ON LEAVES. 



489 



Fig. 33. 



forms of the leaves of climbing monocotyledons are in fact just such 

 as would be obtained by widening more or less the linear, grass-like leaf 

 which is so prevalent in the class. 



This, then, raises the question whether the heart-shaped leaf is the 

 older form from which the palmate type has been gradually evolved. 

 Let us see whether we can find any evidence 

 bearing on this question in what may be called 

 the embryology of plants. The furze, with its 

 spiny prickles, belongs to a group of plants which, 

 as a general rule, have trifoliate or pinnate leaves. 

 Now, if we examine a seedling furze (Fig. 33), we 

 shall find that the cotyledons are succeeded by sev- 

 eral trifoliate leaves, with ovate leaflets. These 

 gradually become narrower, more pointed, and 

 stiffer, thus passing into spines. Hence, we can 

 hardly doubt that the present furze is descended 

 from ancestors with trifoliate leaves. I have al- 

 ready referred to other cases in which the young 

 plants throw light on the previous condition of 

 the species {ante, p. 12). 



Now we shall have no difficulty in finding cases 

 where, while in mature plants the leaves are more 

 or less lobed and palmate, the first leaves succeed- 

 ing the cotyledons are heart-shaped. This would seem to point to the 

 fact that when in any genus we find heart-shaped and lobed leaves, the 

 former may represent the earlier or ancestral condition. 



The advantage of the palmate form may perhaps consist in its 

 bringing the center of gravity nearer to the point of support. Broad 

 leaves, however, are of two types : cordate, with veins following the 

 curvature of the edge ; and palmate or lobed leaves, with veins running 

 straight to the edge. The veins contain vascular bundles which con- 

 duct the nourishment sucked up by the roots, and it is clearly better 

 that they should hold a straight course, rather than wind round in a 

 curve. As the nourishing fluids pass more rapidly along these vascular 

 bundles, the leaf naturally grows there more rapidly, and thus assumes 

 the lobed form, with a vein running to the point of each lobe. 



On the whole, we see, I think, that many at any rate of the forms 

 presented by leaves have reference to the conditions and requirements 

 of the plant. If there was some definite form told off for each species, 

 then, surely, a similar rule ought to hold good for each genus. The 

 species of a genus might well differ more from one another than the 

 varieties of any particular species ; the generic type might be, so to 

 say, less closely limited ; but still there ought to be some type charac- 

 teristic of the genus. Let us see whether this is so. No doubt there 

 are many genera in which the leaves are more or less uniform, but in 

 them the general habit is also, as a rule, more or less similar. Is this 



