l68 GROWTH AND WORK OF PLANTS 



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

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

 in Chapters XVI, XVII. 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 milkweeds, etc., the 

 pollen grains are held in one, or several, more or less compact 

 masses called a pollinium. 



280. 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 flowers (anemophilous flowers). Though in 

 many cases self-pollination takes place, cross-pollination is the 

 rule because of certain peculiarities of the flower (see Darwin, 

 Cross fertilization, 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- 

 pollinated flowers (entomophilous 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 cultivation, especially in greenhouses, it is some- 

 times necessary to hand-pollinate the flowers of some plants, as 

 tomatoes, strawberries, etc. 



281. 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 

 and the flowers of the same tree, but in all the flowers of that par- 



