II 8 Journal of AgricultMral Research voi. x\ai, no. 3 



jfluid. Sections of stigmas show, as mentioned above, that during 

 the later stages of receptiveness pollen may be even partly sunken 

 in among the terminal cells. This, together with the gelatinous or 

 viscous nature of the stigmatic fluid, expecially some time after receptive- 

 ness, largely accounts for the difficulty in washing pollen from the stigma. 

 Also, the inward movement of water would partly counteract the washing 

 action, especially of light rains. In addition, during the early stages of 

 pollen germination the tubes tend to prevent pollen from being washed 

 away. At this time, however, the tube becomes the important considera- 

 tion instead of the pollen. 



All pistils are not subjected alike to rain action. Those on the upper 

 side of limbs and in terminal positions receive the direct impact of rain, 

 while those in the more protected positions, as in the interior parts of the 

 tree and on the under side of clusters, are shielded from the direct force of 

 the rain. 



It will appear from the foregoing that pollen is not so completelv washed 

 away by rain as has heretofore been supposed. This belief has become 

 general on account of the changes which take place in pollen when 

 it is placed upon a receptive stigma. Immediately upon coming in 

 contact with the stigmatic fluid, pollen becomes turgid and is more or less 

 immersed in it. Under these conditions its appearance closely resembles 

 that of the terminal cells of the stigma. If a dilution of the stigmatic 

 fluid and the washing away of pollen are the important inhibiting factors 

 in the setting of fruit, a short dashing rain at blooming time would, 

 at certain stages, do as much dam^age as a prolonged rain, because it 

 would be necessary for the pistil to again become receptive and pollination 

 to again take place. This, however, does not correspond with the general 

 observations of fruit growers nor with the conditions reported here. 



LIMITATIONS UPON FERTILIZATION 



If the statements regarding the effect of rain upon pollen and stigma 

 are correct, the failure of the plum to set fruit during unfavorable weather 

 conditions will have to be explained in another way. At the time the 

 pollen and pistil are maturing and functioning other factors are operating 

 which place certain definite limits upon the time fertilization is possible. 



On account of self-steriUty, the relative time of dehiscence and 

 receptiveness within the variety is not an important factor in the plum. 

 However, because the pollen is mature before the stigma and virtually 

 in a "resting stage" protected by a thick covering in addition to the 

 anther wall, it is less susceptible to injury than the stigma, in which 

 growth changes are still taking place. This difference in the relative 

 maturity of the two structures may largely account for the greater 

 hardiness of pollen during frosts. Upon germination the pollen enters 

 a phase of less resistance, and it shares to a greater extent the lot of the 

 stigma and style, which constitute the substratum for the pollen tube. 



