2 MOSSES AND FERNS chap. 



entirely free, or even raised on a long pedicel ; or it may be 

 completely sunk in the tissue of the plant, or even be formed 

 endogenously. It usually consists of a single outer layer of 

 cells containing chlorophyll, and these enclose a mass of small 

 colourless cells, the sperm cells, each of which gives rise to 

 a single ciliated spermatozoid. The development of the latter 

 is very uniform throughout the Archegoniatae, and differs 

 mainly from the same process in the higher green Algae, 

 especially the Characeae, in the larger amount of nuclear 

 substance in the spermatozoids of the former. 



Fertilisation is only effected when the plants with ripe 

 sexual organs are covered with water. The absorption of 

 water by the mature sexual organs causes them to open, and 

 then, as the spermatozoids are set free, they make their way 

 through the water by means of their cilia and enter the open 

 archegonium, into which they penetrate to the &g%. The 

 sexual cells do not differ essentially from those of the higher 

 Algae, and point unmistakably to. the origin of the Arche- 

 goniatae from similar aquatic forms. Indeed all of the 

 Archegoniatae must still be considered amphibious, inasmuch 

 as the gametophyte or sexual plant is only functional when 

 partially or completely submerged. 



Non-sexual gonidia are known certainly only in Aneura, 

 one of the lower Liverworts, but special reproductive buds or 

 gemmae, both unicellular and multicellular, are common in 

 many forms. 



A very marked characteristic of the whole group is the 

 sharply-marked alternation of sexual and non-sexual stages. 

 The sexual plant or gametophyte varies much in size and 

 complexity. It may be a simple flat thallus comparable in 

 structure to some Algae, and not superior to these in com- 

 plexity so far as the vegetative parts are concerned. In others 

 it becomes larger and shows a high degree of differentiation. 

 Thus among the Liverworts the Marchantiaceae, while the 

 gametophyte still retains a distinctly thalloid form, still show 

 a good deal of variety in the tissues of which the thallus is 

 composed. In others, e.g. the true Mosses, the gametophyte 

 has a distinct axis and leaves, and in the higher ones the 

 tissues are well differentiated for special functions. The 

 gametophyte itself may show two well-marked phases, the 

 protonema and the gametophore. The former is usually 



