The veins.—The leaf is kept taut by a number of branching ribs, somewhat as the silk of an opened umbrella is stretched tightly by the ribs. The ribs or “veins” of the leaf run beneath the skin, but are generally nearer the lower surface than the upper, and are easily seen when the leaf is turned over. If a leaf is allowed to rot in a little soft water, the skin and the soft green stuff of the interior decay and leave these veins as a white “skeleton” ([Fig. 23]). The process may be assisted by gently brushing the leaf from time to time. A skeleton leaf may be obtained still more quickly by putting the leaf in a weak solution of bleaching powder until the skin and interior are soft enough to be brushed away. Care should be taken to use a weak solution, or the veins also will be rotted. The skeleton should be well washed in water before drying.

The arrangement of the veins in a leaf varies widely, but it falls broadly into two classes, according as the main veins run parallel or nearly parallel to each other ([Fig. 24]), or form a less regular network ([Fig. 23]). The venation of a leaf is curiously associated with the number of cotyledons possessed by the seedling; for nearly all dicotyledons ([p. 23]) have net-veined leaves, while the leaves of monocotyledons are almost invariably parallel-veined. Careful drawings of several typical leaves should be made, and the principal veins indicated on them.

Fig. 23.—Net-veined leaf of
a dicotyledon (White Thorn).
(× ½)		 Fig. 24.—Parallel venation of
the leaf of a monocotyledon
(Solomon’s Seal). (× 1)

The shapes of leaves.—Although the blade of a leaf is most commonly flattened, and roughly oval in outline, there are several exceptions. The leaves of pine, spruce, larch, and yew are needle-shaped; those of grasses (Figs. 102-110) are very long in proportion to their width; while the leaves of many moorland plants are rolled up into hollow cylinders. There is some reason—could we find it—for every such variation, and the significance of some of these shapes will be referred to later ([p. 47]). When the dimensions of leaves are carefully measured, the proportion of the length to the width will be found to vary much in the leaves of different plants, but will be found to be pretty constant for the same sort of plant. This holds good, too, for the position of the greatest width (e.g. at, above, or below, the middle of the blade), the form of the apex of the blade (blunt, pointed, spiny, or rounded), the nature of the margin (smooth and “entire,” hairy, saw-edged, doubly saw-edged, lobed, etc.), and the extent and positions of the larger indentations. Thus, while any particular elm leaf ([Fig. 124]) is probably slightly different from every other elm leaf in the world, it resembles every other elm leaf more than it does any leaf from any other plant than an elm. No leaf of this shape ever grows on an oak tree or a sycamore. Thus, in spite of minor variations there is a wonderful conformity to type, and the student will find that by carefully examining the shape, venation, margin, apex, etc., of all his leaves, and above all by drawing them, he will soon be able to recognise them at sight. It is by doing this and noticing in each case the methods of folding and arrangement of young leaves in the bud that it may be possible in the future to explain some variations which are at present not understood. It has already been seen that the peculiar forms of the leaves known as cotyledons are associated to some extent with the shape and size of the seeds containing them, and with the amount, if any, of the food stored in them.

Simple and compound leaves.—Blackberry leaves ([Fig. 76]) well repay close examination. Some of the leaves on the bush will be found to be simple—having one blade only on the leaf stalk. Here and there, however, a leaf may be discovered which is so deeply cut into along one side, that it is almost completely divided into two leaflets; and other leaves will easily be found which consist of three or five leaflets, much resembling the leaves of the rose ([Fig. 25]), a near relative of the blackberry. Here, then, we have a plant which produces simple or compound leaves according to its needs. It seems as if the blackberry were still trying, as an experiment, a device which the rose tree has found so advantageous as to have adopted for good. Some other plants, the ash, for example, have compound leaves broadly similar to the rose leaf—the leaflets springing in pairs from the sides of the midrib.

Fig. 25.—Compound leaf of the Rose.
L, leaflets; P, leaf-stalk;
st, stipules. (× ½)		 Fig. 26.—Compound leaf of the Horse Chestnut. (× ⅙)