CHAPTER V 

 BRYOPHYTA 



The bryoph>'tes. numbering about 20,000 species, form a well-defined 

 division comprising the two classes Hepaticae (liverworts) and :Musci 

 (mosses). They are small, rather inconspicuous, green plants nearly all 

 of which five on land in moist, shaded places. The bryophj^es doubtless 

 have been derived from aquatic ancestors, probably from some group of 

 green algae, but it is uncertain whether they have given rise to any of the 

 higher plant groups. Nevertheless, the bryophnes represent a general 

 condition of structural organization through which the higher plants may 

 have passed in the course of their evolution. Although abundant mois- 

 ture is necessary for vigorous vegetative gro^^'th, some forms live in dry 

 situations and endure considerable desiccation during long rainless 

 periods. A few hver worts and mosses live in fresh water, but the aquatic 

 habit in the bry ophites, as in the higher groups, has undoubtedly been 

 secondarily acquired. 



A well-defined alternation of generations is an established feature of all 

 bryophj^es, the gametophj-te and sporoph>i:e always being morphologi- 

 cally dissimilar. The gametoph^-te, arising from the spore, is the haploid 

 generation, producing sperms and eggs. The sporophyte, arising from 

 the zygote, is the diploid generation. It produces spores, the reduction 

 in chromosome number taking place in connection ^^-ith their formation, 

 as in all the higher plants. S\^-imming spores are entirely ehminated. 

 In the green algae the zygote is liberated into the water and is nearly 

 always a resting cell, while in the bryophytes and all higher groups it 

 germinates at once, without escaping, to produce an embryo sporophyte. 

 In the bryoph>i:es the gametophyte, or haploid generation, is always an 

 independent indi\'idual, while the sporophyte. or diploid generation, is 

 entirely or largely dependent on it for its nutrition. Although the 

 garnet oph^-te is thalloid in some of the liverworts, in most bryophytes it 

 is dift'erentiated into stem and leaves. Growth takes place through the 

 activity of an apical cell. The sex organs, antheridia and archegonia, 

 are always multicellular and provided with an outer sterile jacket. 

 Throughout the algae the gametangia are prevailingly unicellular but, 

 where multicellular, all their cells produce gametes (except in the Charo- 

 phyceae). The antheridium is a stalked, spherical or club-shaped organ 

 consisting of a mass of spermatogenous tissue enclosed by a jacket of 



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