18 



BREEDING CROP PLANTS 



pair being obtained from the male parent and the other from the 

 female parent. At the formation of the sexual cells, or at reduc- 

 tion division, one member of each pair of chromosomes passes to 

 each daughter cell thus reducing the chromosome number to half 

 that in the body cells. Following this reduction division there is 

 an equating division whereby each chromosome is qualitatively 



FIG. 4. Anther and pollen of the lily. 



A, Mature anther, showing the four locules, or chambers, containing pollen grains: the 

 anther opens lengthwise on both sides along the lines of cells shown at a;; B, stages in the 

 formation of pollen grains in a group of four (tetrad) within the pollen mother cell; C, 

 mature pollen grain with early stages in the development of the male gametophyte; t, tube 

 nucleus; g, generative nucleus. (After Bergen and Davis.) 



equally divided. This results in the formation of the male or 

 female sexual cells or gametes as they are called (see Fig. 3). 



The male sexual cells are produced in the anthers and are 

 carried in the pollen grains. A mature pollen grain contains two 

 nuclei, a tube nucleus and a generative nucleus (see Fig. 4). 

 After the pollen grain falls on the pistil the tube cell 



