340 MOSSES LESS. 



aggregate branches, and may form a closely- matted mass 

 of filaments. Sooner or later small lateral buds (o 2 , bd) 

 appear at various places on the protonema : each of these 

 takes on the form of a three-sided pyramidal apical cell, 

 which then proceeds to divide in the characteristic way 

 (p. 335), forming three rows of segments from which leaves 

 spring. In this way each lateral bud of the protonema gives 

 rise to a moss-plant. 



Obviously we have here a somewhat complicated case of 

 alternation of generations (see p. 220). The gamobium or 

 sexual generation is represented by the moss-plant, which 

 originates by budding and produces the sexual organs, while 

 the agamobium consists of the sporogonium, developed from 

 the oosperm and reproducing by means of spores. The 

 protonema, arising from a spore and producing the leafy 

 plant by budding, is merely a stage of the gamobium. 



The nutrition of mosses is holophytic ; but there is a 

 striking differentiation of function correlated with terrestrial 

 habits. In Nitella the entire organism is submerged in 

 water and all the cells contain chlorophyll, so that decom- 

 position of carbon dioxide and absorption of an aqueous 

 solution of salts are performed by all parts alike, every 

 cell being nourished independently of the rest. In the 

 moss, on the other hand, the rootlets are removed from 

 the influence of light and contain no chlorophyll : hence 

 they cannot decompose carbon dioxide ; but, being sur- 

 jounded by moist soil, are in the most favourable position 

 for absorbing water and mineral salts. The stem, again, is 

 converted into an organ of support : the thickness of its 

 external cells prevents absorption and it contains no 

 chlorophyll. Hence the function of decomposing carbon 

 dioxide is confined to the leaves. 



