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THE POPULAR SCIENCE MONTHLY. 



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 charac- 

 terized 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, Eiipleurum fructi- 

 cosum, is a shrub attaining a height of more than six feet, and has the 

 leaves (Fig. 23) coriaceous, and oblong -lanceolate. 



flk 1 



Fig. 23. 



The common groundsel (Fig. 24), again, is a low herb w r ith much 

 cut leaves. Some species of jSenecio, 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, 0. laureola, with 

 leaves like those of a laurel. 



