METHODS OF POLLINATION 183 



293. The pollen grains and how they are shed from the 

 anther. The form and parts of the stamen have been described 

 in Chapters XVIII, XIX. The anthers open in different ways in 

 different flowers. In some the anther opens by long slits, in 

 others by a trap door, and in others by a pore at the end of the 

 locule. The pollen grains are all free, and loose or dusty, or 

 they may be held rather loosely together by a viscid or fibrous 

 substance. In some flowers, especially the orchids, the milk- 

 weeds, etc., the pollen grains are held in one, or several, more 

 or less compact masses.* 



294. Kinds of flowers as regards methods of pollination. 

 Most flowers can be placed in three different groups according 

 to the general method of pollination. First. Those which are 

 self -pollinated, where the anthers lie close to the stigma, or above 

 them, and open at the same time that the stigmas are ready to 

 receive the pollen. The pollen is usually shed directly on the 

 stigma in the same flower. Second. Where the pollen is trans- 

 ported by the wind to the stigmas of a different flower. These 

 are wind-pollinated.-^ Though in many cases self-pollina- 

 tion takes place, cross-pollination is the rule because of 

 certain peculiarities of the flower (see Darwin, Cross-fertili- 

 zation, etc.). The pollen in such flowers is dusty, so that it is 

 easily wafted by the wind. Third. When the pollen is carried 

 by insects from one flower to another. These are insect-polli- 

 nated% flowers. (Self-pollination in some cases may take place 

 here.) The pollen in these flowers is usually held together 

 loosely or firmly by a viscid substance. Humming birds also 

 assist in the pollination of some flowers. In a state of cultiva- 

 tion, especially in greenhouses, it is sometimes necessary to 

 hand-pollinate the flowers of some plants, as tomatoes, straw- 

 berries, etc. 



295. A knowledge of these laws is of great importance to 

 the horticulturist and florist. In many varieties of pears, although 

 the flowers are perfect, it has been found that the pollen is 

 impotent or very weak, not only on the pistil of the same flower 



* A polliniurn. j~ Anemophilous. ^ Entomophilous, 



