Page:Popular Science Monthly Volume 27.djvu/501

 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 observations on record.

Ajuga chamæpitys, 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 nymphœoides, 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 Circæa, the enchanter's nightshade; Ranunculus 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 follow the great orbicular leaves.

Another interesting case is that in which 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 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 external force,