480 FUNDAMENTALS OF FRUIT PRODUCTION 



self pollination is effected without the aid of any outside agency, such 

 as wind or insects, the process is known as autogamy. Many of the 

 peculiarities of form, structure, color and odor of flowers are closely 

 associated with means for securing proper self or cross pollination. 

 Some of the factors which are of importance in aiding or preventing 

 pollination are discussed later. 



Germination of the Pollen Grain. — Pollination is usually followed 

 promptly by the germination of the pollen grain. This is brought 

 about by the absorption of water and various substances in the stigmatie 

 fluid. The grain swells and a tube is pushed out through one of the 

 pores in the outer covering or extine. The tube is formed by the intine 

 or inner covering which pushes out through the germ pore. As it elongates 

 it penetrates the tissues of the style by growing between the cells and as it 

 advances toward the ovarian cavity its rate of growth may increase. The 

 styles of the flowers of many species contain rows of cells that may be 

 looked upon as specialized conducting tissue for the purpose of guiding and 

 facilitating the growth of the pollen tubes. In other species there is 

 no evidence of such tissue. For the most part pollen tubes digest their 

 way as they go, by the secretion of a pectin-digesting enzyme. This 

 dissolves the middle lamella which is composed of pectin-like substances 

 that hold adjoining cells together and thus permits the insertion of the 

 pollen tube between them.^"^ Green^^ has shown that the pollen 

 of many kinds of plants contains diastase and some kinds were found to 

 contain invertase as well ; during the process of germination these enzymes 

 increase in amount. Presumably they are effective in rendering available, 

 for the nutrition of the pollen tube, food materials stored in either pollen 

 grain or style. This assumption is supported by work which showed that 

 pollination produces a rapid rise of respiratory activity in the gynaeceum. ^*^ 



In Pelargonium zonale the amount of carbon dioxide produced by 

 the pollinated flowers is 5.8 times greater than that produced by the 

 unpollinated flowers, though most other cases studied were somewhat 

 less extreme. It was also found that in every case pollination resulted 

 in some change in the respiratory coefficient — the ratio of oxygen taken 

 in to the carbon dioxide given off. 



Course of the Pollen Tube. — For the most part, the growth of the pollen 

 tube is- directed by chemotropic influences supplied by the tissues of 

 the ovary, the ovules and by the style and stigma. Miyoshi^* sowed 

 pollen grains on agar in which were imbedded pieces of stigma, ovary 

 and ovules of different degrees of development. The pollen tubes grew 

 toward the pieces from the vicinity of the stigma, but they were attracted 

 most strongly by ovules ready for fertilization, growing into the micropyle 

 in each instance. In other investigations pieces of stigmatie tissue were 

 observed to influence the direction of pollen tube growth at distances 

 up to 70 times the diameter of the poUen grain. ^*^ Pollen tubes are 



