THE DEVELOPMENT OF THE FRUIT 



523 



by "singeing young buds with a hot platinum wire, by exposure of young 

 plants to chloroform gas, and by cutting away a portion of the pistil and 

 pollinating the stub both with and without the accompaniment of a 

 germinative fluid." The ovaries of certain orchids can be made to 

 develop into fruits by the mechanical irritation of the stigmas. ^^ 



Closely related to the effects of mechanical irritation and of various 

 chemicals on fruit setting are those of the presence or the stings of 

 certain insects. Miiller-Thurgau" stated that the presence of a certain 

 gall insect would cause the setting of pear flowers and a brief rapid 

 growth of the fruit, though these insect-infested specimens fell before 

 reaching maturity. Figure 54 shows a flower cluster of the LeBrun pear 

 shortly after petal fall. The outside flowers had been pollinated, had 

 set fruit, and were developing normally; of 

 the two center specimens one had not been 

 pollinated and was about to drop; the other, 

 infested with the gall insect, had not only 

 set but was enlarging much more rapidly 

 than fruits developing normally. Kraus^^ 

 reports that no.t only fruits but embryo- 

 containing seeds often develop from the 

 flower clusters of self sterile and self barren 

 apple varieties when those flower clusters 

 are attacked by aphids. The same devel- 

 opment has been recorded in the sweet 

 cherry.^" In such instances the resulting 

 fruits are generally much dwarfed and mal- 

 formed and seldom can the seeds be made 

 to germinate; as a rule the fruits contain 

 fewer and smaller seeds than normally devel- 

 oped specimens of the same varieties. ^"^ 



Some observations of Johnson"^ on this point are very interesting. Several 

 species of cacti often retain their fruits long after maturity. They may persist 

 for months or in some cases for years. Johnson, examining a large number of 

 plants of Opuntia versicolor in April and May, found only about 25 per cent, 

 bearing persistent fruits. However, about 9 out of 10 of those plants which did 

 bear apparently normal persistent fruits bore also abnormal gall fruits, the 

 result of the stings of one of the gall insects. This led Johnson to suggest, 

 "that the cause of the persistence of the normal fruits may be the same as the 

 cause of the abnormality as well as of the persistence of the far more common 

 gall fruits." 



One of the most interesting cases of the influence of the presence 

 of insects, independent of their pollen-carrying activities, on fruit 

 setting is found in the male fig, or caprifig.2i)*0;ii2 These are not in 

 fact male trees; their flower clusters contain both staminate and pistillate 



Fig. 54. — Fruit cluster of the 

 LeBrun pear. The central fruit 

 has been parasitized. The outer 

 two have set and are developing 

 normally. The other one is about 

 to fall off. In the cross section, 

 larvse are shown at g. {After 

 Midler-Thurgau.^'') 



