350 



GENETICS IN RELATION TO AGRICULTURE 



ful crosses." The various implements mentioned above are shown in 

 Fig. 147. 



Some of the Difficulties Attending Hybridization. (a) Different 

 Seasons of Maturity. This is a common obstacle to the crossing of 

 different forms. When it involves merely growing periods of unequal 

 length the difficulty can be overcome easily by planting at such times 

 that the various forms will flower simultaneously. When this is not 

 feasible it becomes necessary to resort to some method of preserving the 

 pollen. It has been found that pollen of certain species will retain 

 vitality for weeks or even months if it is kept very dry. Miss Kellerman 



FIG. 147. Implements used in castrating and depollinating hermaphrodite flowers. 

 Right, self-closing forceps, ordinary forceps, scissors and scalpel. Left, chip blowers and 

 syringes. (Courtesy U. S. Department of Agriculture.) 



reports that the most effective method tried by the Bureau of Plant 

 Industry was as follows : anthers were placed in dried vacuum glass tubes, 

 e.g., tube filled with anthers 1-2 inches, cotton % inch, exhausted to about 

 0.5 mm. pressure in the presence of sulfuric acid, the tube then sealed. 

 As far as practicable the pollen was kept at a temperature of 10C. A 

 simpler and very useful method is to make a double container by fitting 

 a small vial inside a larger one and partially filling the space between the 

 two with anhydrous calcium chloride or sulfuric acid, filling in the 

 upper portion with absorbent cotton and tightly corking the larger 

 vial. The anthers or pollen grains are placed in the inner vial after it 

 has been thoroughly sterilized and allowed to dry. 



(6) Failure of Fertilization. This may be due to many causes ranging 

 from simple morphological maladjustments to complex physiological 

 relations amounting to antagonism. Probably a very frequent cause of 

 unsuccessful crosses is failure of the pollen to germinate. When repeated 



