90 DEVELOPMENT OF POLLEN [CH. 



larger size. These cells, the transverse sections of a cor- 

 responding vertical column in each case, are seen to 

 arise by the tangential division of the layer next to the 

 epidermis, and thus lie separated from the epidermis by 

 one cell. They form a mass of spore-producing tissue, 

 and give rise by further division to the pollen mother-celU. 



The cells separating this mass of spore-producing 

 tissue from the epidermis undergo one or two further 

 tangential divisions, until about three rows of cells sepa- 

 rate the sporogenous tissue from the epidermis. The 

 inner of these layers is called the tapetum. 



Meanwhile the cells between the sporogenous mass 

 and the centre of the anther have been growing and 

 dividing, and one layer of cells next the sporogenous mass 

 completes the tapetal layer all round the sporogenous cells. 



The number of spore-forming cells, and of divisions 

 preceding their completion, differ slightly in different 

 plants, in relation with differences in the size and shape 

 of the anther and pollen-sacs, but the principle is the same 

 throughout. 



When the sporogenous mass has divided by walls in all 

 directions, the resulting pollen mother-cells form a tissue 

 with very thin walls, full of highly refringent protoplasm 

 with large nuclei ; these stain well, and yield some of the 

 best examples of nuclear divisions that can be obtained. 



Very soon the pollen mother-cells are found to have 

 thick, swollen and lamellated walls and to be separating, 

 and the surrounding cells also break down to a dis- 

 organised glairy mass ; and we now have the isolated 

 pollen mother-cells floating free in a granular fluid in 

 the four cavities thus scooped out, as it were, in the 

 tissues of the anther-lobes. These cavities are the pollen- 

 sacs. 



The mother-cells, feeding on the nutritious liquid 



