122 Plant Life and Evolution 



to enable it to supply the water necessary for the 

 further development of the sporophyte, which now 

 becomes a perfectly developed land plant, with stem, 

 roots, and leaves and elaborately developed tissues. 

 With the elaboration of this sporophyte, or ter- 

 restrial phase of the plant's life, there has been a 

 gradual reduction of the aquatic phase, and the 

 gametophyte becomes more and more insignificant, 

 culminating in the condition met with in the hetero- 

 sporous pteridophytes, in which the sex of the fu- 

 ture gametophyte is already indicated by the char- 

 acter of the spore. This tendency to heterospory is 

 shown clearly in several quite independent lines, and 

 just as the different types of sporophytes, i.e., ferns, 

 horsetails, and so on, probably have arisen inde- 

 pendently, so heterospory also developed in various 

 quite different lines. In some of these, e.g., the 

 water ferns, no further advance seems to have been 

 made; but in other groups a further development of 

 heterospory resulted in the formation of seeds, the 

 distinguishing mark of the highest plants. As was 

 pointed out in the last chapter, the seed is not a 

 new organ, but is merely an elaboration of one which 

 already existed, the megasporangium, or the 

 sporangium in which the large spores or mega- 

 spores are developed, and from the latter the female 

 gametophyte is produced. There is abundant evi- 

 dence from a study of the Paleozoic pteridophytes 

 that seeds developed in several widely separate 

 groups, and this, together with the structure of the 



