114 STRUCTUKAL BOTANY 



the mother-cells begin to be isolated, the nucleus in 

 each divides into two, and the two daughter-nuclei 

 at once divide again, so that there are four nuclei in 

 the mother-cell (see Fig. 39, B). New cell- walls are 

 formed between the nuclei, and the mother-cell is 

 thus divided into four distinct cells, each with its own 

 nucleus. These four daughter-cells are arranged like 

 four cannon-balls in a heap, the whole group having 

 roughly the form of a four-sided pyramid or a tetra- 

 hedron. 1 Each daughter-cell next forms from its 

 protoplasm a new cell-wall of its own. The wall of 

 the mother-cell is dissolved, and the four pollen-grains 

 are set free. The cavity of each pollen-sac is now 

 filled with a host of free pollen-grains, four times as 

 numerous as the original mother-cells. The grains 

 are at first immersed in a half -fluid substance derived 

 from the protoplasm of the tapetal cells, which break 

 down during the formation of the pollen. This 

 substance is soon used up by the pollen-grains to 

 complete their growth. 



The ripe pollen-grain is somewhat oval in shapa 

 Its wall is considerably thickened, but there are 

 three thinner bands, indicated externally by shallow 

 grooves, which run lengthways of the grain. The 

 pollen-grain in Fig. 42 is shown as if in cross-section, 

 so that it appears round, with three depressions, and 

 we see the three thin places in its wall at the depressed 

 points. The rest of the wall is thick and cuticularised, 

 except a delicate inner layer of cellulose. 



1 Hence only three are seen in one plane. See the mother-cell at the 

 top of Fig. 39, B. 



