MUSCI AND HEPATICAF 477 



gametophyte. In the Bryophytes the second alternative has b< 

 fully exploited. Their characters depend upon the development 

 of the gametophyte to the highest condition in which it is seen in 

 Land Vegetation. The details of this development run parallel 

 with those of the sporophyte in Vascular Plants, so that the two 

 present a series of analogies. The most striking are seen in the 

 organs of photosynthesis, in the conducting tracts, and in the grouping 

 of the organs of sex. In the sporophyte of Vascular Plants the 

 typical photosynthetic organ is the leaf-blade, with its ventilated 

 mesophyll and stomatal control. In the gametophyte of Mosses 

 and Liverworts a similarly ventilated structure is seen in the leaves 

 of some of the larger Mosses (Fig. 359), and in the thallus-structure 

 of the Marchantiales (Fig. 368). These are, however, parts of the 

 gametophyte, and the ventilated structure is here produced mainly 

 by involution of the outer surface, while in Vascular Plants it 

 arises from intercellular splitting of the cell-walls. The physio- 

 logical end is the same in both cases, but the place and 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. 358). 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. Again the similarity of structure to that seen in Vascular 

 Plants must be held as homoplastic. 



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. 256). 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- 



