494 FUNDAMENTALS OF FRUIT PRODUCTION 



the other hand the pistillate flowers of the fig are receptive weeks, or even months, 

 earlier. In this way the wasps, carrying the pollen from one crop {e.g., the pro- 

 fichi) of the fig, enter the flowers of the following crop (mammoni) at a time when 

 their stigmas are receptive. It is possible for self pollination to take place within 

 the tree, but there is at least a crossing between two successive crops of the 

 caprifig and there is often actual cross pollination between trees or varieties. 



Commenting on the significance of dichogamy Kerner and Oliver^^ remark: 

 " From these facts we may infer that every dichogamous plant has an opportunity 

 for illegitimate crossing or hybridization at the beginning or end of its flowering, 

 and that dichogamy — especially incomplete dichogamy — is the most important 

 factor in its production. Of course this does not exclude dichogamy from playing 

 an important part in legitimate crossing as well. On the whole, however, we 

 can maintain the view that the separation of the sexes by the maturation of the 

 sexual organs at different times leads to hybridization, while their separation 

 in space promotes legitimate crossing. The fact that the separation of the sexes 

 in time and space usually occur in conjunction harmonizes with this conclusion, 

 i.e., that the dioecious, monoecious, and pseudo-hermaphrodite flowers, as well 

 as those hermaphrodite flowers whose sexual organs are separated by some little 

 distance, are in addition incompletely dichogamous, because by this contrivance 

 the flowers of any species obtain (1) the possibility of hybridization at the begin- 

 ning or end of their flowering period, and (2) of legitimate crossing during the 

 rest of that time. This also explains why incomplete dichogamy is so much more 

 frequent than complete dichogamy ; why there are no dioecious species of plants 

 with completely dichogamous flowers; and why, if one ever should occur, it 

 would of necessity soon disappear. Let us suppose that somewhere or other 

 there grows a species of Willow with completely protogynous dioecious flowers, 

 that is to say, a species in which the female flowers mature first, and have ceased 

 to be receptive before the male flowers in the same region discharge their pollen. 

 Hybridization only could occur in it, and the young Willow plants resulting from 

 it would all be hybrids whose form would no longer agree absolutely with that 

 of the pistiUiferous plant. The species would therefore not be able to reproduce 

 its own kind by its seed, and it would leave no descendants of similar form; in 

 other words, it would die out." 



Data are not available as to the exact degree of dichogamy char- 

 acteristic of different species and varieties of the deciduous fruits ; therefore 

 it is impossible to state accurately the extent to which it interferes with 

 their self pollination or to what extent it is a factor in determining their 

 fruit setting. Furthermore, as is shown later, the completeness of 

 dichogamy varies considerably with environmental conditions. There 

 can be no question, however, but that in many varieties it explains the 

 failure of numerous blossoms to set. 



Impotence from Degenerating or Aborted Pistils or Ovules. — It is 

 obvious that, if the setting and maturing of fruit usually depend on the 

 union of two properly formed sex cells, anything which occurs to interfere 

 with the development and proper functioning of either gamete probably 

 will result in unfruitfulness or at least in sterility. This occurs in the 



