FLOWERS 117 



the pistil, called the ovary, which contains the one to many 

 ovules (Fig. 21, C and D) in each of which a single egg cell 

 develops. The pollen tubes must grow through the style, 

 which may be quite long as is the corn silk which extends 

 to the kernel. At least one pollen tube must grow through 

 the style into the ovary to enter the micropyle of each ovule 

 and discharge a pair of sperms for fertilization. Careful 

 studies of the petunia were made in which it was learned by 

 counting the seeds of several mature ovaries that each ovary 

 contains at least five hundred seeds (Fig. 21, C and D). 

 Since each of these seeds requires a pollen tube, the slim 

 style (Fig. 21, D) about the size of a large pin, must contain 

 at least five hundred pollen tubes. Other workers have 

 found that usually more than twice as many pollen grains 

 begin to grow pollen tubes as are needed to fertilize all the 

 ovules. Although the diameter of the petunia stigma as 

 shown in Figure 21, D, is about one-tenth of an inch, it is 

 large enough to hold a thousand pollen grains, each of which 

 is only about one-sixth of one-hundredth of an inch in diame- 

 ter. A single layer over the entire stigma would require more 

 than two thousand pollen grains. They are so small that 

 only masses are visible and for that reason the cavity of the 

 anther is shown in Figure 21, E, without pollen. 



Flowers differ in many respects; in fact, they are so differ- 

 ent that most plants can be identified by their flowers. Their 

 differences may be marked, and constitute a group, as the 

 following three which will be described in some detail: the 

 sweet-pea or clover type, the sunflower type, and the grass 

 type. 



The sweet-pea type has two petals forming a keel over 

 the stamens and pistil, two wing-like petals, and a broad 

 petal opposite the keel, called the standard. Nine of the 

 stamens have the lower parts of their filaments united to 

 form a tube around the pistil, the tenth one remains separate. 

 The pistil has the stigma at one side instead of on the end, 

 and the ovary with a single cavity containing several ovules. 



