THE STAMEN AND POLLEN-SAC 287 



organisms, even though they may differ in many respects, it may be 

 assumed that it has seme special significance. In this case the im- 

 portance of the tetrad-division lies partly in the provision of numerous 

 easily transferred bodies, which carry out an essential function in the 

 propagative process. But a more important point is that in the 

 course of the tetrad-division the nuclei undergo the change called 

 REDUCTION. Certain formed bodies, chromosomes, present in con- 

 stant number in each nucleus on division, are reduced, in the course 

 of the process, to half their original number in each nucleus. All 

 the products of further division of nuclei so reduced have the same 

 smaller number. The ordinary nuclei of the plant have a number of 

 chromosomes which may be represented as " 2x," and they are called 

 diploid. The nuclei that result from reduction are found to have only 

 "*" chromosomes, and are described as haploid. The constitution of 

 the nucleus and its behaviour in reduction will be discussed again in 

 detail in Chapter XXXV. Meanwhile we note that an essential change 

 has occurred during the tetrad-division, and that the nuclei produced within 

 the pollen-grain are themselves haploid. It is then not simply a separa- 

 tion of vegetative cells that occurs in the production of pollen. The 

 grain as it leaves the anther conveys with it nuclei that differ in an 

 essential feature from those of the ordinary vegetative cells of the 

 Plant. The process of reduction in the tetrad initiates a sexual phase, 

 or Gametophyte. The result of its further development will be certain 

 cells, which are capable of taking a direct part in sexual reproduction : 

 they are in fact the male gametes, or male sexual cells. 



