142 BOTANY AND PHARMACOGNOSY. . 



duction of fruit, hand-pollination is practiced, as by the growers 

 of vanilla and some other tropical plants of economic importance. 



In the case of unisexual flowers, or those in which the stamens 

 and pistils are in separate flowers, there is of course no chance 

 for self-pollination. Here, as in the case of cross-pollinated 

 hermaphrodite flowers, pollination may be more or less close or 

 it may be remote, as between flowers of the same cluster or inflor- 

 escence, between flowers of different clusters or inflorescences on 

 the same plant, or between flowers on different plants. 



In buckwheat (Fig. 87, illus. i and 2) and partridge berry 

 (Mitchella repens) two kinds of flowers are produced, viz. : (a) 

 one with short styles and long filaments, and another {h) with 

 long styles and short filaments, and thus the flowers appear to be 

 especially adapted for insect cross-pollination and are called 

 DIMORPHIC. In still other cases one species gives rise to three 

 kinds of flowers, depending upon the difference in the relative 

 lengths of the styles and filaments, as in the purple loosestrife 

 (Lythrtim Salicaria), and such flowers are called trimorphic. 



The external agents which are instrumental in carrying pollen 

 from one flower to another and thereby promoting cross-pollina- 

 tion are the wind, water currents, insects, small animals and 

 birds, such as humming-birds, which are, even in temperate 

 regions, to be observed visiting the garden nasturtium. 



In many of the early-flowering trees, as well as pines, Indian 

 corn, etc., the flowers are devoid of showy, attractive features, 

 but produce large quantities of pollen which is more or less dr}' 

 and powdery and carried by the wind to other flowers. Flowers 

 which are wind-pollinated are classed as anemophilous and it is 

 estimated that about one-tenth of all the flower-producing plants 

 belong to this class. 



Plants which are pollinated by the aid of water-currents are 

 known as hydrophilous, and under this head are included those 

 plants which live under the water and those that produce flowers 

 at or near the surface of the water. 



Those plants which depend upon the visitation of insects for 

 the transferral of the pollen in cross-pollination are called ento- 

 MOPHiLOUS (Fig. 86). They frequently possess bright, highly 

 colored flowers and it is considered that these serve as an attrac- 



