PROTHALLUS 181 



349. On the under side of the prothallus two kinds of 

 organs are borne. These are the archegonium and the anthe- 

 ridium. These organs are minute specialized parts of the 

 prothallus. Their positions on a particular prothallus are 



shown at a and 6 in Fig. 339, but in some 

 ferns they are on separate prothalli (plant 

 dioecious). The sperm-cells escape from 

 the antheridium and in the water which 



335. The brake fruits , .in i 



underneath the revo- collects on the prothallus are carried to 



lute edges of the leaf. the arc h e gonium, where fertilization takes 



place. From a fertilized archegonium a plant grows, and 



this plant becomes the "fern." In most cases the prothallus 



dies soon after the fern plant begins to grow. 



350. The fern plant, arising from the fertilized egg in the 

 archegonium, becomes a perennial plant, each year pro- 

 ducing spores from its fronds, as we have seen; 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 fertilized but once in its 

 life-time. The prothallium here is the gametophyte] the "fern" 

 is the sporophyte (phyton is Greek for "plant"). 



351. This succession 

 of generations runs all 

 through the vegetable 

 kingdom, although there 

 are some groups of plants 

 in which it is very ob- 

 scure or apparently want- 

 ing. It is very marked 

 in ferns and mosses. In 



algse (including the sea- m Retod ^^ of a ****** frond - 

 weeds) the gametophyte constitutes the "plant," as the non- 

 botanist knows it. There is a general tendency, in the evo- 

 lution of the vegetable kingdom, for the gametophyte to lose 

 its relative importance and for the sporophyte to become 



