[Chap. XXXVII HEREDITY IN PLANTS 449 



and the megaspore mother cells divide. This particular cell division is 

 called reduction division because the two daughter cells formed have 

 only one-half as many chromosomes as the mother cell. The cell and 

 nucleus divide, but the chromosomes do not "split." This very unique 

 cell division plays an equally unique part in the transmission of heredi- 

 tary factors. It accounts for the principle of heredity known as the 

 "purity of gametes" or the "law of segregation of hereditary factors" with 

 which we shall be concerned presently. 



Some of the features of reduction division are depicted in Fig. 209. 

 Obviously the facts depicted here were difficult to discover. Fortunately 

 certain plants and animals have only a few chromosomes in each cell, 

 and their chromosomes differ sufficiently in form that they may be recog- 

 nized readily and watched during cell division. Here again many de- 

 tailed facts are known; but for our present purposes it will be sufficient 

 if we clearly understand and apply certain basic facts. In contrast to 

 ordinary cell division, the chromosomes during reduction division do not 

 separate longitudinally but assemble at the center of the spore mother 

 cell in pairs, each paternal chromosome pairing with its homologous 

 maternal chromosome. Later the mates of each pair of chromosomes 

 separate and migrate to opposite poles of the cell, resulting finally in 

 two new cells, each having only one-half as many chromosomes as the 

 spore mother cell. 



Usually these two new cells divide immediately by ordinary mitosis, 

 forming a tetrad of spores: a tetrad of microspores if the mother cell was 

 a microsporocyte in the anther; a tetrad of megaspores if the mother 

 cell was a megasporocyte in the ovule. 



Chromosomes in the life cycle of a seed plant. Since the microspore is 

 the forerunner of the pollen grain, and since the generative nucleus in 

 the pollen grain is the forerunner of the two sperms in the pollen tube, 

 the chromosome complement of these two speiTns is identical, and it is 

 also identical with that of the microspore from which the pollen grain 

 developed. Hence in solving problems in heredity that involve the trans- 

 mission of chromosomes by spenus, we can predict the chromosome com- 

 plement of the spenns from that of the microspores. 



Similarly a megaspore is the forerunner of an embryo sac; hence the 

 chromosome complement of each of the eight nuclei in the embryo sac 

 is the same, and it is identical with that of the megaspore. The fusion 

 nucleus formed by the union of two of these nuclei has a double set of 



