23T MORPHOLOG Y. 



etc., are sporophytes, that is they represent the spore -bearing, or sporophytic, 

 stage. Just as we found in the case of the gymnosperms and ferns, this stage 

 is the prominent one, and the one by which we characterize and recognize the 

 plant. We see also that the plants of this group are still more highly special- 

 ized and complex than the gymnosperms, just as they were more specialized 

 and complex than the members of the fern group. From the very simple 

 condition in which we possibly find the sporophyte in some of the algae like 

 spirogyra, vaucheria, and coleochsete, there has been a gradual increase in 

 size, specialization of parts, and complexity of structure through the bryo- 

 phytes, pteridophytes, and gymnosperms, up to the highest types of plant 

 structure found in the angiosperms. Not only do we find that these changes 

 have taken place, but we see that, from a condition of complete dependence of 

 the spore-bearing stage on the sexual stage (gametophyte), as we find it in the 

 liverworts and mosses, it first becomes free from the gametophyte in the mem- 

 bers of the fern group, and is here able to lead an independent existence. 

 The sporophyte, then, might be regarded as the modern phase of plant life, 

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

 times. 



464. 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 algte 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 the pteri- 

 dophytes, the male prothallium 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. 



465. 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. 



466. 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 



