UNFRUITFULNESS ASSOCIATED WITH INTERNAL FACTORS 493 



overlapping in the seasons of maturity of the two sex elements; otherwise 

 it is complete. Complete dichogamy insures pollination with some other 

 flower and perhaps with another plant. Incomplete dichogamy tends 

 in that direction, but still allows opportunity for a certain amount of 

 selfing. 



The frequent occurrence of dichogamy and consequently its impor- 

 tance in influencing the setting of fruit is not generally appreciated. 

 Kerner and Oliver^* state: " . . . It appears that all species of plants 

 whose hermaphrodite flowers are adapted to cross-fertilization by the 

 relative position of anthers and stigmas are, moreover, dichogamous, 

 although this dichogamy may be of slight duration. Plants with hetero- 

 styled flowers are also dichogamous, since those with short-styled and 

 those with long-styled flowers develop at different times. . . . As far as 

 we can tell at present all monoecious plants are protogynous. . . . 

 Alders and Birches, Walnuts, and Planes, Elms and Oaks, Hazels and 

 Beeches are all markedly protogynous. In most of these plants . . . the 

 dust-like pollen is not shed from the anthers until the stigmas on the 

 same plant have been matured 2 to 3 days. Sometimes the interval 

 between the ripening of the sexes is still greater. The majority of 

 dioecious plants are also protogynous." Both Waugh^^* and Dorsey^^ 

 call attention to the existence of dichogamy in the plum. Pecan varieties 

 have been classified in two main groups, those exhibiting dichogamy and 

 those which mature their stamens and pistils simultaneously. ^-'' 



Interesting as illustrating the influence of dichogamy on fruit setting are 

 certain experiments of Wester ^^^ with Anonas. Flowers of the cherimoya 

 (Anona cherimolia) and of the custard apple {A. reticulata) were found to shed 

 their pollen iu the afternoon from about 3:30 to 6:00. Flowers of the sugar 

 apple (.4. squamosa) discharge their pollen from sunrise to about 9:00 a. m. 

 A few trees of this latter species were found to shed their pollen in the afternoon 

 and these same trees did not shed any pollen in the morning. Many pollina- 

 tions were made, the results of all pointing to the same general conclusion. The 

 follo^^^ng account of one of his experiments illustrates the results obtained: 

 "... 1-43 flowers on one sugar apple tree were, in April and May, 1908, pol- 

 linated with their own pollen or that of flowers of other plants of the same 

 species, 41 with pollen of the cherimoya, 31 with pollen of the pond apple, and, 

 51 flowers with pollen of the custard apple. In no instance did fruit set where 

 the pollen was applied to the stigma simultaneously with the discharge of its 

 pollen; practicallj'' all responded where it was applied 15 to 48 hours previous to 

 this act, though here, as in the case of the cherimoya, the tree shed much of the 

 fruit before it matured OA\'ing to its inability to carry it all." 



The flower clusters of the caprifig, the dioecious form of the fig tree, afford 

 an extreme and very interesting instance of dichogamy.^" The stamens and 

 their pollen do not mature until shortly before the ripening of the fig, when the 

 wasps have attained their maturity in the gall flowers of the same flower clusters 

 and are ready to emerge and enter other fruits to which they carry pollen. On 



