PHENOGAMS AND CRYPTOGAMS 179 



taining sperm-cells). These organs are minute specialized 

 parts of the prothallus. Their positions on a particular 

 prothallus are shown at a and b in Fig. 262, but in some 

 ferns they are on separate prothalli (plant dioecious). The 

 sperm-cells escape from the antheridium and in the water 

 that collects on the prothallus are carried to the archegonium, 

 ivhere fertilisation of the egg takes place. From the ferti- 

 lized egg-cell a plant grows, becoming a "fern." In 

 most cases the prothallus soon dies. The prothallus is the 

 gametophyte (from Greek, signifying the fertilized plant). 



The fern plant, arising from the fertilized egg in the 

 archegonium, becomes a perennial plant, each year pro- 

 ducing spores from its fronds (called the sporophyte) ; but 

 these spores which are merely detached special kinds of 

 cells produce the prothallic phase of the fern plant, 

 from which new individuals arise. A fern is fertilised but 

 once in its lifetime. The " fern " bears the spore, the 

 spore gives rise to the prothallus, and the egg-cell of the 

 prothallus (when fertilized) gives rise to the fern. 



A similar alternation of generations runs all through the 

 vegetable kingdom, although there are some groups of 

 plants in which it is very obscure or apparently wanting. 

 It is very marked in ferns and mosses. In algae (includ- 

 ing the seaweeds) the gametophyte is the " plant," as 

 the non-botanist knows it, and the sporophyte is incon- 

 spicuous. There is a general tendency, in the evolution of 

 the vegetable kingdom, for the gametophyte to lose its rela- 

 tive importance and for the sporophyte to become larger and 

 more highly developed. In the seed-bearing plants the 

 sporophyte generation is the only one seen by the non- 

 botanist. The gametophyte stage is of short duration and 

 the parts are small ; it is confined to the time of fertiliza- 

 tion. 



