GAMETOPHYTE AND SPOROPHYTE. - 337 



I 



since it is that which has become and remains the prominent one in later 

 times. 



671. The gametophyte once prominent has become degenerate. On the 



other hand we can see that just as remarkable changes have come upon the 

 other phase of plant life, the sexual stage, or gametophyte. There is reason 

 to believe that the gametophyte was the stage of plant life which in early 

 times existed almost to the exclusion of the sporophyte, since the characteristic 

 thallus of the algae is better adapted to an aquatic life than is the spore-bearing 

 state of plants. At least, we now find in the plants of this group as well as in 

 the liverworts, that the gametophyte is the prominent stage. When we reach the 

 members of the fern group, and the sporophyte becomes independent, we find 

 that the gametophyte is decreasing in size, in the higher members of thepteri- 

 dophytes, the male prothal ium consisting of only a few cells, while the fe- 

 male prothallium completes its development still within the spore wall. And 

 in selaginella it is entirely dependent on the sporophyte for nourishment. 



672. As we pass through the gymnosperms we find that the condition of 

 things which existed in the bryophytes has been reversed, and the gameto- 

 phyte is now entirely dependent on the sporophyte for its nourishment, the 

 female prothallium not even becoming free from the sporangium, which remains 

 attached to the sporophyte, while the remnant of a male prothallium, during 

 the stage of its growth, receives nourishment from the tissues of the nucellus 

 through which it bores its way to the egg-cell. 



673. In the angiosperms this gradual degradation of the male and female 

 prothallia has reached a climax in a one-celled male prothallium with two 

 sperm-cells, and in the embryo-sac with no clearly recognizable traces of an 

 archegonium to identify it as a female prothallium. The development of the 

 endosperm subsequent, in most cases, to fertilization, providing nourishment 

 for the sporophytic embryo at one stage or another, is believed to be the last 

 remnant of the female prothallium in plants. 



674. The seed. The seed is the only important character possessed by 

 the higher plants (the gymnosperms and angiosperms) which is not pos- 

 sessed by one or another of the lower great groups. With the gradual evo- 

 lution of the higher plants from the lower there has been developed at cer- 

 tain periods organs or structural characters which were not present in some 

 of the lower groups. Thus the thallus is the plant body of the algae and 

 fungi, so that these two groups of plants are sometimes called Thattophytes. 

 In the Bryophytes (liverworts and mosses) the thallus is still present, but 

 there is added the highly organized archegonium in place of the simple 

 female gamete or oogonium, or carpogonium of the algae and fungi, and the 

 sporophyte has become a distinct though still dependent structure. In the 

 Pteridophytes the thallus is still present as the prothallium, archegoina are 

 also present, and while both of these structures are retrograding the spo- 

 rophyte has become independent and has organized for the first time a true 



