Page:Popular Science Monthly Volume 27.djvu/507

Rh forms of the leaves of climbing monocotyledons are in fact just such as would be obtained by widening more or less the linear, grass-like leaf which is so prevalent in the class.

This, then, raises the question whether the heart-shaped leaf is the older form from which the palmate type has been gradually evolved. Let us see whether we can find any evidence bearing on this question in what may be called the embryology of plants. The furze, with its spiny prickles, belongs to a group of plants which, as a general rule, have trifoliate or pinnate leaves. Now, if we examine a seedling furze (Fig. 33), we shall find that the cotyledons are succeeded by several trifoliate leaves, with ovate leaflets. These gradually become narrower, more pointed, and stiffer, thus passing into spines. Hence, we can hardly doubt that the present furze is descended from ancestors with trifoliate leaves. I have already referred to other cases in which the young plants throw light on the previous condition of the species (ante, p. 12).

Now we shall have no difficulty in finding cases where, while in mature plants the leaves are more or less lobed and palmate, the first leaves succeeding the cotyledons are heart-shaped. This would seem to point to the fact that when in any genus we find heart-shaped and lobed leaves, the former may represent the earlier or ancestral condition.

The advantage of the palmate form may perhaps consist in its bringing the center of gravity nearer to the point of support. Broad leaves, however, are of two types: cordate, with veins following the curvature of the edge; and palmate or lobed leaves, with veins running straight to the edge. The veins contain vascular bundles which conduct the nourishment sucked up by the roots, and it is clearly better that they should hold a straight course, rather than wind round in a curve. As the nourishing fluids pass more rapidly along these vascular bundles, the leaf naturally grows there more rapidly, and thus assumes the lobed form, with a vein running to the point of each lobe.

On the whole, we see, I think, that many at any rate of the forms presented by leaves have reference to the conditions and requirements of the plant. If there was some definite form told off for each species, then, surely, a similar rule ought to hold good for each genus. The species of a genus might well differ more from one another than the varieties of any particular species; the generic type might be, so to say, less closely limited; but still there ought to be some type characteristic of the genus. Let us see whether this is so. No doubt there are many genera in which the leaves are more or less uniform, but in them the general habit is also, as a rule, more or less similar. Is this