116 PLANT GROWTH 



The petals, which make the corolla or the showy part of 

 most flowers, are either united, as in petunia (Fig. 21, A) 

 and the zinnia (Fig. 20, B, C, D, and E), or separate as in 

 the rose (Fig. 22, A). Such flowers as the sweet pea and 

 snapdragon are called irregular flowers because of the irregu- 

 larity in the shape of their petals. The nectar glands are 

 usually attached at the base of the petals. Flower colors and 

 odors are known, at least in some cases, to attract insects. 



The stamens, inside the petals, vary in number in accord- 

 ance with the kind of flower. Some flowers have fewer sta- 

 mens than petals, some have the same number, and some 

 have many more (Figs. 20, E; 20, B; and 22, A and B) . The 

 enlarged top of the stamen is the anther, which bears the 

 pollen (Fig. 21, E) from which the male sex-cells develop. 

 The anthers may open to shed the pollen before the stigma 

 is "ripe," at the same time, or after, depending on the habit 

 of the plant. When the pollen and stigma are "ripe" at the 

 same time pollination is likely to take place between them, 

 but early- or late-maturing pollen favors cross-pollination. 

 The stamens are attached to the base of the flower through 

 which they get food, but often they adhere to the corolla as 

 in petunia (Fig. 21, B) . In a few flowers, such as the zinnia 

 (Fig. 20, E), they form a tube surrounding the style. 



The pistil is in the center of the flower. In the sedums 

 there are five pistils; the rose has several separate one-seeded 

 pistils in an urn-hke receptacle (Fig. 22, B and C), but in iris 

 there is a single pistil of three united modified leaves (called 

 carpels) as is shown in Figure 3, B, by the three cavities for 

 seeds. The petunia has two united carpels as shown by the 

 division in ovary (Fig. 21, C) and the divided stigma in D. 

 The top of the pistil is the stigma on which pollen must fall 

 in pollination. It usually has a finely irregular surface or 

 fine hairy covering which catches the pollen and which con- 

 tains a substance that stimulates its germination. The style 

 is a solid structure connecting the stigma with the base of 



