ON LEAVES. 483 



and consequently of ripening seeds. There would be a tendency, 

 therefore, according to the well-known principles of Mr. Darwin, to a 

 closer and closer resemblance. I am disposed to suggest whether 

 these resemblances may not serve as a protection, not only from 

 browsing quadrupeds, but also from leaf-eating insects. On this part 

 of the subject we have as yet, however, I think, no sufficient observa- 

 tions on record. 



Ajuga chamcepitys, the yellow bugle, has leaves crowded and 

 divided into three linear lobes, the lateral ones sometimes again 

 divided. They differ, therefore, greatly from those of its allies, and 

 this puzzled me much until one day I found it growing abundantly 

 on the Riviera among Euphorbia cyparissias, and I was much struck 

 by the curious likeness. The Euphorbia has the usual acrid juice 

 of the genus, and it struck me that the yellow ajuga was perhaps 

 protected by its resemblance. 



Leaves which float on the surface of still water tend to be orbicular. 

 The water-lilies are a well-known illustration. I may also mention 

 Limnanthenum nymph ceoides, which, indeed, is often taken for a 

 water-lily, though it really belongs to the family of gentians, and 

 Alisma natans, a species allied to the plantains. In running water, 

 on the contrary, leaves tend to become more or less elongated. 



Subaqueous leaves of fresh-water plants have a great tendency 

 either to become long and grass-like or Jo be divided into more or less 

 hair-like filaments. I might mention, for instance, Myriophyllum ; 

 Hippuris, or mare's-tail, a genus which among English plants comes 

 next to Circaea, the enchanter's nightshade ; 

 Mammculus aquatilis a close ally of the 

 buttercup ; and many others. 



Some, again, which, when mature, have 

 rounded, floating leaves, have long, narrow 

 ones when young. Thus in Victoria regia 

 the first leaves are filiform, then come one 

 or more which are sagittate, and then fol- 

 low the great orbicular leaves. 



Another interesting case is that in which 



Fig 2^ 



the same species has two forms of leaf (Fig. 



22) namely, more or less rounded ones on the surface, and a second 

 series which are subaqueous and composed of more or less linear or 

 finely divided segments. 



Mr. Grant Allen has suggested that this tendency to subdivision in 

 subaqueous leaves is due to the absence or paucity of carbonic acid. I 

 have ventured to suggest a different explanation. Of course it is im- 

 portant 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 consist- 

 ence to support even their own weight, much less any external force, 



