Ths Forms of Leaves

Abstract

THERE are several points in Sir John Lubbock's lecture (NATURE, February 26, p. 398) which seem to invite some little criticism. That “ the size of the leaf . . . is regulated mainly with reference to the thickness of the stem ” seems somewhat self-evident, as a large leaf must have a large stem to carry it, as, e.g., may be seen by comparing the slender shoot of a Dcodar with a cabbage-stalk ; but he adds: “ The size once determined exercises much influence on the form.” This is a deduction which seems to require verification. Sir John gives the area of a beech-leaf as about 3 square inches, but the form remains the same whatever the size. Size rather depends on vigorous growth, as in the following instances: Populus alba leaves on a vigorous basal shoot were 61/2 × 31/2 inches, the diameter of the shoot being 1/8inch; on the upper branches of the same tree many leaves were only 11/2 to 21/2 inches long, the diameter of the shoot being also inch. Similarly growing oak leaves of the same shape were 6 ×3 inches and 2×3/4 inches respectively. An Ancuba japonica bore rounded leaves on a basal shoot 4 × 31/4 inches, but those on the stem were 4 ×1 inch. In this case, as in other plants with (normally) dimorphic leaves, as ivy, it is difficult to see what connection there is between size and form. Indeed leaves of every degree of superficial area can be found amongst the lobed ones on the climbing stem of ivy, and the entire ones of the flowering branch. Sir John adds that “the form of the inner edge [of the beech] . . . decides that of the outer one.” He does not seem to have verified this deduction. The two edges are symmetrical in this leaf, but they are not so in the elm and lime. How will the inner edge explain the cause of their obliquity? If, however, the buds of the lime be examined, a more probable cause (as it seems to me) will be discovered in the conditions of development. He describes the Eucalyptus, when young, as having “horizontal leaves, which in older ones are replaced by scimitar-shaped phyllodes.” Bentham and Hooker say of Eucalyptus: “Folia in arbore juniore sæpe opposita, in adulto pleraque alterna,” but makes no mention of phyllodes. Speaking of evergreen leaves, he says: “Glossy leaves have a tendency to throw [snow] off, and thus escape, hence evergreen leaves are very generally smooth and glossy.” This sentence appears to imply that such leaves are glossy in anticipation of snow! a deduction which certainly requires verification. Again: “Evergreen leaves often have special protection ... by thorns and spines. Of this the holly is a familiar illustration; and it was pointed out that in old plants above the range of browsing quadrupeds, the leaves tend to lose their spines and become unarmed.” The inference the reader draws from this is that when the holly grows out of reach of browsing animals it has no necessity to produce prickly leaves, and so changes them accordingly, thereby implying that unarmed leaves were in some way preferable. This is another instance of deductive reasoning which requires verification, for it seems to be attributing to the holly a very unexpected process of ratiocination! But it is not at all usual for hollies to do this. I have several from six to nearly twenty feet high, and not one has borne an unarmed leaf. Though my cows do not touch a holly hedge, yet one young bush lately planted has taken their fancy, and they have bitten it all to pieces. On the other hand one bush (in the garden), a variety with unarmed foliage, occasionally throws out a branch with prickly leaves, though the cows are not admitted where it grows.