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A TEXTBOOK OF BOTANY 



[Ch. XI 



respondingly modified, while at the opposite extreme, those 

 in very dry, cold, mountainous regions form dense, compact, 

 radiate tussocks. A European form, living in caves, has a 

 very remarkable protonema, which has been much studied 

 from a physiological point of view (Fig. 348). 



Ecologically the Bryophytes represent the typical carpet 

 vegetation of the land, — the Liverworts in the moister and 

 the Mosses in the drier places; and both become also 

 epiphytes. Phylogenetically they are the lowest of true land 



plants, and no doubt 

 represent a survival of 

 the first land forms which 

 evolved from the Algse. 

 Presumably they were 

 never taller than now, 

 their lack of a vascular 

 system being prohibitive 

 of such growth. It is 

 probable, however, that 

 in a period of geological 

 history prior to the 

 development of any other land vegetation, the surface of the 

 earth was covered with a dense green carpet of Liverworts 

 and Mosses, which became in large part displaced by the later 

 development of Pteridophytes and Spermatophytes. As 

 to the group of Algse from which they arose, there is much 

 uncertainty, since connecting links are missing. In some 

 features of their reproduction the Bryophytes show resem- 

 blance to the Brown Algse, though much other evidence im- 

 plies descent from the higher Green Algse. Probably Brown 

 Algse and Bryophytes originated close together among the 

 higher Green Algse, somewhere near Coleochcete, as suggested 

 in our phylogenetic tree (Fig. 275). Indeed, a transition 

 is not difficult to suppose from the submerged disc-form 

 thallus of Coleochcete (page 424), to the floating, and later 

 land-dwelling, thallus of simple Liverworts like Riccia; 



Fig. 348. — The protonema of Schistostega 

 osmundacea, a cave-dwelling moss ; X 200. 



The cells are so shaped as to concentrate 

 the feeble light upon chloroplasts in the 

 depression at the bottom of each cell. 

 (From Strasburger.) 



