STERILITY. 445 



It denotes, not the non-production of fruit, but of 

 seed, the entire or partial loss of which, though a 

 disease strictly considered, is generally attended by 

 the production Of fruit, because the resources of the 

 tree are not exhausted. The Vicar of Winkfield 

 pear seldom produces perfect seed ; and hence it is 

 a very regular and abundant bearer. There are 

 some varieties of grapes which do not produce seed, 

 and this is regarded as a point of excellence. 



But, in a practical view of sterility, it will be well 

 to consider it only affecting the production of fruit. 

 Though all the parts of the flower may be perfect, 

 and proper fecundation take place, and even an 

 embryo be formed, yet the plant may be sterile as 

 to seed from habitual abortion, which is especially 

 liable to take place in those varieties in which the 

 walls of the ovary are highly developed. Hence, 

 in some of the finer varieties of pears, perfect seed 

 is a comparatively rare product. As the properties 

 which make the fruit valuable are altogether inde- 

 pendent of the seed, it is sufficient for all ordinary 

 purposes if impregnation has taken place to such 

 an extent as to insure the swelling of the sarcocarp 

 or intermediate flesh. Many cases of sterility result 

 from the fact that fertilization takes place only suf- 

 ficient to stimulate temporarily the contents of the 

 embryo sac and the fleshy w^alls of the fruit, but 

 not suflicient to ensure its growth to maturity. 

 Hence, after a time, the embryo ceases to grow, and 

 88 



