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 [Pg 40] 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)