juneie. 1919 Relation of Weather to Fruitjulness in Plum 115 



immediately preceding a rain, owing to the rapidity with which they 

 close. Anthers open as a result of drying, a condition which is brought 

 about by cutting off the water supply at the constriction of the filament, 

 and by evaporation, particularly from the suture. Anthers which have 

 dehisced close quickly when brought in contact with water, and, like 

 those which have not dehisced, remain closed as long as wet. Con- 

 sequently, pollen is not available for dissemination during a rain. A 

 careful distinction must be made between the normal shedding of pollen, 

 which takes place for the most part the first day or even the first few 

 hours an anther is open, and the washing away of pollen by rain, for the 

 reason that empty anthers close when wet but open again after a rain 

 when dry. Insect visits are reduced to a minimum, if not prevented, 

 under the same conditions that impede pollen dispersal. The cytological 

 studies show that plum pollen does not burst when wet by rain and 

 crossing tests show that it is not killed by moderate exposures to water, 

 although the results of Sandsten {14) indicate that humidity decreases 

 its longevity. x\s far as the pollen is concerned, therefore, a prolonged 

 rain acts primarily to delay pollination until conditions are again restored 

 which are favorable to dehiscence and dissem.ination. 



THE STIGMATIC SURFACE 



As in the case of anther and pollen, a study has been made of the 

 changes of the pistil during the functional period, which may be regarded 

 as a critical stage viewed from the standpoint of the relation of adverse 

 weather to the setting of fruit. 



Immediately before the receptive period the outer cells of the stigma 

 are turgid (PI. 14, C and D) and their papillate structure gives to the 

 surface a characteristic velvety appeal ance Vv^hich is readily distinguished 

 from the glossy, moist surface when receptive. Where the suture termi- 

 nates, the stigma has a distinct depression, and in the plum its surface 

 is more or less oblique to the axis of the style, with the higher margin 

 opposite the marginal suture fold. 



The terminal cells are one layer thick, and in longitudinal sections are 

 clearly distinct from the cells below on account of their large size, scant 

 cytoplasm, and conspicuous vacuoles. There is a slight variation in 

 the length of these cells in different species. In some, as in Sapa (P. 

 BesseyiXP. triflora), they contain spherical bodies, which stain deeply 

 and vary much in size, the larger ones being somewhat greater in cross 

 section than the nucleus. The scant cytoplasm in the terminal cells is 

 mostly located at the extreme terminal end in the form of a crescent. 



THE RECEPTIVE STIGMA 



Decided changes are noticeable in the terminal cells after the stigma 

 has become receptive. In sections made from stigmas 48 hours after 

 first becoming receptive the papillate cells are very irregular in outline 



