THE ORIGIN OF SEED PLANTS 889 



THE OKIGIN OF SEED PLANTS AND THE 

 SEED HABIT* 



366. The origin of seed plants. We shall never know exactly 

 when and how seed plants arose, for that important event in 

 plant evolution probably took place earlier than the Carbon- 

 iferous Age. We can, however, form some idea of the chief 

 factors that brought about the seed habit from a study of 

 the life histories of living pteridophytes and spermatophytes. 

 As with a number of other forward steps in the evolution of 

 plants, such as the origin of sex, alternation of generations, 

 and heterospory, the seed habit probably was developed by a 

 number of different groups of pteridophytes independently one 

 of another. Thus the cycads and the conifers among the gym- 

 nosperms are so widely separated that it seems possible that 

 they may have come from different pteridophyte parentage. 

 Therefore the gymnosperms are generally regarded as a group of 

 divergent evolutionary lines. The angiosperms are even more 

 puzzling. Some botanists believe that they arose quite inde- 

 pendently of the gymnosperms, but others hold that they may 

 be distantly related to Gnetum. Some think that the mono- 

 cotyledons and dicotyledons even have had independent origins. 

 However, the view which seems to be finding greatest favor at 

 present regards the monocotyledons not as the ancestors of the 

 dicotyledons, but as derived from primitive dicotyledons. 1 



367. The origin of the seed habit. The most important fac- 

 tors leading to the seed habit appear to have been (1) heter- 

 ospory, (2) the retention of the megaspore in the megasporangium 

 to become the embryo sac in which the female gametophyte 

 develops parasitically, and (3) the development of the pollen tube 

 and its parasitic habit of growth through the tissues of the 



* To THE INSTRUCTOR : This subject is very difficult and may be omitted. 



1 These topics are far too technical for consideration here. For reviews of 

 the various theories with their evidence, the reader is referred to Coulter 

 and Chamberlain, Morphology of Spermatophytes (Gymnosperms), 1901 and 

 Morphology of Angiosperms^ 1903. 



