ON LEAVES. 



35i 



somewhat to overlap ; but it must be remembered that in temperate 

 regions the sun is never vertical. Moreover, while alternate leaves are 

 more convenient in such an arrangement as that of the beech, where 

 there would be no room for a second leaf, it is more suitable in such 

 cases as the sycamores and maples that the leaves should be opposite, 

 because, if, other things remaining the same, the leaves of the sycamore 

 were alternate, the sixth leaf would require an inconvenient length of 

 petiole. 



Perhaps it will be said that the plane-tree, which has leaves so 

 like a maple that one species of the latter genus is named after it 

 {Acer platanoides), has, nevertheless, alternate leaves. In reality, 

 however, I think this rather supports my argument, because the 

 leaves of the plane, instead of being at right angles to the stem, lie 

 more nearly parallel with it. Moreover, as any one can see, the leaves 

 are not arranged so successfully with reference to exposure as those of 

 the species we have hitherto been considering, perhaps because, living 

 as it does in more southern localities, the economy of sunshine is less 

 important than in more northern regions. 



The shoot of the horse-chestnut is even stouter than that of the 

 sycamore, and has a diameter below the sixth leaf of no less than three 

 tenths of an inch. With this increase of strength is, I think, con- 

 nected the greater size of the leaves, which attain to as much as eight- 

 een inches in diameter, and this greater size, again, has perhaps led to 

 the dissection of the leaves into five or seven distinct segments, each 

 of which has a form somewhat peculiar in itself, but which fits in 

 admirably with the other leaflets. However this may be, we have in 

 the horse-chestnut, as in the sycamores and maples, a beautiful dome 

 of leaves, each standing free from the rest, and expanding to the fresh 

 air and sunlight a surface of foliage in proportion to the stout, bold 

 stem on which they are borne. 



Now, if we place the leaves of one tree on the branches of another, 

 we shall at once see how unsuitable they would be. I do not speak 

 of putting a small leaf such as that of a beech on a large-leaved tree 

 such as the horse-chestnut ; but if we place, for instance, beech on 

 lime, or vice versa, the contrast is sufficiently striking. 



The lime-leaves would overlap one another, while, on the other 

 hand, the beech-leaves would leave considerable interspaces. Or let 

 us in the same way transpose those of the Spanish chestnut ( Castanea) 

 and those of Acer platanoides, a species of maple. I have taken 

 specimens in which the six terminal leaves of a shoot of the two 

 species occupy approximately the same area. Figs. 4 and 11 show the 

 leaves in their natural position, those of Castanea lying along the stalk, 

 while those of Acer are ranged round it. In both cases it will be 

 seen that there is practically no overlapping, and very little waste of 

 space. In Castanea the stalks are just long enough to give a certain 

 play to the leaves. In Acer they are much longer, bringing the leaves 



