364 Plant Physiology 



scended from some one ancestral or stock individual, and 

 there have been, so to speak, nothing but generations of 

 this original individual. Pollination between the different 

 trees in a block of Bartlett pears would, therefore, be con- 

 sidered self-pollination. 



According to Waite, some of the common varieties of 

 pear generally self-sterile are the following : Angou, Bart- 

 lett, Clairgeau, and many others, 22 in all. The varieties 

 showing a capacity for self-fertilization were 14 in number; 

 among these being the Flemish Beauty, Keiffer, Le Conte, 

 and Seckel. Climatic conditions have been shown to be 

 of some importance with respect to the general problem 

 of self-sterility (cf . Fletcher) . 



213. Self-sterility in other orchard trees. The apple, 

 plum, peach, and other fruits are similarly more or less 

 self-sterile. The investigation of self-sterility, which was 

 given a special impetus through the work of Waite, has 

 resulted in a modification in orchard-practices of im- 

 mense economic value. It is certain that the grower 

 needs to consider an adequate distribution of varieties 

 so that pollination with effective pollen may be secured to 

 all. A particular variety may not be constantly self-sterile 

 under diverse conditions, and it is clear that many prob- 

 lems respecting pollination await careful investigation. 



214. Parthenogenesis. There are exceptions to the 

 rule that the egg cell must be fertilized in order to develop 

 the embryo of the seed. The maturation of a cell occupy- 

 ing the position of an egg with or without the usual segre- 

 gation, or reduction, division and its development without 

 fertilization constitute parthenogenesis. This phenome- 

 non is reported to be characteristic of several forms oi 



