BOTANICAL SUBJECTS. 135 



natures. These undulations of the margin have been given various 

 names, according to their size and form. We have the terms 

 serrate, serrulate, dentate or toothed, crenate or scalloped, repancl 

 or undulate, sinuate, incised, and so forth. What apparentlv are 

 more different in nature than a rose, a holly, and a chestnut tree ? 

 Yet the margins of the leaves of all three are dentate or serrate, 

 the serrations being of different size. 



I am not aware that anvone has tried to work out the genesis 

 of this feature, so general in leaves. In the " Cultivated Oranges 

 and Lemons of India and Ceylon " (p. 177), I threw out the notion 

 drawn from other soitrces, that the crenations and serrations of 

 Citrus leaves meant leaflets or branches, but then I did not know 

 that in seaweeds I could find the cradle of a feature so common in 

 the leaves of ph^enogams. Seaweeds must have been the forms 

 of j)lants, from which all our land plants have descended. 



I feel certain that the serrations in the seaweed cladophyl are 

 due to atrophy of branches, which in these ancestral plants were 

 often given off from the margins of the cladophyl, as in Fucus 



Fig. 23. Caulerpa taxifolin (var.), A^^ (" Haney's Thyc. Austr.," pi. 178)— 

 part of a froud emerging from a rhizome. 



palmatus, marginijier (Fig. 20), and Lenormandia marginafa 

 (Fig. 22). In Caulerpa taxifoUa (Fig. 23) and Ptilota 

 plumosa, Ag.,"* we actually find the simple di^dsions of the 

 margin transformed into more compound branches. And in 

 I>esmarp!itia ligulata (Fig. 12G), we find even the minute " .«;pine- 

 like teeth" transformed into branches. So that branches and 

 teeth may be regarded as homologous organs. 



* " Harvey's Pbyc. Brit.»" pi. 80. 



