June i6. 1919 Relation of Weather to Fruit fulness in Plum 117 



in contact with water before the stigmatic fluid, this, as has been shown, 

 would not be prohibitive of subsequent normal development. Further- 

 more, since tests show that germination takes place in a considerable 

 range of concentration in a sugar solution, a partial dilution of the stig- 

 matic fluid as a result of water absorption would probably not alone 

 prohibit tube growth. Under greenhouse conditions and in the orchard 

 under favorable conditions a stigma, like a leaf gland, has more than 

 one period of active secretion. If the first fluid to be secreted was com- 

 pletely removed by rain, it would be again renewed under favorable 

 conditions, so that a short rain alone would not necessarily be detri- 

 mental. Even if the secretion were considerably diluted following a 

 rain, evaporation from the surface would result in a gradual concentra- 

 tion. Furthermore, the influence of rain upon the stigmatic secretion 

 could be considered of more importance if the stigma had only a single, 

 short period of activity. 



WASHING OF POLLEN FROM THE STIGMA 



The adherence of pollen to the stigma was first noticed in pistils which 

 had gone through the washing and numerous changes of solution in the 

 preparation for sectioning by the usual cytological procedure. Stigmas 

 which had passed through a 2 -day rain, in addition to the cytological 

 process, still held as many as 40 to 50 pollen grains. 



An examination under orchard conditions of stigmas which had been 

 subjected to a heavy rain of over 14 hours duration, showed that most 

 of the stigmas still retained a considerable quantity of pollen (PI. 14, B). 

 On one stigma 42 grains were counted. On another, which had passed 

 through a 2 -day rain while in bloom, there were 32 pollen grains, and 6 

 days afterward on still another there were 176. However, in the last 

 instance a part or all of the pollen could have reached the stigma after 

 the rain. 



In order to determine how readily pollen can be washed away, an 

 abundance of pollen was placed on a stigma which was then immersed 

 in water, the results being observed with a binocular miscroscope. At 

 the first impact of the water a few of the outlying grains were washed 

 away, but at the end of 10 minutes of vigorous stirring and dipping in a 

 pail of water, 73 grains still adhered to the stigma. While the number of 

 grains at the start was not counted, it was estimated that less than one- 

 fourth were lost. The outstanding fact is that not all of the pollen 

 was removed by a washing action, certainly as vigorous if not as pro- 

 longed as a rain. 



An explanation of the adhesion of pollen is found in the condition 

 of the respective stigma. In some of the fixed preparations there is a 

 slight staining area beyond the terminal cells of the stigma (PI. 14, A 

 and B), in depth about equal to the thickness of two or three pollen grains. 

 This undoubtedly represents the area in cross section of the stigmatic 



