STAMEN 461 
Stamen. — The stamen (microsporophyll) 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 
Fic. 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 while 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 cling together for a short time after they are 
formed, but soon separate and each becomes a pollen grain. The 
pollen grains are in reality the one-celled stages of the male 
gametophytes, since they have the reduced or gametophytic 
number 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 
