CHAPTER XV 

 THE BRYOPHYTA 



The second of the four great divisions of the plant kingdom 

 is the Bryophyta or bryophytes, the members of which we know 

 commonly as the liverworts and mosses. This group is much 

 smaller than the thallophytes, containing only about 16,000 species, 

 nor does it approach them in the diversity of plant types which it 

 displays. All of its members are lowly and inconspicuous, the 

 tallest mosses rarely attaining a decimeter in height. From the 

 economic point of view the group is of very minor consequence. 

 To the botanist, however, the bryophytes are of particular 

 interest in helping to picture for us those ancient plants which 

 first crept out of the sea to invade the dry land, and which 

 therefore took the first steps along the evolutionary road leading 

 up to our dominant and familiar seed plants. 



The bryophytes are undoubtedly a very ancient group and 

 their history is necessarily obscure. We have good reason 

 to believe, however, that they arose from plants resembling 

 some of our higher algae of today, and several connecting links 

 between algae and liverworts have accordingly been suggested. 

 The two main characteristics which distinguish the bryophytes 

 as a whole from these ancestral thallophytes are the establish- 

 ment of a clearly marked alternation of generations and the 

 possession of multicellular sexual organs. 



Alternation of Generations. — As outlined in a previous chapter, 

 these plants possess a definite sexual gamete-producing member, 

 the gametuphyte, which is followed in the life history by an equally 

 definite non-sexual, spore-producing member, the sporophyte. 

 The sporophyte here is little more than a spore case and is always 

 attached to the tissues of the gametophyte, never becoming an in- 

 dependent individual as it does among the ferns and their allies. 

 The V)eginnings of this alternation of generations, as we have seen, 

 make their appearance here and there among the thallophytes, 

 but when we reach the liverworts and mosses it becomes 

 regularly established and is henceforth a distinctive feature in 

 the life histories of all species throughout the plant kingdom. 



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