144 EXPLANATION OF FORMS OF LEAVES. [CHAP.

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

FIG. 94.-Passiflora cærulea. Seedling.

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

VI.] EXPLANATION OF FORMS OF LEAVES.

145

0

them the general habit is also, as a rule, more or less similar. Is this the case in genera where the various species differ greatly in habit? I have already incidentally given cases which show that this is not so, but let us take some group-for instance, the genus Senecio, to which the common Groundsel (Fig. 82) belongs, as a type well known to all of us -and look at it a little more closely.

The leaves of the common Groundsel I need not describe, because they are familiar to us all. This type occurs in various other species of more or less similar habit. On the other hand, the fen Senecio (S. paludosus) and the marsh Senecio (S. palustris), which live in marshy and wet places, have long, narrow, sword-shaped leaves, like those of so many other plants which are found in such localities. The field Senecio (S. campestris, Fig. 95), which lives in meadows and pastures, has a small terminal head of flowers springing from a rosette of leaves much like those of a common Daisy (Bellis perennis); a Madagascar species, as yet I believe unnamed, is even more like a Daisy. Senecio junceus looks much like a Rush; S. hypocharideus of South Africa strikingly resembles a Hypochæris, as its name denotes. A considerable number of species attain to a larger size and become woody, so as to form regular bushes; S. buxifolius has very much the general look of a Box, S. vagans of a Privet, S. laurifolius of a Laurel, ericafolius of a Heath, pinifolius of a Fir, or rather, a Yew.

Again, some species are climbers; S. scandens and S. macroglossus have leaves like a Bryony;

L

146 TREE SPECIES. SUCCULENT SPECIES. [CHAP.

S. araneosus and S. tamoides like a smilax or tamus (Yam); S. tropaolifolius like a tropaolum.

Among the species inhabiting hot, dry regions are some with swollen fleshy leaves, such as S. haworthii, from the Cape of Good Hope, and S. pteroneura, from Mogador. Senecio rosmarinifolius, of the Cape, is curiously like a Rosemary or Lavender. Lastly, some species may almost be called small trees, such

as S. populifolius, with leaves like a Poplar; and S. amygdaloides, like an Almond.

I might mention, if space permitted, many other species which, as their names denote, closely resemble forms belonging to other groups-such, for instance, as Senecio lobelioides, erysimoides, bupleurioides, verbascifolius, juniperinus, ilicifolius, acanthifolius, linifolius, platanifolius, graminifolius, verbenefolius,

VI.]

CONCLUSION.

147

rosmarinifolius, coronopifolius, chenopodifolius, lavanderiæ folius, salicifolius, mesembryanthemoides, digitalifolius,abietinus,arbutifolius,malvæfolius, erodiifolius, halimifolius, hakeæfolius, resedæfolius, hederæfolius, acerifolius, plantigineus, castaniæfolius, spiræifolius, bryoniæfolius, primulifolius, and many more. These names, however, indicate similarities to over thirty other perfectly distinct families.

It seems clear, then, that these differences have reference not to any inherent tendency, but to the structure and organisation, the habits and requirements, of the plant. Of course it may be that the present form has reference not to existing, but to ancient, conditions, which renders the problem all the more difficult. Nor do I at all intend to maintain that every form of leaf is, or ever has been, necessarily that best adapted to the circumstances, but only that they are constantly tending to become so, just as water always tends to find its own level.

But, however this may be, if my main argument is correct, it opens out a very wide and interesting field of study, for every one of the almost infinite forms of leaves must have some cause and explanation.

THE END.