratology (270). A danger in the assumption that every morphological feature must be explainable within the classical scheme is that an actual departure from the ‘norm’ within the plant kingdom will not be recognized as such or is passed off as an organ ‘sui generis’ (literally ‘of its own kind’, i.e. a one off, ‘atypical’, an ‘inexplicable structure’, e.g. 122) if they defy classical interpretation. A more flexible approach is possibly advisable (Groff and Kaplan 1988). Sattler (1974) for example advocates the recognition and expectation of structures that in their development fall between the rigid bounds of leaf and stem (see phylloclades 126). There is no doubt that a few plants are evolving forms that cannot sensibly be accommodated in traditional descriptions (see Streptocarpus for example 208). Some of these are outlined here and described as ‘misfits’ (208-212); misfits, that is, to a botanical discipline not misfits for a successful existence. Similarly, the haustoria of many parasitic plants do not have a conventional morphology (108) and the leaves of the Lentibulariaceae are difficult to reconcile with the classical mould (Sculthorpe 1967); they are included here under indeterminate growth (90). Fig. 207. Mourera weddelliana (Podostemaceae) entire plant (210). Redrawn from Tulasne (1852). Misfits: theoretical background | 207 ( 208 | Misfits: Gesneriaceae Most plants in the family Gesneriaceae (dicotyledons) have a conventional morphology, although there is a tendency in many for there to be an inequality in cotyledon size (anisocotyly 32). In some genera (especially Streptocarpus subgen. Streptocarpus (209a-f), but also Acanthonema, Trachystigma, Monophyllaea, Moultonia, and Epithema 209g, h) one cotyledon completely outgrows the other and the plant has a growth form that is not compatible with traditional concepts (206). Jong and Burtt (1975) suggest that Streptocarpus fanniniae for example would have to be described in conventional terms as follows: ‘The plant composed entirely of numerous petiolate leaves (i.e. no stem), the long trailing petioles rooting from the lower surface as they creep over the substrate forming a dense tangled mat. Accessory leaves arising at regular intervals usually on the upper surface of the long petioles and they in turn forming further accessory leaves. Inflorescences developing at the junction of lamina and petiole’. Jong and Burtt (1975) avoid any attempt to describe such a structure by making homological (1) comparisons with ‘ordinary’ plants, but identify the basic unit of construction of these plants as a ‘phyllomorph’, i.e. a leaf blade (or lamina) plus its proximal petiole (or ‘petiolode’, as it has a much more elaborate morphology than most conventional petioles). When a Streptocarpus seed germinates one cotyledon only enlarges and becomes the first phyllomorph of the plant (209a-d). The apical meristem of the seedling, which would normally continue development to form the epicotyl, becomes incorporated in the tissue of the upper (‘adaxial’) surface of the enlarging cotyledon and will be found at the junction of the petiolode and lamina (208). This meristem, typical of a phyllomorph, is termed the groove meristem (as it is visible as an elongated depression). In some species the groove meristem gives rise to an inflorescence and the plant, which only consists of the one cotyledonary phyllomorph, dies after fruiting. In other species the groove meristem produces few or many additional phyllomorphs and inflorescences with varying degrees of regularity and second order phyllomorphs may give rise to third order LSS. SSS... phyllomorphs and so on. Root primordia form in the under surface of the petiolode. In Epithema (Hallé and Delmotte 1973) the single large cotyledon dies in the dry season and it is the third leaf of the plant that forms a fertile phyllomorph (209g, h). Phyllomorphs are capable of extended growth over a number of seasons. This is due to the activity of two additional meristems. The basal meristem (209d) is situated at the proximal end of the lamina, next to the groove meristem, and continued cell division in this area increases the length of the lamina in favourable conditions. Conversely, the lamina has the ability to jettison its distal end by the formation of an abscission layer across the lamina in unfavourable (dry) conditions. The petiolode can also increase in length due to the activity of a petiolode meristem (209e) situated transversely across the petiolode beneath the groove meristem. Elongation at this point not only adds to the length of the petiolode but also breaks the groove meristem into a number of separate regions, termed by Jong and Burtt (1975) detached meristems (209d). Each Fig. 208. Streptocarpus rexii, base of single phyllomorph with daughter phyllomorphs arising from meristematic region. Misfits: Gesneriaceae | 209 detached meristem can continue to produce additional phyllomorphs and/or inflorescences. These various phyllomorph characteristics occur with different emphasis in different species. The phyllomorphic construction within the Gesneriaceae is quite unlike the leaf with axillary bud format of most flowering plants and must represent an alternative evolutionary trend (cf. Lemnaceae 212). (c) (e) Fig. 209. a-e) Streptocarpus fanniniae, stages of development of seedling; f) Streptocarpus rexii, base of plant; g) Epithema tenue, sterile whole plant; h) Epithema tenue, fertile whole plant. Bm: basal meristem. Co: cotyledon. Dm: detached meristem. Gm: groove meristem. Hy: hypocotyl. |: inflorescence. L: leaf no. 3. Pet: petiolode. Ph: phyllomorph. Pm: petiolode meristem. R: root. g, h) redrawn from Hallé and Delmotte (1973), remainder from Jong and Burtt (1975). 210 | Misfits: Podostemaceae and Tristichaceae Plants belonging to the two families Podostemaceae (dicotyledons, flowers without perianth segments but enclosed in a spathe 140) and Tristichaceae (dicotyledons, flowers with 3 to 5 perianth segments) live in fast-flowing tropical streams of Asia, Africa, and South America. They vary enormously in morphology (207, 211) and in the absence of flowers are not recognizable as Angiosperms. Just as the vegetative body of some parasitic plants (e.g. Rafflesia 108) is described as ‘mycelial’ reflecting its fungal appearance, so the structure of these plants is described as ‘thalloid’ due to its superficial similarity to that of various algae or liverworts. Some members of these families do give the appearance of having a stem bearing leaves but these structures may merge in their appearance; in some species for example the ‘leaves’ are indeterminate (90) and retain an active apical meristem. The genera of the Tristichaceae bear non-vascularized scale leaves only. A germinating seed does not produce a radicle, but the hypocotyl bears adventitious roots (98). These roots may then develop into an elaborate structure, fixed to rock surface (a ‘hapteron’, again terminology borrowed from algae description) and can become dorsiventrally flattened and contain chlorophyll (cf. the roots of some orchids 198). In common with studies of other plants that have a construction that cannot be reconciled with traditional morphological expectations (206), it is probably pointless to search for homological comparison (1) between these plants and conventional Angiosperms. Nevertheless in theory they are generally Fig. 210. A 19th century engraving by Thuret (1878) of the thallus of the marine alga Cutleria multifida. Compare with Figs. 207 and 211. supposed to be basically of root origin bearing endogenous shoot/leaf structures in the manner of root buds (178) (Schnell 1967). An extensive bibliography of the Podostemales is to be found in Cusset and Cusset (1988) together with an account of the morphology of members of the Tristichaceae. Fig. 211. Examples of variation of growth form in the Podostemaceae. a) Rhyncholacis hydrocichorum, b) Marathrum utile, c) Caste/navia princeps. Redrawn from Tulasne (1852). (See also 207.) Misfits: Podostemaceae and Tristichaceae 242 | Misfits: Lemnaceae The family Lemnaceae (monocotyledons with close affinities to the Araceae) is composed of four genera: Spirodela, Lemna, Wolffiella, and Wolffia. All species are represented by very small aquatic plants floating on or at the surface of fresh water. Each plant consists of either a single ‘frond’ or ‘thallus’ or a more or less temporarily connected series of these structures. Fronds vary in size in different species from about 10 mm in length (Spirodela species 213e) to 1.5 mm in length (Wolffia species 213b, c) and are generally flattened distally with a narrow proximal end. Roots are either absent, solitary, or few in number and develop from the under surface. The edges of the frond bear two (usually one in Wolffia) meristematic zones each sunken into ‘pockets’ more or less protected by a flap of tissue. New fronds develop within these pockets from which they sooner or later become detached, forming clones of fronds. In adverse conditions very small ‘resting’ fronds are produced (turions 172). From time to time a pocket becomes reproductive and male and female flowers consisting of androecium and gynoecium only (213b) are formed. Lemna clones (e.g. Lemna perpusilla) can show considerable symmetry of organization (228): fronds emerge from pockets in strict sequence from one side to the other and clones are either always left- or right-handed depending upon which side produces the first frond. Fronds of left-handed clones will have their reproductive pockets on the right and vice versa. Such small and simple plants do not lend themselves to morphological interpretation even by developmental studies. Each frond has been considered to consist of a distal leaf lamina plus a proximal narrow region of combined stem and leaf origin together with meristematic zones (and thus similar in concept to the phyllomorph of Streptocarpus 208). Studies of the larger species of Spirodela and Lemna have suggested a conventional arrangement of distichous buds on a very short stem more or less lacking subtending leaves and with either a terminal flattened stem end (cladode 126) or a terminal leaf, the apical meristem being lost. Fig. 212. Lemna minor Fronds viewed from below. One root per frond. Misfits: Lemnaceae | 213 Fig. 213. a) Lemna valdiviana, b) Wolffia microscopia, c) Wolffia papulifera, d) Lemna trisulca, e) Spirodela oligorhiza, f) Wolffiella floridana. Redrawn from Daubs (1965). Whole plant in each case. Part II Constructional organization ‘What a complex matter in its summation, but what a simple one in its graduated steps, the shaping of a tree is.’ Ward (1909) Trees: Form. ‘(Actually) we need a solid geometry of tree form to show how systems with apical growth and axillary branching, rooted in the ground and displaying foliage, pervade space.’ Corner (1946) Suggestions for botanical progress. Fig. 215. The prototype “Raft of the treetops” on its inaugural flight in South America. A well organized construction designed to allow botanists to study, /n situ, the architectural details of the surface of the tropical rain forest for the very first time 216 | Constructional organization: introduction Fig. 216a. Pinus sp. (a Gymnosperm, 14) Fig. 216b. Populus sp. These botanically accurate computer images were synthesized at the Laboratory of Biomodelization at the Centre de Coopération Internationale en Récherche Agronomique pour le Développement, Montpellier, France. (Reffye et a/. 1988). The aspects of plant morphology described in the first part of this guide are to a large extent static features that can be identified in a plant (although sometimes a study of the development of the structure over time is needed in order to understand the final construction). However, a flowering plant is not a static object. It is a dynamic organism constantly growing and becoming more elaborate. Its continued construction is represented by progressive accumulation (and loss) of the morphological features described in Part I. Plants do not grow in a haphazard way but in an organized flexible manner controlled by internal and environmental factors. Part II considers aspects of the dynamic morphology of plants which are not necessarily the features that can be appreciated by studying a plant at one point in its life span. Happily, morphological clues to earlier sequences of events can often be found on a plant such as scars of jettisoned organs or progressive changes in comparable organs of different ages. As a plant grows and becomes more elaborate, it is possible to monitor these sequences at many levels, such as increase in cell numbers, increase in weight, or leaf number and area. However, greater insight into the developmental morphology or ‘architecture’ of the plant is revealed by a study of bud activity. New structural components in a plant’s framework are developed from buds. A bud develops into a shoot (also termed here a shoot unit 286). The term bud implies a dormant phase but this does not always take place (262). It is therefore more accurate to talk in terms of apical meristems rather than of buds (see page 16 for an introductory discussion of bud and meristem terminology). The contribution of buds or apical meristems to the progressive development of a plant’s growth form can be considered under three related headings. Firstly the position of the bud within the plant’s framework, secondly the potential of that bud if it grows, i.e. what it will develop into and how fixed is its fate (topophysis 242), and thirdly, the timing and duration of the bud’s growth and that of the resulting shoot with respect to the rest of the plant. The morphological consequences of apical meristem position, potential, and time of activity are described in Part II together with an indication of what can go wrong (meristem disruption 270-278) and an example of what can be discovered about the overall morphological entity of a plant (plant branch construction 280-314). An understanding of plant construction necessitates a recognition of the units of that construction (280-286) (plus an awareness that some plants will inevitably not conform to the general flowering plant format 206-212). This section is confined to a consideration of shoot construction (the details to be seen in rooting systems are not yet sufficiently understood 96), and uses advances in the study of tree architecture as an illustration (288-304). ee “d * at AY A A | § VA I a x, A : NSt! di Ky i Fig. 217. Platanus b fi orientalis. Manipulation of ey) WW) j yy branching pattern by W yg pruning. An example of 4 traumatic reiteration (298). SS 4 aie Batis. ase aoe catia, NUseeam a ‘ a yi is ett? a a 218 | Meristem position: phyllotaxis, arrangement of leaves on stem Phyllotaxis (alternative phyllotaxy) is the term applied to the sequence of origin of leaves on a stem (cf. rhizotaxy 96). The phyllotaxis of any one plant, or at least any one shoot on a plant, is usually constant and often of diagnostic value. In monocotyledons only one leaf is borne at a node although a succession of short internodes interspersed between long may mask this condition. Leaves of dicotyledons are present one to many at a node. The relative positions of leaves on a plant must affect the interception of light and, more importantly, the position of a leaf usually fixes the position of its subtended axillary bud (or apical meristem 16). Thus the phyllotaxis of a plant can play a considerable role in determining the branching pattern of a plant, particularly for woody perennials (288-304). The study of phyllotaxis has led to an extensive terminology and also to a preoccupation with the Fibonacci series (220). Fig. 218. Ravenala madagascariensis Leaf primordia and therefore leaves develop in two rows on the apical meristem exactly 180° apart giving a distichous phyllotaxis (Fig. 219c). Phyllotaxis terminology A. One leaf per node (sometimes referred to as ‘alternate’ in contrast to two per node ‘opposite’, see below) Monostichous. All leaves on one side of stem, i.e. one row as seen from above (219a). This is a very rare phyllotaxis and is most often accompanied by asymmetrical internode growth between successive leaves resulting in a slight twist. As a result the leaves are arranged in a shallow helix and the phyllotaxis is termed spiromonostichous (219b, 226). Distichous. The leaves are arranged in two rows seen from above, usually with 180° between the rows (219c, 218). This is a common condition and is a diagnostic feature of grasses (180). If a slight twist is superimposed on this phyllotaxis, the result is a spirodistichy (219d, 220). Tristichous. Leaves in three rows with 120° between rows (219e). Typical of the Cyperaceae (196). Twisting may occur—spirotristichous phyllotaxis (219f). Spiral. This term is applied if more than three longitudinal rows of leaves are present, e.g. 5 rows as seen from above (219g) or 8 rows as seen from above (219h, 132, 246). The exact nature of the spiral is described in terms of a fraction indicating the angle between any two successive leaves (220). B. Two leaves per node Opposite. The two leaves at each node are 180° apart and form two rows as seen from above (219i) (the same arrangement often results from internode twisting e.g. 225a). When successive pairs are orientated at 90° to each other, four rows of leaves will be visible from above and the phyllotaxis is opposite decussate (219j, 233). In some plants, successive pairs are less than 90° apart and the phyllotaxis is described as bijugate (219k), leading to a double spiral having two rows of leaves (genetic spirals 220). This arrangement can be referred to as spiral decussate. C. Three or more leaves per node Whorled. A fixed or variable number of leaves arises at each node. Leaves in successive whorls may or may not be arranged in discrete rows as seen from above. If so, then the whorls are often neatly interspaced (2191, 229c). (A pseudowhorl results in plants with one leaf per node if series of very short internodes are separated by single long internodes 260a). Meristem position: phyllotaxis, arrangement of leaves on stem | 219 (a) (c) Fi rz) topmost leaf (e) (g) spiromonostichous, c)distichous, d) spirodistichous, e) tristichous, f) spirotristichous, g, h) spiral, i) opposite, j) opposite decussate, k) bijugate (spiral decussate), |) whorled. (i) (i) (k) bg a