402 IOWA ACADEMY OF SCIENCE Vol. XXV, 1918 



Duriiiu- the blooming- period of 1916 there were a number of 

 warm, bright days which afforded an opportunity to study the 

 stigmas. Warm, bright days favor insect pollination and statis- 

 tics show that such days during the blooming period are most 

 favorable for the setting of fruit. On warm, bright days the 

 stigmas should be free from condensed atmosplieric moisture and 

 thus any liquids present could be regarded as secretions. In the 

 study of the stigma, pistils left exposed and pistils enclosed in 

 paper bags and cheese cloth were used. 



Stigmas ready for pollination and some that were pollinated 

 under control were brought into the laboratory and examined 

 for secretions. On bright, warm days when the pollen was ger- 

 minating well on the stigma, no liquids could be seen on the sur- 

 face of the stig;ma with the compound microscope. When 

 moTinted in oil there was no evidence of any liquid in the space 

 between the papillae. The liquid was being lost from the pa- 

 pillae by evaporation and not by exudation. Even on stigmas 

 kept enclosed in paper bags no liquid was found. 



Only a smiall percentage of the pollen on a well pollinated 

 stigma is suitably located for germination. The favorable loca- 

 tion is between the papillas as shown on figure 166. Those on the 

 tips of the papillse were only rarely observed to absorb enough 

 moisture to become turgid. The observations on the five varie- 

 ties studied, showed that the germination of the pollen does not 

 depend upon stigmatic secretions, which exude from the stigma 

 but upon the ability of the pollen to draw liquids from the pa- 

 pillae, and it is obvious from the nature of the pollen that water 

 is the only liquid necessary to start germination. Pollen lodged 

 between the papillae was able to absorb from the papillge the re- 

 quired amount of water for germination. The problem as to what 

 effect liquids have on germination when present on the stigma 

 was not worked out. Since apple pollen germinates poorly in 

 water, it is reasonable to infer that pollen germinates poorly on a 

 stigma wet with rain, and the fact that rain during the bloom- 

 ing period is unfavorable to the setting of fruit may be attrib- 

 uted in part to this. Of course such weather is usually accom- 

 panied by a low temperature, which retards the germination of 

 the pollen, ajid such weather is also unfavorable to pollination 

 by insects. A number of factors are concerned, but so far as 

 we could determine, there is no basis for the theorv that rains 



