198 BOTANY FOR BEGINNERS CHAP. 



correspond to the four pollen sacs. The cells formed by their 

 division give rise to 



i. The cells of the fibrous layer. 

 2. The cells of the tapetum layer. 



3. The pollen mother cells, formed from the inner cells. 

 4. The epidermis of the anther, formed from the cells above 

 the hypodermic layer. 



The filament is the last portion of the stamen to be produced ; 

 as a rule, it is not fully developed until just before the pollen is 

 ripe. 



The Development of Pollen. Dicotyledons. The pol- 

 len mother-cells are large, thin-walled, filled with protoplasm and 

 contain a large nucleus. They are more or less rounded in out- 

 line and a very large number of them occur in each pollen sac. 



FIG. 206. Diagram illustrating the development of pollen in a dicotyledonous anther. 



Each mother-cell divides into four pollen grains in the following 

 manner. The nucleus divides (p, 89) into two, and each half 

 again divides into two, so that there are four nuclei in the 

 mother-cell. The protoplasm becomes rounded off so as to 

 form separate masses round each nucleus. (Fig. 206). A new 

 cell wall is produced round each nucleus from the protoplasm, 

 thus separating the daughter-cells, as they are called. Each 

 cell after ripening forms a pollen grain. The pollen grains are 

 set at liberty by the breaking down of the wall of the mother- 

 cell. This method of formation of pollen is the common one for 

 all dicotyledonous plants. 



Monocotyledons. -In monocotyledonous plants the formation 

 of pollen differs from the method just described. The nucleus 

 of the mother-cell divides into two parts, and between -these 

 parts a cell wall is formed which extends right across the cell. 

 Each nucleus again divides and new cell walls are formed be- 

 tween them. Thus, out of the mother-cell four daughter-cells 



