THE WALLFLOWER 113 



cells produced by this division form what is called the 

 archesporium, i.e. the layer from which the spores, here 

 called the pollen-grains, 1 ultimately arise. Each arche- 

 sporium thus consists at first of a single strand or layer 

 of cells running the whole length of the anther. The 

 archesporial cells grow and divide up in all directions, 

 and we soon have, in place of the original layer, a solid 

 cylindrical column of tissue, consisting of large cells 

 (see Fig. 41, B and C), which are densely filled with 

 protoplasm, and have large nuclei. In the meantime 

 the cells immediately outside the archesporium have 

 undergone several divisions, so that the wall of the 

 pollen-sac becomes about four cells thick (see Fig. 41, 

 A, B, and C), including the epidermis, which does not 

 itself divide. The cells which are actually in contact 

 with the archesporium become elongated radially, and 

 form a well-marked internal lining to each pollen-sac, 

 called the tapetum (see Figs. 39 A, and 41 C, t). The 

 tapetum extends all round the sac. Its cells are at first 

 rich in protoplasm. The function of this layer is^to 

 feed the developing pollen-grains, by which it is entirely 

 used up. 



To return to the archesporium : after its cells have 

 divided a great many times, the divisions cease for 

 a while, the pollen-sac continues growing, its cavity 

 becomes more roomy, and so the cells derived from the 

 archesporium become isolated from one another and 

 round off their corners (see Fig. 39, A). These cells 

 now bear the name of the pollen mother-cells, because 

 each of them divides up into four pollen-grains. The 

 division takes place as follows : at the time when 



1 See Part II., Flowerless Plants, pp. 18 and 31. 



