Page:Hardwicke's Science-Gossip - Volume 1.pdf/71

1, 1865.] of progression or development all throughout nature, and beautifully marked upon a branch of Araucaria, or the cone of a fir. The volute may be greater or it may be less, and the investigations of Braun and Henslow have reduced the spiral regulation of leaves to mathematical precision. It is well known that the leaf upon the stem or branch originates in a node, the leaves being "alternate" when each node produces a single leaf on different sides in succession; and "opposite" when two leaves originate in a node and stand out face to face. Two spirals are in the latter case to be considered as passing up the stem; and in the case of three or more leaves from the same node forming a whorl, the spiral is held to be reduced to a circle. The stems of twinning-plants, like honeysuckle and convulvus, evince the disposition of the entire structure to take the spiral twist. The leaves of banana exhibit the same tendency, in common with the leaves of many plants while in the bud, and in common also with tendrils and similar modifications of leaves which follow the same law, and even with the flower-stalk in Cyclamen, and the seed-vessel in Streptocarpus. In the ripe strawberry the numerous small pistils, erroneously termed seeds, dotted over its surface follow the spiral arrangement; for as the soft juicy part of the strawberry is only an enlarged foot-stalk, the numerous minute pistils or seed-vessels which it bears are arranged according to the law which regulates the position of leaves. The peas in the interior of a pod alternate with each other on the opposite sides on the true spiral arrangement, and even the seeds in the seed-vessel of the wallflower, though far more numerous; nay, those in the common foxglove (Digitalis), which are more abundant still.

The leaf of every tree possesses individuality. Its aggregate of leaves make up the character by which the tree is known. No doubt the tree possesses also what arboriculturists term its own "habit," and by that in some measure it is known and recognised. Nevertheless, the leaf is its characteristic element and emblem, and in all probability imparts the habit in question. In the following figures we have the cherry leaf and the leaves of the beech, the lime, and the poplar (figs. 7, 8, 9, 10):—And it has been ascertained from minute comparisons that the leaf is to a great extent a model of the tree. For instance, the beech leaf, it will be seen (fig. 7), has little or no leaf-stalk; now, the beech tree in its growth is naturally feathered, from its base, with branches. Accordingly the leaves of the beech, the oak, the elm, the box, the holly, the laurel, the bay, arbutus, laurustinus, privet, and snowberry, whose natural tendency is to cover the ground from the base, have

either a very short petiole or none at all. The beech leaf, too, like its branches, will be observed to throw out a venation nearly parallel—the angle being from 45° to 50°, whilst

the midrib is rather inclined to zigzag. The cherry leaf (fig. 8), and the leaves of the lime and poplar (figs. 9 and 10), are endowed with leaf-stalks, and represent trees with a bare trunk, as do the leaves of the apple, the pear, the birch, the chestnut, and sycamore. The lime leaf (fig. 9) affords an illustration of the