Part 33
The form and arrangement of the parts of a typical foliage leaf are intimately associated with the part played by the leaf in the life of the plant. The flat surface is spread to allow the maximum amount of sunlight to fall upon it, as it is by the absorption of energy from the sun's rays by means of the chlorophyll contained in the cells of the leaf that the building up of plant food is rendered possible; this process is known as photo-synthesis; the first stage is the combination of carbon dioxide, absorbed from the air taken in through the stomata into the living cells of the leaf, with water which is brought into the leaf by the wood-vessels. The wood-vessels form part of the fibro-vascular bundles or veins of the leaf and are continuous throughout the leaf-stalk and stem with the root by which water is absorbed from the soil. The palisade layers of the mesophyll contain the larger number of chlorophyll grains (or corpuscles) while the absorption of carbon dioxide is carried on chiefly through the lower epidermis which is generally much richer in stomata. The water taken up by the root from the soil contains nitrogenous and mineral salts which combine with the first product of photo-synthesis--a carbohydrate--to form more complicated nitrogen-containing food substances of a proteid nature; these are then distributed by other elements of the vascular bundles (the _phloem_) through the leaf to the stem and so throughout the plant to wherever growth or development is going on. A large proportion of the water which ascends to the leaf acts merely as a carrier for the other raw food materials and is got rid of from the leaf in the form of water vapour through the stomata--this process is known as _transpiration_. Hence the extended surface of the leaf exposing a large area to light and air is eminently adapted for the carrying out of the process of photo-synthesis and transpiration. The arrangement of the leaves on the stem and branches (see _Phyllotaxy_, below) is such as to prevent the upper leaves shading the lower, and the shape of the leaf serves towards the same end--the disposition of leaves on a branch or stem is often seen to form a "mosaic," each leaf fitting into the space between neighbouring leaves and the branch on which they are borne without overlapping.
Submerged leaves, or leaves which are developed under water, differ in structure from aerial leaves. They have usually no fibro-vascular system, but consist of a congeries of cells, which sometimes become elongated and compressed so as to resemble veins. They have a layer of compact cells on their surface, but no true epidermis, and no stomata. Their internal structure consists of cells, disposed irregularly, and sometimes leaving spaces which are filled with air for the purpose of floating the leaf. When exposed to the air these leaves easily part with their moisture, and become shrivelled and dry. In some cases there is only a network of filament-like cells, the spaces between which are not filled with parenchyma, giving a skeleton appearance to the leaf, as in _Ouvirandra fenestralis_ (Lattice plant).
A leaf, whether aerial or submerged, generally consists of a flat expanded portion, called the _blade_, or _lamina_, of a narrower portion called the _petiole_ or _stalk_, and sometimes of a portion at the base of the petiole, which forms a _sheath_ or _vagina_ (fig. 5, s), or is developed in the form of outgrowths, called _stipules_ (fig. 24, s). All these portions are not always present. The sheathing or stipulary portion is frequently wanting. When a leaf has a distinct stalk it is _petiolate_; when it has none, it is _sessile_, and if in this case it embraces the stem it is said to be _amplexicaul_. The part of the leaf next the petiole or the axis is the _base_, while the opposite extremity is the _apex_. The leaf is usually flattened and expanded horizontally, i.e. at right angles to the longitudinal axis of the shoot, so that the upper face is directed towards the heavens, and the lower towards the earth. In some cases leaves, as in Iris, or leaf-like petioles, as in Australian acacias and eucalypti, have their plane of expansion parallel to the axis of the shoot, there is then no distinction into an upper and a lower face, but the two sides are developed alike; or the leaf may have a cylindrical or polyhedral form, as in mesembryanthemum. The upper angle formed between the leaf and the stem is called its _axil_; it is there that leaf-buds are normally developed. The leaf is sometimes articulated with the stem, and when it falls off a _scar_ remains; at other times it is continuous with it, and then decays, while still attached to the axis. In their early state all leaves are continuous with the stem, and it is only in their after growth that articulations are formed. When leaves fall off annually they are called _deciduous_; when they remain for two or more years they are _persistent_, and the plant is _evergreen_. The laminar portion of a leaf is occasionally articulated with the petiole, as in the orange, and a joint at times exists between the vaginal or stipulary portion and the petiole.
[Illustration: FIG. 3.--Leaf of Elm (_Ulmus_). Reticulated venation; primary veins going to the margin, which is serrated. Leaf unequal at the base.]
[Illustration: FIG. 4.--Multicostate leaf of Castor-oil plant (_Ricinus communis_). It is palmately-cleft, and exhibits seven lobes at the margin. The petiole is inserted a little above the base, and hence the leaf is called peltate or shield-like.]
Venation.
The arrangement of the fibro-vascular system in the lamina constitutes the _venation_ or _nervation_. In an ordinary leaf, as that of the elm, there is observed a large central vein running from the base to the apex of the leaf, this is the _midrib_ (fig. 3); it gives off veins laterally (_primary veins_). A leaf with only a single midrib is said to be _unicostate_ and the venation is described as pinnate or feather-veined. In some cases, as sycamore or castor oil (fig. 4), in place of there being only a single midrib there are several large veins (_ribs_) of nearly equal size, which diverge from the point where the blade joins the petiole or stem, giving off lateral veins. The leaf in this case is _multicostate_ and the venation palmate. The primary veins give off secondary veins, and these in their turn give off tertiary veins, and so on until a complete network of vessels is produced, and those veins usually project on the under surface of the leaf. To a distribution of veins such as this the name of _reticulated_ or _netted_ venation has been applied. In the leaves of some plants there exists a midrib with large veins running nearly parallel to it from the base to the apex of the lamina, as in grasses (fig. 5); or with veins diverging from the base of the lamina in more or less parallel lines, as in fan palms (fig. 6), or with veins coming off from it throughout its whole course, and running parallel to each other in a straight or curved direction towards the margin of the leaf, as in plantain and banana. In these cases the veins are often united by cross veinlets, which do not, however, form an angular network. Such leaves are said to be _parallel-veined_. The leaves of Monocotyledons have generally this kind of venation, while reticulated venation most usually occurs amongst Dicotyledons. Some plants, which in most points of their structure are monocotyledonous, yet have reticulated venation; as in _Smilax_ and _Dioscorea_. In vascular acotyledonous plants there is frequently a tendency to fork exhibited by the fibro-vascular bundles in the leaf; and when this is the case we have _fork-veined_ leaves. This is well seen in many ferns. The distribution of the system of vessels in the leaf is usually easily traced, but in the case of succulent plants, as _Hoya_, agave, stonecrop and mesembryanthemum, the veins are obscure. The function of the veins which consist of vessels and fibres is to form a rigid framework for the leaf and to conduct liquids.
[Illustration: FIG. 5.--Stem of a Grass (_Poa_) with leaf. The sheaths ending in a process l, called a ligule; the blade of the leaf, f.]
[Illustration: FIG. 6.--Leaf of a Fan Palm (_Chamaerops_), showing the veins running from the base to the margin, and not forming an angular network.]
In all plants, except Thallophytes, leaves are present at some period of their existence. In _Cuscuta_ (Dodder) (q.v.), however, we have an exception. The forms assumed by leaves vary much, not only in different plants, but in the same plant. It is only amongst the lower classes of plants--Mosses, Characeae, &c.--that all the leaves on a plant are similar. As we pass up the scale of plant life we find them becoming more and more variable. The structures in ordinary language designated as leaves are considered so _par excellence_, and they are frequently spoken of as _foliage leaves_. In relation to their production on the stem we may observe that when they are small they are always produced in great number, and as they increase in size their number diminishes correspondingly. The cellular process from the axis which develops into a leaf is simple and undivided; it rarely remains so, but in progress of growth becomes segmented in various ways, either longitudinally or laterally, or in both ways. By longitudinal segmentation we have a leaf formed consisting of sheath, stalk and blade; or one or other of these may be absent, and thus stalked, sessile, sheathing, &c., leaves are produced. Lateral segmentation affects the lamina, producing indentations, lobings or fissuring of its margins. In this way two marked forms of leaf are produced--(1) _Simple_ form, in which the segmentation, however deeply it extends into the lamina, does not separate portions of the lamina which become articulated with the midrib or petiole; and (2) _Compound_ form, where portions of the lamina are separated as detached _leaflets_, which become articulated with the midrib or petiole. In both simple and compound leaves, according to the amount of segmentation and the mode of development of the parenchyma and direction of the fibro-vascular bundles, many forms are produced.
Simple leaves.
_Simple Leaves._--When the parenchyma is developed symmetrically on each side of the midrib or stalk, the leaf is _equal_; if otherwise, the leaf is _unequal_ or _oblique_ (fig. 3). If the margins are even and present no divisions, the leaf is _entire_ (fig. 7); if there are slight projections which are more or less pointed, the leaf is _dentate_ or toothed; when the projections lie regularly over each other, like the teeth of a saw, the leaf is _serrate_ (fig. 3); when they are rounded the leaf is _crenate_. If the divisions extend more deeply into the lamina than the margin, the leaf receives different names according to the nature of the segments; thus, when the divisions extend about half-way down (fig. 8), it is _cleft_; when the divisions extend nearly to the base or to the midrib the leaf is _partite_.
If these divisions take place in a simple _feather-veined_ leaf it becomes either _pinnatifid_ (fig. 9), when the segments extend to about the middle, or _pinnatipartite_, when the divisions extend nearly to the midrib. These primary divisions may be again subdivided in a similar manner, and thus a feather-veined leaf will become _bipinnatifid_ or _bipinnatipartite_; still further subdivisions give origin to _tripinnatifid_ and _laciniated_ leaves. The same kinds of divisions taking place in a simple leaf with palmate or _radiating_ venation, give origin to _lobed_, _cleft_ and _partite_ forms. The name _palmate_ or _palmatifid_ (fig. 4) is the general term applied to leaves with radiating venation, in which there are several lobes united by a broad expansion of parenchyma, like the palm of the hand, as in the sycamore, castor-oil plant, &c. The divisions of leaves with radiating venation may extend to near the base of the leaf, and the names _bipartite_, _tripartite_, _quinquepartite_, &c., are given according as the partitions are two, three, five or more. The term _dissected_ is applied to leaves with radiating venation, having numerous narrow divisions, as in _Geranium dissectum_.
[Illustration:
FIG. 7.--Ovate acute leaf of _Coriara myrtifolia_. Besides the midrib there are two intra-marginal ribs which converge to the apex. The leaf is therefore tricostate.
FIG. 8.--Runcinate leaf of Dandelion. It is a pinnatifid leaf, with the divisions pointing towards the petiole and a large triangular apex.
FIG. 9.--Pinnatifid leaf of _Valeriana dioica_.]
[Illustration: FIG. 10.--Five-partite leaf of Aconite.]
[Illustration: FIG. 11.--Pedate leaf of Stinking Hellebore (_Helleborus foetidus_). The venation is radiating. It is a palmately-partite leaf, in which the lateral lobes are deeply divided. When the leaf hangs down it resembles the foot of a bird, and hence the name.]
When in a radiating leaf there are three primary partitions, and the two lateral lobes are again cleft, as in hellebore (fig. 11), the leaf is called _pedate_ or _pedatifid_, from a fancied resemblance to the claw of a bird. In all the instances already alluded to the leaves have been considered as flat expansions, in which the ribs or veins spread out on the same plane with the stalk. In some cases, however, the veins spread at right angles to the stalk, forming a _peltate_ leaf as in Indian cress (fig. 12).
The form of the leaf shows a very great variety ranging from the narrow _linear_ form with parallel sides, as in grasses or the needle-like leaves of pines and firs to more or less rounded or _orbicular_--descriptions of these will be found in works on descriptive botany--a few examples are illustrated here (figs. 7, 13, 14, 15). The apex also varies considerably, being rounded, or _obtuse_, sharp or _acute_ (fig. 7), notched (fig. 15), &c. Similarly the shape of the base may vary, when rounded lobes are formed, as in dog-violet, the leaf is cordate or heart-shaped; or kidney-shaped or _reniform_ (fig. 16), when the apex is rounded as in ground ivy. When the lobes are prolonged downwards and are acute, the leaf is _sagittate_ (fig. 17); when they proceed at right angles, as in _Rumex Acetosella_, the leaf is _hastate_ or halbert-shaped. When a simple leaf is divided at the base into two leaf-like appendages, it is called _auriculate_. When the development of parenchyma is such that it more than fills up the spaces between the veins, the margins become _wavy_, _crisp_ or _undulated_, as in _Rumex crispus_ and _Rheum undulatum_. By cultivation the cellular tissue is often much increased, giving rise to the _curled_ leaves of greens, savoys, cresses, lettuce, &c.
[Illustration: FIG. 12.--Peltate leaves of Indian Cress (_Tropaeolum majus_).]
[Illustration: FIG. 13.--Lanceolate leaf of a species of Senna.]
Compound leaves.
Compound leaves are those in which the divisions extend to the midrib or petiole, and the separated portions become each articulated with it, and receive the name of _leaflets_. The midrib, or petiole, has thus the appearance of a branch with separate leaves attached to it, but it is considered properly as one leaf, because in its earliest state it arises from the axis as a single piece, and its subsequent divisions in the form of leaflets are all in one plane. The leaflets are either sessile (fig. 18) or have stalks, called _petiolules_ (fig. 19). Compound leaves are pinnate (fig. 19) or palmate (fig. 18) according to the arrangement of leaflets. When a pinnate leaf ends in a pair of pinnae it is _equally_ or _abruptly pinnate_ (paripinnate); when there is a single terminal leaflet (fig. 19), the leaf is _unequally pinnate_ (imparipinnate); when the leaflets or pinnae are placed alternately on either side of the midrib, and not directly opposite to each other, the leaf is _alternately pinnate_; and when the pinnae are of different sizes, the leaf is _interruptedly pinnate_. When the division is carried into the second degree, and the pinnae of a compound leaf are themselves pinnately compound, a bipinnate leaf is formed.
[Illustration:
FIG. 14.--Oblong leaf of a species of Senna.
FIG. 15.--Emarginate leaf of a species of Senna. The leaf in its contour is somewhat obovate, or inversely egg-shaped, and its base is oblique.
FIG. 16.--Reniform leaf of _Nepeta Glechoma_, margin crenate.
FIG. 17.--Sagittate leaf of Convolvulus.]
[Illustration: FIG. 18.--Palmately compound leaf of the Horse-chestnut (_Aesculus Hippocastanum_).]
[Illustration: FIG. 19.--Imparipinnate (unequal pinnate) leaf of Robinia. There are nine pairs of shortly-stalked leaflets (foliola, pinnae), and an odd one at the extremity. At the base of the leaf the spiny stipules are seen.]
Petiole.
The _petiole_ or leaf-stalk is the part which unites the limb or blade of the leaf to the stem. It is absent in _sessile_ leaves, and this is also frequently the case when a sheath is present, as in grasses (fig. 5). It consists of the fibro-vascular bundles with a varying amount of cellular tissue. When the vascular bundles reach the base of the lamina they separate and spread out in various ways, as already described under venation. The lower part of the petiole is often swollen (fig. 20, _p_), forming the _pulvinus_, formed of cellular tissue, the cells of which exhibit the phenomenon of irritability. In _Mimosa pudica_ (fig. 20) a sensitiveness is located in the pulvinus which upon irritation induces a depression of the whole bipinnate leaf, a similar property exists in the pulvini at the base of the leaflets which fold upwards. The petiole varies in length, being usually shorter than the lamina, but sometimes much longer. In some palms it is 15 or 20 ft. long, and is so firm as to be used for poles or walking-sticks. In general, the petiole is more or less rounded in its form, the upper surface being flattened or grooved. Sometimes it is compressed laterally, as in the aspen, and to this peculiarity the trembling of the leaves of this tree is due. In aquatic plants the leaf-stalk is sometimes distended with air, as in _Pontederia_ and _Trapa_, so as to float the leaf. At other times it is _winged_, and is either leafy, as in the orange (fig. 21, p), lemon and _Dionaea_, or pitcher-like, as in _Sarracenia_ (fig. 22). In some Australian acacias, and in some species of _Oxalis_ and _Bupleurum_, the petiole is flattened in a vertical direction, the vascular bundles separating immediately after quitting the stem and running nearly parallel from base to apex. This kind of petiole (fig. 23, p) has been called a _phyllode_. In these plants the laminae or blades of the leaves are pinnate or bipinnate, and are produced at the extremities of the phyllodes in a horizontal direction; but in many instances they are not developed, and the phyllode serves the purpose of a leaf. These phyllodes, by their vertical position and their peculiar form, give a remarkable aspect to vegetation. On the same acacia there occur leaves with the petiole and lamina perfect; others having the petiole slightly expanded or winged, and the lamina imperfectly developed; and others in which there is no lamina, and the petiole becomes large and broad. Some petioles are long, slender and sensitive to contact, and function as tendrils by means of which the plant climbs; as in the nasturtiums (_Tropaeolum_), clematis and others; and in compound leaves the midrib and some of the leaflets may similarly be transformed into tendrils, as in the pea and vetch.
[Illustration: FIG. 20.--Branch and leaves of the Sensitive plant (_Mimosa pudica_), showing the petiole in its erect state, a, and in its depressed state, b; also the leaflets closed, c, and the leaflets expanded, d. Irritability resides in the pulvinus, p.]
Leaf base.
The leaf base is often developed as a _sheath_ (_vagina_), which embraces the whole or part of the circumference of the stem (fig. 5). This sheath is comparatively rare in dicotyledons, but is seen in umbelliferous plants. It is much more common amongst monocotyledons. In sedges the sheath forms a complete investment of the stem, whilst in grasses it is split on one side. In the latter plants there is also a membranous outgrowth, the _ligule_, at right angles to the median plane of the leaf from the point where the sheath passes into the lamina, there being no petiole (fig. 5, _l_).
[Illustration: FIG. 21.--Leaf of Orange (_Citrus Aurantium_), showing a winged leafy petiole p, which is articulated to the lamina l.]
[Illustration: FIG. 22.--Pitcher (_ascidium_) of a species of Side-saddle plant (_Sarracenia purpurea_). The pitcher is formed from the petiole, which is prolonged.]
In leaves in which no sheath is produced we not infrequently find small foliar organs, _stipules_, at the base of the petiole (fig. 24, s). The stipules are generally two in number, and they are important as supplying characters in certain natural orders. Thus they occur in the pea and bean family, in rosaceous plants and the family Rubiaceae. They are not common in dicotyledons with opposite leaves. Plants having stipules are called _stipulate_; those having none are _exstipulate_. Stipules may be large or small, entire or divided, deciduous or persistent. They are not usually of the same form as the ordinary foliage leaves of the plant, from which they are distinguished by their lateral position at the base of the petiole. In the pansy (fig. 24) the true leaves are stalked and crenate, while the stipules s are large, sessile and pinnatifid. In _Lathyrus Aphaca_ and some other plants the true pinnate leaves are abortive, the petiole forms a tendril, and the stipules alone are developed, performing the office of leaves. When stipulate leaves are opposite to each other, at the same height on the stem, it occasionally happens that the stipules on the two sides unite wholly or partially, so as to form an _interpetiolary_ or _interfoliar_ stipule, as in members of the family Rubiaceae. In the case of alternate leaves, the stipules at the base of each leaf are sometimes united to the petiole and to each other, so as to form an _adnate_, _adherent_ or _petiolary_ stipule, as in the rose, or an _axillary_ stipule, as in _Houttuynia cordata_. In other instances the stipules unite together on the side of the stem opposite the leaf forming an _ocrea_, as in the dock family (fig. 25).
[Illustration: FIG. 23.--Leaf of an Acacia (_Acacia heterophylla_), showing a flattened leaf-like petiole p, called a phyllode, with straight venation, and a bipinnate lamina.]
In the development of the leaf the stipules frequently play a most important part. They begin to be formed after the origin of the leaves, but grow much more rapidly than the leaves, and in this way they arch over the young leaves and form protective chambers wherein the parts of the leaf may develop. In the figs, magnolia and pondweeds they are very large and completely envelop the young leaf-bud. The stipules are sometimes so minute as to be scarcely distinguishable without the aid of a lens, and so fugacious as to be visible only in the very young state of the leaf. They may assume a hard and spiny character, as in _Robinia Pseudacacia_ (fig. 19), or may be cirrose, as in _Smilax_, where each stipule is represented by a tendril. At the base of the leaflets of a compound leaf, small stipules (_stipels_) are occasionally produced.
[Illustration: FIG. 24.--Leaf of Pansy. s, Stipules.]
[Illustration: FIG. 25.--Leaf of Polygonum, with part of stem. o, Ocrea.]
Modifications.