STERILITY. 447 



one of the most powerful agents in drying up and 

 destroying the delicate tissue of the blossoms ; and 

 sometimes this takes place in flowers which have 

 been impregnated, as well as those which have not. 

 Such cases of failure are repeatedly noticed in this 

 country, as well as in England. There is no doubt 

 that sterility sometimes occurs from a superabun- 

 dance of this viscous fiuid. 



Fungi and insects often attack the organs of fruc- 

 tification, and cause more or less injury, according 

 to the stage of progress in which the blossom then is. 

 Sterility is sometimes occasioned by injury of the 

 embryo, before the time oi\Aoss,OYa,hy extreme changes 

 in the temperature. Those seasons which are usually 

 called the most severe, are generally best suited to 

 plants. A low degree of the thermometer is not 

 necessarily injurious if it is regular, and not inter- 

 mitted by warm seasons, which excite the bud and 

 cause it to swell, and sometimes even to put forth 

 blossom. If such seasons are succeeded by extreme 

 cold, they destroy the embryo. Such extremes are 

 not so liable to injure the bud in the early part 

 of winter, because then it would not be natural for 

 the bud to start ; but towards spring, when the sap 

 is just beginning to flow, such changes are more 

 serious. 



Sensitiveness to extremes is almost always con- 

 fined to foreign fruits, and is never noticed in native 

 varieties unless such extremes are very severe. 



