188 



BOTANY: PRINCIPLES AND PROBLEMS 



to the flower those insects which are important in effecting polH- 

 nation. Finally, outside the corolla is the calyx or circle of sepals. 

 These are usually green or greenish structures which protect the 

 delicate inner organs of the flower while in the bud. All floral 



Jl 



H I 



P*IG. 101. — Diagram of the "reduction division", which occurs in the develop- 

 ment of reproductive cells. Formation of a tetrad or group of four pollen grains. 

 A, resting nucleus in the pollen mother-cell. B, the six chromosomes become 

 distinct, each with an individuality of its own. C, these chromosomes group 

 themselves into three pairs. D, the three pairs become arranged across the 

 equator of the cell. E, the members of these pairs separate, three going to one 

 pole and three to the other. F, two new cells are formed, each of which has just 

 half the chromosome content of the mother-cell, as a result of this reduction 

 division. G, H, and /: these cells each divide in two normally, producing four 

 cells each with the reduced chromosome number. J, four pollen grains which 

 have thus arisen from the pollen mother-cell. (Modified from Sharp). 



parts are attached to the receptacle or enlarged tip of the flower 

 stalk. 



Variations in Floral Parts. — These four groups of organs 

 exhibit such great differences in the number, shape, size, color, 

 texture and relative position of their parts as to give the flower a 



