THE WALLFLOWER 



115 



The pollen-grain at first, like most other cells, has 

 a single nucleus. This nucleus divides while the 

 grain is ripening, and one of. the two daughter-nuclei 

 divides again. Thus in the mature pollen-grain 

 there are three nuclei, one of which is larger than 

 the other two ; the two smaller nuclei have arisen 

 from the second division. From observations on 

 other plants which are more favour- 

 able for examination than the Wall- 

 flower, it is probable that the 

 protoplasm around each of the 

 smaller nuclei forms a distinct cell, 

 so that the mature pollen-grain 

 thus consists of three cells. In 

 many other plants the pollen-grain 

 has only two cells at this stage 

 (cf. the Lily, p. 178, Fig. 80). 



The two layers of cells next out- 

 side the tapetum, like it, become 

 absorbed. One of them often dis- 

 appears even before the tapetum 

 itself, as shown in Fig. 3 9, A. When 

 the anther is ripe, the wall of each 

 of the four pollen-sacs consists on 

 its free outer side of one or two layers besides the 

 epidermis. These persistent layers are distinguished 

 by a strongly-marked spiral or netted thickening on 

 their membranes. These cells form \hzfibrous layer (see 

 Fig. 40,/). It completely surrounds each pollen-sac, 

 except at the point just opposite the partition which 

 separates the two pollen-sacs in the same lobe. Here 

 the fibrous layer is interrupted by one or two thin- 



FIG. 42. A single 

 pollen-grain of the 

 Wallflower, show- 

 ing the three thin 

 places in the wall, 

 at any one of which 

 the pollen-tube may 

 subsequently grow 

 out. Observe the 

 three nuclei, ___ of 

 which the largest 

 is vegetative and 

 the two smaller are 

 generative. Magni- 

 fied about 



450. 



(R. S.) 



