This is just one example of what usually happens in the history of plants with compound leaves, or leaves with any special shape; the young seedling’s earlier leaves are much more simple than the later ones. You should collect as many seedlings as possible and make drawings of them if you can, to show the various stages leaves pass through before reaching the full-grown complex form.
Fig. 55. Skeleton of a leaf, showing the fine network of the small veins.
Now let us look again at the actual structure of leaves. Hold up those of the rose, or lilac, or lime tree to the light, and look at the “veins” running in them. There is a chief central vein or mid-rib, and from it a number of side branches come off and divide and branch again and again till they form a fine net-work throughout the whole of the leaf blade (see fig. 55). If you now look at a grass or lily leaf, you will find that there are very many veins about equally important, running from end to end of the leaf and remaining nearly parallel to each other. This difference between parallel veins and net-work (or reticulate) veins is quite important, and is one of the characters which help to separate two very big families of flowering plants (see Chapter XXIII).
Fig. 56. Alternating pairs of leaves of the Dead Nettle.
