306 



ELEMENTARY BIOLOGY 



stamens may be in one flower and the pistils in a different 

 flower, either on the same plant or on a different one. 



Some common moncecious plants (that is, plants having the stami- 

 nate and the pistillate flowers on the same individual) are birch, hazel, 

 chestnut, oak, walnut, hickory, squash, maize, and the cone-bearing trees. 



Some common dioecious plants are poplar, willow, box elder, tape- 

 grass ( Vallisneria), begonia, sassafras, and virgin's bower. 



FIG. 138. Pollenation by water 



The tape-grass ( Vallisneria) 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 indi- 

 vidual remains beneath the surface. The staminate flowers, b, are detached 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. When one of these float- 

 ing stamen flowers comes close to the pistillate flower of the species, the anther is brought 

 into direct contact with the stigma, and thus pollenation is effected 



2. Time relations. The stamens and pistils of some species 

 of plants do not ripen at the same time, close pollenation 

 being thus impossible in these species. 



The pollen ripens before the pistil in maize, in the mallows, in 

 many species of the aster family, in the creeping crowfoot, and 

 in the sage. 



The pistils ripen ahead of the stamens in the common plantain, 

 in the potentilla, or cinquefoil, and in the oriental grass known as 

 Job's tears. 



