Page:Popular Science Monthly Volume 27.djvu/498

480 the later years they gradually dry and wither; still, under these circumstances they naturally require special protection. They are, as a general rule, tough, and even leathery. In many species, again, as is the case with our holly, they are spinose. This serves as a protection from browsing animals; and in this way we can, I think, explain the curious fact that, while young hollies have spiny leaves, those of older trees, which are out of the reach of browsing animals, tend to become quite unarmed.

In confirmation of this I may also adduce the fact that while in the evergreen-oak the leaves on well-grown trees are entire and smooth edged like those of the laurel, specimens which are cropped and kept low form scrubby brushes with hard prickly leaves.

Mr. Grindon, in his "Echoes on Plant and Flower Life" (p. 30), says that "the occurrence of prickles only here and there among plants shows them to be unconnected with any general and ruling requirement of vegetation. We can only fall back upon the principle laid down at the outset, that they are illustrations of the unity of design in Nature, leading us away from the earth to Him who is 'the end of problems and the font of certainties.'" Surely, however, it is obvious that the existence of spines and prickles serves as a protection.

Another point of much importance in the economy of leaves is the presence or absence of hairs. I have already observed that most evergreens are glossy and smooth, and have suggested that this may be an advantage, as tending to prevent the adherence of snow, which might otherwise accumulate and break them down.

The hairs which occur on so many leaves are of several different types. Thus, leaves are called silky when clothed with long, even, shining hairs (silver-weed); pubescent or downy, when they are clothed with soft, short hairs (strawberry); pilose, when the hairs are long and scattered (herb-robert); villous, when the hairs are rather long, soft, white, and close (forget-me-not); hirsute, when the hairs are long and numerous (rose-campion); hispid, when they are erect and stiff (borage); setose, when they are long, spreading, and bristly (poppy); tomentose, when they are rather short, soft, and matted; woolly, when long, appressed, curly, but not matted (corn-centaury); velvety, when the pubescence is short and soft to the touch (foxglove); cobwebby, when the hairs are long, very fine, and interlaced like a cobweb (thistle, cobwebby houseleek). The arrangement of the hairs is also interesting. In some plants there is a double row of hairs along the stem. In the chickweed only one. This, perhaps, serves to collect rain and dew, and it is significant that the row of hairs is always opposite to the flower-stalk, which also has a single row. Now, the flower-stalk is for a considerable part of its life turned downward, with the row of hair outward. This, perhaps, may account for the absence of hairs on that side of the stem.