REPRODUCTION: ALTERNATION OF GENERATIONS 273 



common flowering plant there is an alternation of generations, 

 based upon the same principle as that just described in the 

 fern; but it is so obscured by certain modifications that it is 

 extremely difficult to understand. The difficulty lies in three 

 facts: (1) Two kinds of spores are produced instead of one, as 

 in the fern; one of them becomes the female gametophyte, 

 ; producing the equivalent of the archegonium of the prothallium 

 Vth its egg, while the other becomes a male gametophyte, 

 producing the equivalent of the antheridium of the prothallium 

 with its sperms. (2) Both of these gametophytes have become 

 very much reduced in size and are only distinguishable by micro- 

 scopic examination with special methods. (3) These two 

 gametophytes grow attached to the plant that produces the spores 

 instead of detached from it, as does the gametophyte of the 

 fern. If these differences be kept in mind the alternation of 

 generations in the flowering plant is plain. It is as follows: 

 We usually speak of the flower as containing sexual organs, 

 the stamens being spoken of as the male and the pistil as the 

 female organs. When the pollen is carried to the pistil it has 

 commonly been spoken of as fertilizing the stigma, the infer- 

 ence being that the pollen is the male cell and actually fertil- 

 izes the female cell in the pistil. When the flower is studied 

 by modern methods, however, it is found that in reality it is 

 not a sexual plant at all, and does not produce sexual organs. 

 The stamens are not male organs and the pollen is not a male 

 cell; the pistil itself produces no eggs. The pollen is really a 

 mass of spores, called microspores. In the pistil, as already 

 noticed (see Fig. 64), are several ovules and inside of each 

 ovule is a single large cell, formerly called the embryo sac, but 

 now known as a macrospore ; Fig. 126 sp. The flower thus 

 produces large numbers of microspores and a smaller number 

 of macrospores, which together correspond to the spores of 

 the fern. These cells are known to be spores rather than 

 gametes, since they do not unite with each other. That 

 ihe pollen is a spore rather than a sex cell is proved by 



