VI.] FLOATING AND SUBMERGED LEAVES. 131 



to subdivision in subaqueous leaves is due to the 

 absence or paucity of carbonic acid. I have ventured 

 to suggest a dififerent explanation. Of course it is 

 important to expose as large a surface as may be to 

 the action of the water. We know that the gills of 

 fish consist of a number of thin plates, which while in 

 water float apart, but have not sufficient consistence 

 to support even their own weight, much less any ex- 

 ternal force, 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 sub- 

 aqueous 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 characterised 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 



K 2 



