Forms of Leaves

Abstract

IN a recent issue of NATURE, in the discussion on the forms of leaves, Mr. Henslow seems to doubt the assertion of Sir John Lubbock that the holly produces prickly leaves on the lower branches, and smooth leaves without spines above; but this is a fact which may easily be verified in numerous localities (selected gardens varieties are of course not intended). I know of a large tree at Kew which altogether confirms the statement. The explanation, however, that the spines of the lower leaves may be produced to prevent animals from browsing on them, and that they are not developed on the upper branches because these are beyond the reach of animals, seems to me to require some modification, if not to be given up altogether, in this limited sense. It seems to me to admit of a much simpler explanation, namely, that it is an approximation—or reversion, if indeed the term be applicable—to the ancestral type. It is a well-known fact that in the embryonic stage of an organism the affinity with the ancestral type is best seen, and that in the mature stage the greatest amount of specialisation takes place; and, viewed in this light, the case of the holly does not appear to present much difficulty. A young seedling is seen to have very spiny leaves but with increasing age the leaves becoming comparatively spineless. In the case of the furze we have the most overwhelming evidence that the spiny character has been developed to repel the attacks of herbivorous animals, and a young seedling is seen to have trifoliate leaves—like the laburnam—from which we infer that its ancestral type was spineless, and had trifoliate leaves. The large group of phyllodineous Acacias bear an equally unmistakable stamp of their origin in the bipinnate leaves which the seedlings at first produce. In most cases these leaves are very early superseded by phyllodes, but in A. melanoxylon the habit of producing true leaves is never quite lost. There is a large tree of this species about 40 feet high at Kew, at the south end of the Temperate House, close to the spiral staircase. It is thus in an admirable position for examination. At the base of this tree the leaves predominate over the phyllodes, but in ascending the staircase the proportion is seen to gradually diminish, till at the top of the tree—a few feet above the gallery—scarcely a true leaf is to be seen. Assuming the mature stage to be the more highly specialised, we have in the holly a preisely parallel case. This necessarily involves the opinion that he ancestral type of the genus Ilex had spiny leaves; and, if so, it seems highly probable that the character was developed as a protection against the attacks of herbivorous animals. A possible objection which at first struck me was that many of the pecies have quite smooth leaves; but this has been removed by a search through the specimens in the Kew Herbarium. In the first place, species with spiny leaves occur in each great centre of distribution of the genus–in North and South America, India, China and Japan, the Atlantic Islands, as well as Europe—and in the second, although no seedling plants were found, there are three species which show very spiny leaves on barren branches, and smooth leaves on the more mature flowering branches. These are I. insignis and I. dipyrena, from India, and I. Perado, from the Atlantic Islands. I have little doubt that seedlings of many species would present the spiny character if we could only see them. The presence of spines—the nerves being extended beyond the margin of the leaf—seems to indicate an excess of vascular over cellular tissue; a condition which is either modified with increasing maturity or is not exhibited in the same phenomena. In any case a severe pruning—or reduction of the parts to be nourished—is followed by a temporary reversion to the more spiny character. If this explanation be the correct one the question naturally arises, Why are the hollies losing the property of producing spiny leaves? rather than, Why does the holly produce spiny leaves on its lower branches? The answer to the first query would perhaps be, Because they no longer need the protection afforded by the spines. To the second, Long-continued habits are not often instantly laid aside.