xxix GAMOBIUM AND AGAMOBIUM 361 



production of fresh segments from its apical cell : leaves (/) are 

 developed from the segments, and grow upwards parallel with 

 the cotyledon. The leaves first formed are small and 

 simple in structure, but those arising later become succes- 

 sively larger and more complicated, until they finally attain 

 the size and complexity of the ordinary leaves of the fern. 

 In the meantime new roots are formed ; the cotyledon, the 

 foot, and the prothallus wither, and thus the phyllula, by the 

 successive formation of new parts from its constantly growing 

 stem, becomes a fern-plant. 



We see that the life-history of the fern resembles in 

 essentials that of the moss. In both, alternation of genera- 

 tion occurs, a gamobium or sexual generation giving rise, by 

 the conjugation of ovum and sperm, to an agamobium or 

 asexual generation, which, by an asexual process of spore - 

 formation, produces the gamobium. But in the relative 

 proportions of the two generations the difference is very great. 

 What we know as the moss plant is the gamobium, and the 

 agamobium is a mere spore-producing structure, never getting 

 beyond the stage of a highly differentiated polyplast, and 

 dependent throughout its existence upon the gamobium, to 

 which it is permanently attached. What we know as the 

 fern plant is the agamobium, a large and complex structure 

 dependent only for a brief period of its early life upon the 

 small and insignificant gamobium. Thus while the gamobium 

 is the dominant phase in the life-history of mosses, the 

 agamobium appearing like a mere organ, in ferns the 

 positions are more than reversed the agamobium may 

 assume the proportions of a tree, while the gamobium is so 

 small that its very existence is unknown to a large propor- 

 tion of fern-collectors. 



It follows from what has just been said that the various 

 organs of a fern do not severally correspond with those of a 



