and consequently collapse in air. The same thing happens with thin, finely cut leaves. In still water they afford the greatest possible extent of surface with the least expenditure of effort in the formation of skeleton. This is, I believe, the explanation of the prevalence of this form in subaqueous leaves.
Again, in still air the conditions, except so far as they are modified by the weight, would approximate to those of water; but the more the plant is exposed to wind the more would it require strengthening. Hence, perhaps, the fact that herbs so much oftener have finely cut leaves than is the case with trees. In the Umbellifers, for instance, almost all the species have the leaves much divided—more, I need hardly say, than is the case with trees. Shrubs and trees are characterized by more or less entire leaves, such as those of the laurel, beech, hornbeam, lime, or by similarly shaped leaflets, as in the ash, horse-chestnut, walnut.
There are, however, many groups of plants which, while habitually herbaceous, contain some shrubby species, or vice versa. Let us take some groups of this description in which the herbaceous species have their leaves much cut up, and see what is the character of the foliage in the shrubby species.
The vast majority of Umbellifers, as I have just observed, are herbaceous, and with leaves much divided, the common carrot being a typical example. One European species, however, Bupleurum fructicosum, is a shrub attaining a height of more than six feet, and has the leaves (Fig. 23) coriaceous, and oblong-lanceolate.
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| Fig. 23. | Fig. 24. | |
The common groundsel (Fig. 24), again, is a low herb with much cut leaves. Some species of Senecio, however, are shrubby, and their leaves assume a totally different character, Senecio laurifolius and S. populifolius having, as their specific names denote, leaves respectively resembling the laurel and poplar. In the genus Oxalis, again, to which the shamrock belongs, there is a shrubby species, O. laureola, with leaves like those of a laurel.
