Moore: Self-Sterility 



205 



Thus a larger percentage of the 

 flowers was fertihzed from the cross- 

 pollinations than from the self-pollina- 

 tions. From this, alfalfa may be said 

 to be partly self-sterile. 



With the Shirley poppy {Papaver 

 rhoeas var. Shirley), there were eighty- 

 six flowers self-pollinated, using sixty- 

 eight different plants. There were 

 seventy-five flowers cross-pollinated, us- 

 ing seventy different plants. The fol- 

 lowing table gives the results of the 

 pollinations. 



For cross-pollination, the flowers were 

 emasculated about two days prior to the 

 opening of the flower. All flowers were 

 bagged with glacine bags. Since but 

 84% of the cross-pollinations set seed, 

 it suggests that a number of plants 

 may have been cross-sterile. However, 

 this cannot be definitely stated until the 

 work is again checked up as considerable 

 experimental error may be involved, 

 tests of the germination of pollen on each 

 stigma not having been made. How- 

 ever, on all of the stigmas from cross- 

 pollinations that were examined good 

 germination of pollen was observed. 



The species of Tradescantia, alsike 

 clover, alfalfa, and Shirley poppy showed 

 different degrees of self-sterility. Tra- 

 descantia was completely self-sterile; 

 in alsike clover about 2% of the flowers 

 set seed when self-pollinated; in alfalfa 

 27% of the flowers were fertilized with 

 self -pollen, and when the Shirley poppies 

 were self -pollinated 39% of the flowers 

 set seed. 



FUNCTION OF POLLEN TUBE 



From a morphological standpoint, the 

 pollen tube of the angiosperms has two 

 functions; first, the transferring of the 

 male generative nucleus to the embryo 

 sac, and second, the function of growth 

 which is one of food assimilation. The 

 pollen tubes of the Cycadales of the 

 gymnosperms are much branched and 

 according to Coulter (1910, p. 201) 

 always function as an absorptive organ. 



They do not have the function of 

 transferring the male nucleus since this 

 cell is a swimming sperm. In the higher 

 groups of the gymnosperms and angio- 

 sperms, the nucleus loses the power of 

 motility and the pollen tube has taken 

 on the function of transferring the 



THE TUBE GROWS 



Constantty absorbing nourishment from 

 the surrounding tissues, the tube 

 grows longer with great rapidity. It 

 may be that its elongation is due to 

 the need of seeking farther for food, 

 and that if the food supply around it 

 is very suitable for assimilation, it will 

 grow wide as in the case of Tradescan- 

 tia instead of long. This hypothesis 

 has been advanced to account for 

 cases of self-sterility. Photomicro- 

 graph by John Howard Paine. (Fig. 3.) 



nucleus to the embryo sac. In the 

 manner of assimilating food and growth, 

 the pollen tube acts in a similar manner 

 to the mycelium of a fungus. 



Compton (1913, p. 200) has suggested 



