502 FUNDAMENTALS OF FRUIT PRODUCTION 



are not available to show to what extent, if at all, the degree of inter- 

 unfruitfulness can be modified by environmental conditions and it is 

 not possible to tell, without trial, which varieties are and which are not 

 interfruitful. 



In Reciprocal Crossings. — In the investigations with tobacco to which 

 reference has just been made, there was found a uniformity of behavior 

 between reciprocal crossings. ^^ That is, if a certain crossing proved 

 sterile, its reciprocal was likewise sterile and if one variety proved incom- 

 patible with two others, those two were likewise sterile to each other. 

 On the other hand, all grades of opposite results in interfertility have 

 been obtained in Verhascum phceniceum when reciprocal crossings were 

 made.^^^ In some instances when one plant was used as the male and 

 the other as the female parent there was complete compatibility and 

 when the reverse combination was attempted there was complete incom- 

 patibility. A similar condition has been reported in chicory. ^^^ Vitis 

 vinifera, V. bourquiniana, V. labrusca and V. cordijolia hybridize freely 

 with V. routundifolia and V. munsoniana when the latter two are used 

 as the pollen parent, but they hybridize much less freely when the re- 

 ciprocal crossing is made.^^ 



An interesting case of interf ruitf ulness of a reciprocal crossing but of intersteril- 

 ity when the crossing was made one way and interfertility when made the other 

 appeared in work done at the Georgia Experiment Station. ^^ Flowers of the 

 upland cotton, Gossypiiwi Barbadense, were crossed with pollen of the okra. 

 Hibiscus esculentns. Perfect cotton bolls were produced but the seeds were non- 

 viable. The reciprocal crossing resulted in normal appearing okra fruits and in 

 viable seeds. WeUington^'''^ secured seedless tomatoes by using pollen of the 

 Jerusalem cherry, Solanum pseudocapsicum, but no fruit was formed when the 

 reciprocal crossing was made. 



DUE PRINCIPALLY TO PHYSIOLOGICAL INFLUENCES 



Besides the effects of evolutionary and genetic influences in hmiting 

 the set of fruit there are a number of others that can be conveniently 

 grouped as physiological, though exact demarcation is impossible. 



Unfruitfulness Due to Slow Growth of the Pollen Tube. — Closely 

 related to the unfruitfulness and the sterility due to incompatibility is 

 that caused by the very slow growth of the pollen tubes in the style. 

 Indeed, this may be considered one type of incompatibility, due to 

 chemotropic influences. 



Darwin^^ made many crossings between different forms of heterostyled 

 dimorphic and trimorphic plants. He found that when pistils were 

 pollinated with pollen from stamens of corresponding height there was 

 a high degree of fertility; when pollinated from stamens of a different 

 height there were varying degrees of sterility. This sterility ranged 

 from slight to absolute. Pollen from stamens of a height corresponding 



