Page:Popular Science Monthly Volume 27.djvu/368

352 approximately to the same level, and carrying the lower and outer ones free from the upper and younger ones.

Now, if we arrange the Spanish-chestnut leaves round a center, as in Fig. 12, it is at once obvious how much space is wasted. On the other hand, if we attach the leaves of the Acer to the stalk of Castanea at the points from which the leaves of Castanea came off, as in Fig. 13,



we shall see that the stalks are useless, and even mischievous, as a cause of weakness and of waste of space; while, on the other hand, if we omit the stalks, or shorten them to the same length as those of Castanea, as in Fig. 14, the leaves would greatly overlap one another.

Once more, for leaves arranged as in the beech the gentle swell at the base is admirably suited; but in a crown of leaves, such as those of the sycamore, space would be wasted, and it is better that they should expand at once as soon as their stalks have borne them free from those within. Moreover, the spreading lobes leave a triangular space (Fig. 11) with the insertion of the stalk at the apex, which seems as if expressly designed to leave room for the pointed end of the leaf within.

Hence we see how beautifully the whole form of these leaves is adapted to the mode of growth of the trees themselves and the arrangement of their buds.

Before we proceed to consider the next series of species to which I wish to direct attention, it will be necessary for me to say a few words on the microscopical structure of the leaf. Although so thin, the leaf consists of several layers of cells. Speaking roughly, and as a general rule, we may say that on each side is a thin membrane, or epidermis, underneath which on the upper side are one or more layers of elongated