THE EVOLUTIONARY HISTORY OF PLANTS 



303 



•■a 



development the macrospore produces an endosperm whicli 

 is really a small cellular prothallium, concealed in the ovule. 

 The microspore contains vestiges of a minute prothallium. 



In the angiosperms the macrospore and its prothallium 

 are still less developed, and the 

 microspore, or pollen grain, has 

 lost all traces of a prothallium 

 and is merely an antheridium 

 which contains two generative 

 ceUs.^ These are most easily 

 seen in the pollen grain, but 

 sometimes they are plainly visi- 

 ble in the pollen tube (Fig. 164). 

 Phanerogams are distinguLshed 

 from all other plants by their 

 power of producing seeds, or 

 enclosed macrosporangia, with 

 embryos. 



375. The Law of Biogenesis 

 and the Relationships of the Great 

 Groups of Plants. — On summing 

 up Sects. 872-374 it is evident 

 that the sexual generation in 

 general occupies a less and less 

 important share in the life of the 

 plant as one goes higher in the scale of plant life.^ In the 

 case of the rockweed, for instance, the sexual generation 

 is the plant. Among mosses and liverworts the sexual 



1 Sometimes only one generative cell escapes from the pollen grain into the 

 pollen tuhe, and there it divides into two cells. 



2 A good many plants of low organization, however, are not known to pass 

 through any sexual stage. 



Fig. 217.— Longitudinal Section 

 through Fertilized Ovule of a 

 Spruce. 



^fPolleu grains ; t, pollen tubes ; 

 n, neck of the archegonlum ; 

 a, body of archegonium with 

 nucleus ; e, embryo sac filled 

 with endosperm. 



