MALE REPRODUCTIVE CELLS 



431 



Here it is most clearly to be seen that four cells result from the 

 two reduction mitoses. 



This can be demonstrated clearly and plainly because at the begin- 

 ning of this period each 

 mother cell becomes incased 

 in a tough-walled sac or cell- 

 wall, and remains in this same 

 envelope until its four descen- 

 dants become full-grown pol- 

 len grains. This feature 

 serves to show that the proc- 

 ess of reduction in plants 

 and animals is much the 

 same and is possibly an ho- 

 mologous process. 



The origin of the pollen 

 mother cells is a more or less 

 late (in the life of the indi- 

 vidual tree) differentiation, 

 which occurs each year from 

 the ever young cells of the 

 plant's upper growing point. 

 As member after member, like 

 leaf, bract, petal, etc., is pro- 

 duced by these cells, a time 

 comes when the season or 

 stage of growth determines 

 the differentiation of a group 

 of members called the flower, 

 and in this group are certain 

 members called the stamens. 



Four longitudinal, parallel 



FIG. 383. A, outer nurse cells in pollen sac of 

 Magnolia soulangeana beginning to differentiate 

 from inner reproductive cells. B, nurse cells in 

 their vigor feeding pollen mother cells, one of which 

 is shown in outline. One nurse cell in mitosis. 

 C, nurse cells degenerating after pollen cells are 

 formed. Two groups of pollen cells in outline. 

 Low magnification. 



regions called the pollen sacs 

 are early marked out in such 

 a stamen member, and the 

 cells within them become the 

 primitive, pollen-forming cells. 

 These cells are all alike at this 

 time (early fall in Princeton) and lie dormant all winter, perhaps pursuing 

 a very slow growth on the least cold days. In February, with the warm- 

 ing weather, the pollen sacs begin to grow, and now it will be noticed 

 that the central cells are increasing in size much faster than the two outer 

 layers. This is well shown in Figure 383, A, where the centrally situated 



