THE STAMEN AND POLLEN-SAC 251 



lias some special significance. In this case the importance of the 

 tetrad-division may in part lie in the provision of numerous easily 

 transferred bodies, whicli have an essential part to play in the 

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

 course of the tetrad-division the nuclei tuidergo the cJiayige 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 m 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 " 2.r," and they are called 

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

 " X " chromosomes, and are described as haploid. The constitution 

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

 in detail in a later chapter. Meanwhile we note that aw essential change 

 has occurred in 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 Ganietophyte. The result of its further development will be certain 

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

 viz. the male gametes, or sexual cells. 



