50 BOTANY OF CROP PLANTS 



or stalk. When the filament is absent, the anther is said 

 to be sessile. A cross-section of an immature anther is 

 seen to have four chambers or locules, each with a number 

 of pollen mother cells; each pollen mother cell normally 

 divides to form four pollen grains. As the anther matures 

 the pairs of locules unite, thus forming two pollen sacs in 

 each anther. Finally, each sac splits open (dehisces) allow- 

 ing the pollen to escape (Fig. 22). 



Matiire Pollen Grain. — When the pollen grain is mature, 

 it consists of a wall surrounding a protoplasmic mass, the 

 essential parts of which are a tube nucleus and a generative 

 nucleus. At the time of pollen germination the latter di- 

 vides into two sperm or male nuclei. 



Pistil. — The pistil usually consists of an ovary, style and 

 stigma. The seeds are borne in the ovary. A cross-section 

 of a simple ovary shows it to have one locule or chamber 

 with one or more ovules attached to the wall. The tissue to 

 which the ovule or ovules are attached is the placenta. 

 A compound ovary (Figs. 21 and 131) usually has two or 

 more compartments, with an ovule-bearing tissue (placenta) 

 in each. We may also speak of the pistil as simple or com- 

 pound. A simple pistil has one carpel, which is in reality a 

 modified leaf bearing one or more seeds. A compound pistil 

 has two or more carpels. When the carpels are separate, 

 as in the strawberry (Fig. 151) the flower is said to be 

 apocarpous; when united, as in asparagus (Fig. 99), 

 syncarpous. 



Ovule. — Fig. 23 shows an ovule just before fertilization. 

 A central mass of tissue, the nucellus, is surrounded by an 

 inner and an outer integument, except for a small opening, 

 the micropyle. Within the nucellus is the embryo sac, at 

 this stage consisting of eight nuclei: two synergids, one egg 

 nucleus, three antipodals, and two polar nuclei. They occupy 



