298 



PRACTICAL COURSE IN BOTANY 



bearing somewhat the aspect of Hchens, met with everywhere 



on wet rocks and banks around shady watercourses. The 

 name is a reminiscence of their former use 

 in medicine as a specific for diseases of the 

 liver, and not, as in the case of the hver leaf, 

 of a fancied resemblan3e to that organ. 



Mosses are one of the best defined of 

 botanical orders, and ire easily recognized 

 by their slender, leafy iruiting stalks, grow- 

 ing usually in dense, spreading mats, and 

 presenting every appearance of a highly 

 organized structure, well differentiated into 

 root, stem, and leaves. 



The liverworts represent 

 the more primitive division 

 of the group, and in some 

 of their forms approach so 

 near the thallophytes that 

 it is not difficult to recog- 

 nize them as connecting 

 links in the same chain oi 



life. Their relationship to the next higher 



group is not clear, but while they represent 



a more primitive stage of evolution than 



the mosses, the development of the latter 



has followed a course divergent from the 



main line of evolutionary progress. 



335. III. Pteridophytes, or fern plants, are 



classed roughly in the three divisions of 



ferns, horsetails, and club mosses. They 



differ greatly in structure, but all possess a 



vascular system, and a well-organized struc- 

 ture of root, stem, and leaves. They rank 



next to the spermatophytes in the order of 



development, and the group is of especial interest on account 



of its relationship to the higher plants. One of its divisions, 



Fig. 427.—. 

 shoot of peat mos 

 with ripe s p o r 

 fniits, /, /. 



Fig. 428. — A com- 

 mon fern {Poly po- 

 dium vulgare). 



