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N. H. Agricultural Experiment Station [Bulletin 274 



and fertilizes an ovule ; so it takes the blossoms on one whole tree of a 

 pollenizer to insure the production of a good crop on ten other trees. 



Under natural conditions in the orchard many factors prevent the effi- 

 cient utilization of pollen. Weather and the presence or lack of insects are 

 most important in this respect. The honey and bumble bees are the principal 

 insects involved in the transfer of apple pollen. 



The mature pollen grain is a specialized cell containing a great amount of 

 stored food. A large nucleus occupies a portion of it, or in the case of apple 

 pollen, two nuclei, one of which is known as the "tube" nucleus because it 

 directs the course of the pollen tube growth dow-n the style into the ovary. 

 The other is known as the generative nucleus because it contains the male 

 elements that function later in the fertilization process. 



Fig. 10. Terminal and undcTlying cells of stigma (OOOx). 



The nuclei and the stored food are enclosed by two layers of material. 

 The outer one is composed chiefly of cutin, a substance very resistant to 

 moisture and to the penetration of chemical substances. 



The bee visiting the opened flower in search of either nectar or pollen 

 brushes against the opened anthers and becomes covered with pollen grains 

 whicli stick to the hairs covering the body of the insect. Some of these 

 grains mav be deposited on the surface of the stigma of other flowers or 

 possibly the same flower. 



On reaching the stigma the normal pollen grain germinates by sending 

 out a so-called "tube" (Fig. 9.). This penetrates the tissue of the stigma 

 and grows downward through the central portion of the style in the direc- 

 tion of the ovary. This i)rogress is accompanied by the movement of the 

 tube nucleus down through the stylar tissue, with the generative nucleus 

 traveling liehind or l)eside it. During this period the latter divides into two 

 sperm nuclei. 



