66 NATURE STUDY AND AGRICULTURE 
Protection of Stamens and Pistil.— We have not yet 
mentioned the use of the calyx, but that is easy to under- 
stand. Before the flower opens we see the calyx covering 
and protecting all the other organs until they have reached 
their full development. Next to the calyx lies the corolla, 
which we now see serves another useful purpose, for within 
its soft and delicate folds are wrapped up the essential organs 
of the flower,—the stamens and pistils. In many plants, 
however, the calyx and corolla are wanting, and the grow- 
ing stamens and pistil are protected in some other way. 
Imperfect Flowers.— In many plants cross-pollination 
is the only method of fertilization possible, because the 
IMPERFECT FLOWERS OF Box ELDER 
a. Cluster of staminate flowers. 6. Three of the same flowers enlarged. 
c. Pistillate flowers from another tree. d. The two ripened seeds from one of 
these flowers. 
stamens and pistil are not found on the same flower but on 
separate flowers. Such flowers are said to be imperfect. 
Willows, cottonwood, and box elders produce only staminate 
flowers on some trees and only pistillate flowers on others. 
In popular language the former kind are said to be male 
trees and the latter female. There are many other plants in 
which the same arrangement is found. Still others, like 
the pumpkin, squash, melon, and corn, bear the two kinds 
of flowers, the staminate and the pistillate, on the same plant. 
