Leaf

The pitchers of insectivorous plants are formed either by petioles or by laminae, and they are composed of one or more leaves. In Sarracenia and Heliamphora the pitcher is composed of the petiole of the leaf. In Nepenthes the pitcher is a modifica tion of the lamina, the petiole often plays the part of a tendril, while the leaf base is flat and leaf-like.

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—cataphyl lary leaves; they are produced abundantly upon underground shoots. In parasites (Lathraea, Orobanche) and in plants growing on decaying vegetable matter (saprophytes), in which no chloro phyll 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, phyllodes 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 pro duced 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 de veloped are known as seed leaves or cotyledons. The arrangement of the leaves on the axis and its appendages is called phyllotaxis. 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 two 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. 18) ; 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 suc cessive 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. 17)'. 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. 18). 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 coin cide they form a straight row or orthostichy. On every axis there are usually two or more orthostichies. In fig. 19 the leaves arise from nodes and the nodes are separated from each other by inter nodes, the arrangement resulting in one leaf being placed above the other. In this case, then, there are two orthostichies, and the ar rangement is said to be distichous. When the fourth leaf is di rectly above the first, the arrangement is tristichous. The same arrangement continues throughout the branch, so that in the lat ter case the 7th leaf is above the 4th, the loth 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 reach ing the leaf directly above that from which the enumeration com menced. If this arrangement is expressed by a fraction, the numer ator 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. 20 A, B, the leaf cycle consists of five leaves, the 6th leaf being placed vertically over the 1st, the 7th over the second and so on; while the number of turns between the 1st and 6th leaf is two; hence this arrangement is indicated by the fraction 5. In other words, the distance or divergence between the first and second leaf, expressed in parts of a circle, is s of a circle or X i= In fig. 19 the spiral is 1, 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, 36o° x ---- 18o°.