Part 34

Variations in the structure and forms of leaves and leafstalks are produced by the increased development of cellular tissue, by the abortion or degeneration of parts, by the multiplication or repetition of parts and by adhesion. When cellular tissue is developed to a great extent, leaves become succulent and occasionally assume a crisp or curled appearance. Such changes take place naturally, but they are often increased by the art of the gardener, and the object of many horticultural operations is to increase the bulk and succulence of leaves. It is in this way that cabbages and savoys are rendered more delicate and nutritious. By a deficiency in development of parenchyma and an increase in the mechanical tissue, leaves are liable to become hardened and spinescent. The leaves of barberry and of some species of _Astragalus_, and the stipules of the false acacia (_Robinia_) are spiny. To the same cause is due the spiny margin of the holly-leaf. When two lobes at the base of a leaf are prolonged beyond the stem and unite (fig. 26), the leaf is _perfoliate_, the stem appearing to pass through it, as in _Bupleurum perfoliatum_ and _Chlora perfoliata_; when two leaves unite by their bases they become _connate_ (fig. 27), as in _Lonicera Caprifolium_; and when leaves adhere to the stem, forming a sort of winged or leafy appendage, they are _decurrent_, as in thistles. The formation of peltate leaves has been traced to the union of the lobes of a cleft leaf. In the leaf of the _Victoria regia_ the transformation may be traced during germination. The first leaves produced by the young plant are linear, the second are sagittate and hastate, the third are rounded-cordate and the next are orbicular. The cleft indicating the union of the lobes remains in the large leaves. The parts of the leaf are frequently transformed into _tendrils_, with the view of enabling the plants to twine round others for support. In Leguminous plants (the pea tribe) the pinnae are frequently modified to form tendrils, as in _Lathyrus Aphaca_, in which the stipules perform the function of true leaves. In _Flagellaria indica_, _Gloriosa superba_ and others, the midrib of the leaf ends in a tendril. In _Smilax_ there are two stipulary tendrils.

[Illustration: FIG. 26.--Perfoliate leaf of a species of Hare's-ear (_Bupleurum rotundifolium_). The two lobes at the base of the leaf are united, so that the stalk appears to come through the leaf.]

[Illustration: FIG. 27.--Connate leaves of a species of Honeysuckle (_Lonicera Caprifolium_). Two leaves are united by their bases.]

The vascular bundles and cellular tissue are sometimes developed in such a way as to form a circle, with a hollow in the centre, and thus give rise to what are called _fistular_ or hollow leaves, as in the onion, and to _ascidia_ or _pitchers_. Pitchers are formed either by petioles or by laminae, and they are composed of one or more leaves. In _Sarracenia_ (fig. 22) and _Heliamphora_ the pitcher is composed of the petiole of the leaf. In the pitcher plant, _Nepenthes_, the pitcher is a modification of the lamina, the petiole often plays the part of a tendril, while the leaf base is flat and leaf-like (fig. 28).

[Illustration: FIG. 28.--Pitcher of a species of pitcher-plant (_Nepenthes distillatoria_).]

In _Utricularia_ bladder-like sacs are formed by a modification of leaflets on the submerged leaves.

In some cases the leaves are reduced to mere _scales_--_cataphyllary_ leaves; they are produced abundantly upon underground shoots. In parasites (_Lathraea_, _Orobanche_) and in plants growing on decaying vegetable matter (_saprophytes_), in which no chlorophyll is formed, these scales are the only leaves produced. In _Pinus_ the only leaves produced on the main stem and the lateral shoots are scales, the acicular leaves of the tree growing from axillary shoots. In _Cycas_ whorls of scales alternate with large pinnate leaves. In many plants, as already noticed, phyllodia or stipules perform the function of leaves. The production of leaf-buds from leaves sometimes occurs as in _Bryophyllum_, and many plants of the order Gesneraceae. The leaf of Venus's fly-trap (_Dionaea muscipula_) when cut off and placed in damp moss, with a pan of water underneath and a bell-glass for a cover, has produced buds from which young plants were obtained. Some species of saxifrage and of ferns also produce buds on their leaves and fronds. In _Nymphaea micrantha_ buds appear at the upper part of the petiole.

Phyllotaxis.

Leaves occupy various positions on the stem and branches, and have received different names according to their situation. Thus leaves arising from the crown of the root, as in the primrose, are called _radical_; those on the stem are _cauline_; on flower-stalks, _floral_ leaves (see FLOWER). The first leaves developed are known as seed leaves or _cotyledons_. The arrangement of the leaves on the axis and its appendages is called _phyllotaxis_.

[Illustration: FIG. 29.--A stem with opposite leaves. The pairs are placed at right angles alternately, or in what is called a decussate manner. In the lowest pair one leaf is in front and the other at the back; in the second pair the leaves are placed laterally, and so on.]

[Illustration: FIG. 30.--A stem with alternate leaves, arranged in a pentastichous or quincuncial manner. The sixth leaf is directly above the first, and commences the second cycle. The fraction of the circumference of the stem expressing the divergence of the leaves is two-fifths.]

In their arrangement leaves follow a definite order. The points on the stem at which leaves appear are called nodes; the part of the stem between the nodes is the _internode_. When two leaves are produced at the same node, one on each side of the stem or axis, and at the same level, they are _opposite_ (fig. 29); when more than two are produced they are _verticillate_, and the circle of leaves is then called a _verticil_ or _whorl_. When leaves are opposite, each successive pair may be placed at right angles to the pair immediately preceding. They are then said to _decussate_, following thus a law of alternation (fig. 29). The same occurs in the verticillate arrangement, the leaves of each whorl rarely being _superposed_ on those of the whorl next it, but usually alternating so that each leaf in a whorl occupies the space between two leaves of the whorl next to it. There are considerable irregularities, however, in this respect, and the number of leaves in different whorls is not always uniform, as may be seen in _Lysimachia vulgaris_. When a single leaf is produced at a node, and the nodes are separated so that each leaf is placed at a different height on the stem, the leaves are _alternate_ (fig. 30). A plane passing through the point of insertion of the leaf in the node, dividing the leaf into similar halves, is the median plane of the leaf; and when the leaves are arranged alternately on an axis so that their median planes coincide they form a straight row or _orthostichy_. On every axis there are usually two or more orthostichies. In fig. 31, leaf 1 arises from a node n; leaf 2 is separated from it by an internode m, and is placed to the right or left; while leaf 3 is situated directly above leaf 1. In this case, then, there are two orthostichies, and the arrangement is said to be _distichous_. When the fourth leaf is directly above the first, the arrangement is _tristichous_. The same arrangement continues throughout the branch, so that in the latter case the 7th leaf is above the 4th, the 10th above the 7th; also the 5th above the 2nd, the 6th above the 3rd and so on. The size of the angle between the median planes of two consecutive leaves in an alternate arrangement is their _divergence_; and it is expressed in fractions of the circumference of the axis which is supposed to be a circle. In a regularly-formed straight branch covered with leaves, if a thread is passed from one to the other, turning always in the same direction, a spiral is described, and a certain number of leaves and of complete turns occur before reaching the leaf directly above that from which the enumeration commenced. If this arrangement is expressed by a fraction, the numerator of which indicates the number of turns, and the denominator the number of internodes in the spiral cycle, the fraction will be found to represent the angle of divergence of the consecutive leaves on the axis. Thus, in fig. 32, a, b, the cycle consists of five leaves, the 6th leaf being placed vertically over the 1st, the 7th over the 2nd and so on; while the number of turns between the 1st and 6th leaf is two; hence this arrangement is indicated by the fraction 2/5. In other words, the distance or divergence between the first and second leaf, expressed in parts of a circle, is 2/5 of a circle or 360° × 2/5 = 144°. In fig. 31, a, b, the spiral is ½, i.e. one turn and two leaves; the third leaf being placed vertically over the first, and the divergence between the first and second leaf being one-half the circumference of a circle, 360° × ½ = 180°. Again, in a tristichous arrangement the number is 1/3, or one turn and three leaves, the angular divergence being 120°.

[Illustration: FIG. 31.--Portion of a branch of a Lime tree, with four leaves arranged in a distichous manner, or in two rows. a, The branch with the leaves numbered in their order, n being the node and m the internode; b is a magnified representation of the branch, showing the points of insertion of the leaves and their spiral arrangement, which is expressed by the fraction ½, or one turn of the spiral for two internodes.]

By this means we have a convenient mode of expressing on paper the exact position of the leaves upon an axis. And in many cases such a mode of expression is of excellent service in enabling us readily to understand the relations of the leaves. The divergences may also be represented diagrammatically on a horizontal projection of the vertical axis, as in fig. 33. Here the outermost circle represents a section of that portion of the axis bearing the lowest leaf, the innermost represents the highest. The broad dark lines represent the leaves, and they are numbered according to their age and position. It will be seen at once that the leaves are arranged in orthostichies marked I.-V., and that these divide the circumference into five equal portions. But the divergence between leaf 1 and leaf 2 is equal to (2/5)ths of the circumference, and the same is the case between 2 and 3, 3 and 4, &c. The divergence, then, is 2/5, and from this we learn that, starting from any leaf on the axis, we must pass twice round the stem in a spiral through five leaves before reaching one directly over that with which we started. The line which, winding round an axis either to the right or to the left, passes through the points of insertion of all the leaves on the axis is termed the _genetic_ or _generating spiral_; and that margin of each leaf which is towards the direction from which the spiral proceeds is the _kathodic_ side, the other margin facing the point whither the spiral passes being the _anodic_ side.

[Illustration: FIG. 32.--Part of a branch of a Cherry with six leaves, the sixth being placed vertically over the first, after two turns of the spiral. This is expressed by two-fifths. a, The branch, with the leaves numbered in order; b, a magnified representation of the branch, showing the points of insertion of the leaves and their spiral arrangement.]

In cases where the internodes are very short and the leaves are closely applied to each other, as in the house-leek, it is difficult to trace the _generating spiral_. Thus, in fig. 34 there are thirteen leaves which are numbered in their order, and five turns of the spiral marked by circles in the centre (5/13 indicating the arrangement); but this could not be detected at once. So also in fir cones (fig. 35), which are composed of scales or modified leaves, the generating spiral cannot be determined easily. But in such cases a series of _secondary spirals_ or _parastichies_ are seen running parallel with each other both right and left, which to a certain extent conceal the genetic spiral.

The spiral is not always constant throughout the whole length of an axis. The angle of divergence may alter either abruptly or gradually, and the phyllotaxis thus becomes very complicated. This change may be brought about by arrest of development, by increased development of parts or by a torsion of the axis. The former are exemplified in many Crassulaceae and aloes. The latter is seen well in the screw-pine (_Pandanus_). In the bud of the screw-pine the leaves are arranged in three orthostichies with the phyllotaxis 1/3, but by torsion the developed leaves become arranged in three strong spiral rows running round the stem. These causes of change in phyllotaxis are also well exemplified in the alteration of an opposite or verticillate arrangement to an alternate, and vice versa; thus the effect of interruption of growth, in causing alternate leaves to become opposite and verticillate, can be distinctly shown in _Rhododendron ponticum_. The primitive or generating spiral may pass either from right to left or from left to right. It sometimes follows a different direction in the branches from that pursued in the stem. When it follows the same course in the stem and branches, they are _homodromous_; when the direction differs, they are _heterodromous_. In different species of the same genus the phyllotaxis frequently varies.

[Illustration: FIG. 33.--Diagram of a phyllotaxis represented by the fraction 2/5.]

All modifications of leaves follow the same laws of arrangement as true leaves--a fact which is of importance in a morphological point of view. In dicotyledonous plants the first leaves produced (the cotyledons) are opposite. This arrangement often continues during the life of the plant, but at other times it changes, passing into distichous and spiral forms. Some tribes of plants are distinguished by their opposite or verticillate, others by their alternate, leaves. Labiate plants have decussate leaves, while Boraginaceae have alternate leaves, and Tiliaceae usually have distichous leaves; Rubiaceae have opposite leaves. Such arrangements as 2/5, 3/8, 5/13 and 8/21 are common in Dicotyledons. The first of these, called a _quincunx_, is met with in the apple, pear and cherry (fig. 32); the second, in the bay, holly, _Plantago media_; the third, in the cones of _Picea alba_ (fig. 35); and the fourth in those of the silver fir. In monocotyledonous plants there is only one seed-leaf or cotyledon, and hence the arrangement is at first alternate; and it generally continues so more or less, rarely being verticillate. Such arrangements as ½, 1/3 and 2/3 are common in Monocotyledons, as in grasses, sedges and lilies. It has been found in general that, while the number 5 occurs in the phyllotaxis of Dicotyledons, 3 is common in that of Monocotyledons.

[Illustration: FIG. 34.--Cycle of thirteen leaves placed closely together so as to form a rosette, as in _Sempervivum_. A is the very short axis to which the leaves are attached. The leaves are numbered in their order, from below upwards. The circles in the centre indicate the five turns of the spiral, and show the insertion of each of the leaves. The divergence is expressed by the fraction (5/13)ths._]

[Illustration: FIG. 35.--Cone of _Picea alba_ with the scales or modified leaves numbered in the order of their arrangement on the axis of the cone. The lines indicate a rectilinear series of scales and two lateral secondary spirals, one turning from left to right, the other from right to left.]

In the axil of previously formed leaves leaf-buds arise. These leaf-buds contain the rudiments of a shoot, and consist of leaves covering a growing point. The buds of trees of temperate climates, which lie dormant during the winter, are protected by scale leaves. These scales or protective appendages of the bud consist either of the altered laminae or of the enlarged petiolary sheath, or of stipules, as in the fig and magnolia, or of one or two of these parts combined. These are often of a coarse nature, serving a temporary purpose, and then falling off when the leaf is expanded. They are frequently covered with a resinous matter, as in balsam-poplar and horse-chestnut, or by a thick downy covering as in the willow. In plants of warm climates the buds have often no protective appendages, and are then said to be _naked_.

[Illustration: FIG. 36.--Circinate vernation.

FIG. 37.--Transverse section of a conduplicate leaf.

FIG. 38.--Transverse section of a plicate or plaited leaf.

FIG. 39.--Transverse section of a convolute leaf.

FIG. 40.--Transverse section of an involute leaf.

FIG. 41.--Transverse section of a revolute leaf.]

[Illustration: FIG. 42.--Transverse section of a bud, in which the leaves are arranged in an accumbent manner.

FIG. 43.--Transverse section of a bud, in which the leaves are arranged in an equitant manner.

FIG. 44.--Transverse section of a bud, showing two leaves folded in an obvolute manner. Each is conduplicate, and one embraces the edge of the other.

FIG. 45.--Transverse section of a bud, showing two leaves arranged in a supervolute manner.]

The arrangement of the leaves in the bud is termed _vernation_ or _prefoliation_. In considering vernation we must take into account both the manner in which each individual leaf is folded and also the arrangement of the leaves in relation to each other. These vary in different plants, but in each species they follow a regular law. The leaves in the bud are either placed simply in apposition, as in the mistletoe, or they are folded or rolled up longitudinally or laterally, giving rise to different kinds of vernation, as delineated in figs. 36 to 45, where the folded or curved lines represent the leaves, the thickened part being the midrib. The leaf taken individually is either folded longitudinally from apex to base, as in the tulip-tree, and called _reclinate_ or _replicate_; or rolled up in a circular manner from apex to base, as in ferns (fig. 36), and called _circinate_; or folded laterally, _conduplicate_ (fig. 37), as in oak; or it has several folds like a fan, _plicate_ or _plaited_ (fig. 38), as in vine and sycamore, and in leaves with radiating vernation, where the ribs mark the foldings; or it is rolled upon itself, _convolute_ (fig. 39), as in banana and apricot; or its edges are rolled inwards, _involute_ (fig. 40), as in violet; or outwards, _revolute_ (fig. 41), as in rosemary. The different divisions of a cut leaf may be folded or rolled up separately, as in ferns, while the entire leaf may have either the same or a different kind of vernation. The leaves have a definite relation to each other in the bud, being either opposite, alternate or verticillate; and thus different kinds of vernation are produced. Sometimes they are nearly in a circle at the same level, remaining flat or only slightly convex externally, and placed so as to touch each other by their edges, thus giving rise to _valvate_ vernation. At other times they are at different levels, and are applied over each other, so as to be _imbricated_, as in lilac, and in the outer scales of sycamore; and occasionally the margin of one leaf overlaps that of another, while it in its turn is overlapped by a third, so as to be _twisted_, _spiral_ or _contortive_. When leaves are applied to each other face to face, without being folded or rolled together, they are _appressed_. When the leaves are more completely folded they either touch at their extremities and are _accumbent_ or _opposite_ (fig. 42), or are folded inwards by their margin and become _induplicate_; or a conduplicate leaf covers another similarly folded, which in turn covers a third, and thus the vernation is _equitant_ (fig. 43), as in privet; or conduplicate leaves are placed so that the half of the one covers the half of another, and thus they become _half-equitant_ or _obvolute_ (fig. 44), as in sage. When in the case of convolute leaves one leaf is rolled up within the other, it is _supervolute_ (fig. 45). The scales of a bud sometimes exhibit one kind of vernation and the leaves another. The same modes of arrangement occur in the flower-buds.

Leaves, after performing their functions for a certain time, wither and die. In doing so they frequently change colour, and hence arise the beautiful and varied tints of the autumnal foliage. This change of colour is chiefly occasioned by the diminished circulation in the leaves, and the higher degree of oxidation to which their chlorophyll has been submitted.

Leaves which are articulated with the stem, as in the walnut and horse-chestnut, fall and leave a scar, while those which are continuous with it remain attached for some time after they have lost their vitality. Most of the trees of Great Britain have deciduous leaves, their duration not extending over more than a few months, while in trees of warm climates the leaves often remain for two or more years. In tropical countries, however, many trees lose their leaves in the dry season. The period of defoliation varies in different countries according to the nature of their climate. Trees which are called evergreen, as pines and evergreen-oak, are always deprived of a certain number of leaves at intervals, sufficient being left, however, to preserve their green appearance. The cause of the fall of the leaf in cold climates seems to be deficiency of light and heat in winter, which causes a cessation in the functions of the cells of the leaf. The fall is directly caused by the formation of a layer of tissue across the base of the leaf-stalk; the cells of this layer separate from one another and the leaf remains attached only by the fibres of the veins until it becomes finally detached by the wind or frost. Before its fall the leaf has become dry owing to loss of water and the removal of the protoplasm and food substances to the stem for use next season; the red and yellow colouring matters are products of decomposition of the chlorophyll. Inorganic and other waste matters are stored in the leaf-tissue and thus got rid of by the plant. The leaf scar is protected by a corky change (suberization) in the walls of the exposed cells. (A. B. R.)