CHAPTER II — BRYOPHYTES 



Introductory. — This great division of plants comprisesj' the liver- 

 worts (Hepaticae) and mosses (Musci). The conspicuous features of 

 the group as contrasted with thallophytes are as follows: 



1. The establishment of a definite alternation of generations. Distinct 

 sexual and sexless individuals alternately produce each other, the ga- 

 metophyte producing the sex organs (containing gametes), the sporo- 

 phyte producing the asexual spores. The two generations are further 

 distinguished by their chromosome numbers : the 2x number arises 

 from the fusion of the sexual cells, and occurs in all the cells of the sporo- 

 phyte ; and the x number occurs in all the cells of the gametophyte, the 

 reduction taking place in connection with the formation of the tetrad of 

 spores by the mother cell. 



2. The appearance of the archegonium. This female sex organ is 

 very characteristic of the groups that possess it (bryophytes, pterido- 

 phytes, and gymnosperms). On this account they are often spoken 

 of collectively as archegoniates, but the groups are too unrelated to de- 

 serve a collective name. The archegonium is a flask-shaped organ, con- 

 sisting of a jacket of sterile cells (neck and venter) surrounding an axial 

 row of cells (neck canal cells, ventral canal cell, and egg) (fig. 219). 

 The cells of the axial row are doubtless to be regarded as potential eggs, 

 only the innermost one maturing and functioning as an egg, the others 

 breaking down and leaving an open canal to the egg. 



3. The appearance of a multicellular antheridium. Multicellular sex 

 organs and even multicellular antheridia appear among the algae, as in 

 Ectocarpus (see p. 46) and Charales (see p. 42), but the antheridium 

 of bryophytes is a very uniform and characteristic structure. It is 

 more or less stalked, and consists of a single layer of sterile jacket 

 cells investing a mass of small cubical sperm mother cells (fig. 210). 

 The sperm is also of a definite kind, consisting of a small, more 

 or less spirally curved body bearing a pair of long terminal cilia 

 (fig. 211). 



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