STAMEN 



461 



Stamen. — The stamen (miorosporophyll) has its pollen sacs 

 (microsporangia), usually four in number, joined into the struc- 

 ture called anther. The pollen grains (microspores) are numer- 

 ous in each sac and are formed before the flower opens. Like 

 the spores of Gymnosperms, Pteridophytes, and Bryophytes, 

 they are formed by special cells known as mother cells of which 

 there are many in each pollen sac as shown at A in Figure 408. 

 These mother cells also divide by the reduction division, that is, 

 by the kind of cell division in which the daughter nuclei get only 

 half the sporophytic number of chromosomes. The mother cells 



A B 



Fig. 408. — The spore mother cells of Angiosperms. A, cross section of 

 a young anther, .showing the microspore mother cells (m). B, section through 

 an ovule, showing the megaspore mother cell (m). Both are highly magnified. 



are formed and undergo the reduction division whUe the flowers 

 are still small buds. Immediately following the division of the 

 mother cell, the daughter nuclei resulting from this division 

 divide and consequently there are four spores or pollen grains 

 formed from each mother cell. The four spores constituting 

 the progeny of a mother cell are called a tetrad. The cells of the 

 tetrad commonly chng together for a short time after they are 

 formed, but soon separate and each becomes a pollen grain. The 

 pollen grains are in reahty the one-celled stages of the male 

 gametophytes, since they have the reduced or gametophytic 

 niunber of chromosomes. Usually before the pollen grain 

 leaves the anther its nucleus divides, forming a tube and genera- 

 tive nucleus. In this condition the pollen grain is carried to the 



