46 



BIOLOGY AND HUMAN LIFE 



flowers, some bearing only stamens and others bearing only 

 pistils, as the corn and other grasses, birch, hazel, chestnut, 

 oak, squash, and the cone-bearing trees. Such plants are some- 

 times called monoecious, meaning "of one household." It is of 

 course impossible for close-pollenation to take place in these. 



There are still other plants in which the stamen-bearing flowers are 

 borne on one individual and the pistil flowers on a different one, as in 

 poplar, wiUow, box elder, tape grass {V allisneria) (see Fig. 20), begonia, 



Fig. 20. Pollenation by water 



The tape grass {V allisneria) is a dioecious water plant. The pistillate individual 

 grows up to the surface of the water, where the flowers, a, are opened, while the 

 staminate individual remains beneath the surface. The staminate flowers, b, are de- 

 tached from the stalks and rise to the surface, where they float about and gather in 

 large numbers in the quiet stretches of water close to solid objects of various kinds. 

 Whenever one of these floating stamen flowers comes close to the pistillate flower of 

 the species, the anther is brought into direct contact with the stigma, and in this 



way pollenation is effected 



sassafras, and virgin's bower {Clematis). Such plants are sometimes 

 called dicecioiis, meaning "of two households." Close-pollenation must 

 be impossible in these plants also. 



2. Time relations. In some species of plants the stamens and 

 the stigmas do not ripen at the same time, close-pollenation 

 being thus impossible. 



The pollen ripens before the stigma in maize, in the mallows, in many 

 species of the aster family, in the creeping crowfoot, and in the sage. 



