PLANT BREEDING 269 



from the pollen tube unites with the nucleus of the cell 

 in which the female, or egg, cell is located, and this, though 

 unable to form an embryo, may nevertheless grow and form 

 part or all of the endosperm, or albumen, which usually sur- 

 rounds the embryo in the seeds. In the corn, xenia affects 

 only the endosperm, though the fact that the second union 

 of cells has been made is proof that the embryo in such 

 seeds has also been produced from the sexual cells of two 

 different strains. 



Parthenogenesis. While it is the rule that neither seeds nor 

 young plants result from flowers unless fertilization takes place, 

 there are not a few species that are able to produce new embryos 

 without this process. The production of young plants in this 

 way, from what are essentially unfertilized eggs, is called par- 

 thenogenesis. This phenomenon is not entirely confined to plant 

 life. The aphids, or plant lice, reproduce by parthenogenesis, 

 and there are sometimes as many as thirteen generations of 

 parthenogenetically produced females before a generation con- 

 taining males is produced. Parthenogenesis differs from ordi- 

 nary vegetative reproduction in plants in that it always results 

 from an egg cell, while in vegetative reproduction any part of 

 the plant may give rise to a new plant. 



PRACTICAL EXERCISES 



1. Find the amount of variation that is exhibited by a hundred 

 specimens of one kind selected at random, counting or measuring the 

 parts as necessary. The following list is suggestive : pods of catalpa 

 (variation in length and diameter) ; peas or beans (number in pod) ; 

 sepals (colored organs) in hepatica (variations in number ; in color) ; 

 petals of bloodroot (number) ; ray flowers of daisy, sunflower, or other 

 composite (number); lobes of the leaves in mulberry (number) ; leaves 

 in poplar or apple (width and breadth) ; leaflets in mountain ash, locust, 

 or rose (number) ; fruits in a cluster of currants or grapes (number). 

 In counting or measuring make a column of figures in numerical order, 

 and opposite each figure make a straight mark each time a count falls 

 upon it. Every fifth mark is; made across the preceding four, so that the 



