482 BOTANY PART n 



the higher Green Algae and Characeae on the other, no transitional forms are 

 known. Morphological comparison points rather to a connection between the 

 Bryophyta and the Brown Algae, the multilocular gametangia of which (in some 

 genera already differentiated into oogonia and antheridia) may be regarded as 

 homologous structures leading to the archegonia and antheridia of the Arche- 

 goniatae. Thus the antheridium of the lower Liverworts shows a cellular con- 

 struction in agreement with that of the gametangia of Brown Algae (cf. Figs. 434, 

 354, 356) ; it is distinguished by the possession of a sterile, protective layer of 

 cells forming the wall, and the differentiation of this can be regarded as an 

 adaptation to a terrestrial mode of existence. Further, among the Brown Algae, 

 in Dictyota. there is an alternation of generations agreeing with that of Bryophyta, 

 although the gametophyte and sporophyte are similar in their vegetative structure. 

 The tetrasporangia of the sporophyte of Dictyota correspond to the spore-mother - 

 cells of the sporophyte of the Bryophyta ; their endogenous position in the latter 

 may be related to the influence of a terrestrial mode of life. While the form of 

 the gametophyte in the thalloid Liverworts shows many points in common witli 

 the thallus of certain Brown Algae, the sporophyte of the Bryophyta proceeds 

 early to the development of its spores, and ceases growth without a .segmenta- 

 tion into vegetative organs. It thus becomes essentially different from the 

 gametophyte ( 98 ). 



With the exception of a few forms which have secondarily assumed an aquatic 

 life, the Bryophyta in contrast to the Algae are land-plants and exhibit corre- 

 sponding adaptations in their structure. Thus all the above-ground parts are 

 covered with a cuticle. The small size of the Bryophyta as compared with 

 Pteridophyta stands in connection with their simple cellular construction from 

 which true vessels are absent. True roots are also wanting. Some are minute 

 plants, while the largest Mosses, represented by the Dawsonieae of New Zealand, 

 have leafy stems attaining a height of 50 cm. 



The two very distinct classes of Bryophytes may be briefly charac- 

 terised as follows : 



1. Hepaticae (Liverworts). The sexual generation, with poorly 

 developed and generally not distinctly differentiated proton ema, is 

 either a dichotomously-divided thallus or is developed as a leafy and, 

 with few exceptions, dorsiventral shoot. In the majority of Hepaticae, 

 in addition to spores, the capsule produces also elaters ; only in one 

 order, Anthoceroteae, does the capsule have a columella. 



2. Musci (Mosses). The protonema of the sexual generation is 

 usually well developed and distinctly defined, and the moss plant is 

 always segmented into stem and leaves. The leaves are arranged 

 spirally in polysymrnetrical, less frequently in bisymmetrical, rows. 

 The capsule is always without elaters, but with a columella. 



Fossil Bryophyta. The Liverworts are more primitive in their organisation 

 than the Mosses and appear to be more ancient, since their fossil remains are 

 occasionally met with back to the Carboniferous period, while the earliest known 

 Mosses are from the Upper Cretaceous. Most fossil Bryophytes are from the 

 Tertiary rocks and closely resemble existing forms, 



