370 BOTANY OF THE LIVING PLANT 



the means are different. Plainly these are the results of parallel 

 evolution, or homoplasy. 



So also the conducting tissues seen in the stem of large Mosses, such 

 as Polytrichum, show in their connections with the leaves, as well as 

 in their construction of hydrom and leptom, similarities with the 

 conducting system of Vascular Plants (Fig. 299). But again the 

 comparison is between the gametophyte on the one hand and the 

 sporophyte on the other ; while the isolation of such phenomena in 

 the larger Mosses indicates that the conducting tissues are an adaptive 

 feature specially developed in them, and not general for all Mosses. 



There is also a very peculiar analogy between the flowers of Angio- 

 sperms and the so-called " flowers " of Mosses, where the perichaetial 

 leaves surround the sexual organs, as the perianth surrounds the 

 androecium and gynoecium in Flowering Plants. There is even 

 parallelism in the distribution of the sexes, for such " flowers " in 

 Mosses may be hermaphrodite or unisexual. Notwithstanding this 

 likeness it is necessary to keep clearly in mind that such comparisons 

 deal with essentially different things, though both involve the sex- 

 distinction (p. 360). The interest of them lies in the fact that the 

 similarity exists at all. 



Such comparisons show how nearly the evolution of the gameto- 

 phyte in the Bryophyta may follow along the same lines of adapta- 

 tion as the sporophyte of Vascular Plants. The end is the same for 

 both, viz. to develop on land as large a vegetative system as possible, 

 so as to provide material for the largest possible number of germs. 

 The one phylum has solved it by enlargement of the sporophyte, 

 which thus becomes the substantive " Plant " of Vascular types. 

 The other has solved it by elaboration of the gametophyte, which has 

 similarly become the substantive " Plant " of the Bryophytes. Both 

 have one foot in water and the other on land. In Vascular Plants 

 it is the sporophyte foot that is more firmly set on land. In the 

 Bryophytes it is the gametophyte foot that is more securely placed, 

 and the sporophyte is dependent upon it, not temporarily, but up to 

 the time of maturity of its spores. 



