616 



WEED FLORA OF IOWA 



regions are conspicuous because the cells contained are larger and 

 have a denser content. Each of these cells will produce four pollen 

 grains and for this reason are called pollen mother cells. 



Surrounding each group of pollen mother cells is usually one 

 layer of cells whose content is quite dense. These are sacrificed 

 as food material for the pollen mother cells and are designated as 

 tapetal cells. 



After the pollen grains are formed, they lie loose in these cavities. 

 Each cavity is considered as a case or angium and since a pollen 

 grain is a spore, this case is called a sporangium. 



The partition between each pair of sporangia usually breaks 

 down, and two spore-containing cavities are formed. These are 

 generally called pollen sacs. The pollen sacs are now ready to 

 open or dehisce as the process is called. This is due to especially 

 modified cells, which produce such strains through the variation 

 of moisture, that usually longitudinal slits or terminal pores are 

 produced. 



Fig. 477. Thrift, ov, ovary ; s, style ; st, papillary stigmas. 

 (After Thome.) 



The pollen is now either by wind, insects, or water carried to 

 the stigma of the pistil where it begins the performance of its 

 important function. This process of transference is pollination. 

 An examination of a pollen grain at the time of pollination will 

 usually show that it has two nuclei; one of these has to do with 

 the production of a tube which traverses the tissues of the pistil 

 and furnishes a passage way to the embryo sac which contains the 



