528 



Journal of Agricultural Research voi. xvin. No. lo 



this pollen, either by artificial means or on the stigma. On the other 

 hand, normal pollen grains will burst from too rapid and too extensive 

 water absorption if the water condensation is too rapid. Weather con- 

 ditions affect the growth of pollen both in the field and in the laboratory. 

 Germination is accomplished with difficulty on cold, wet days. There 

 is very little fertiUzation in the field under such conditions; and ger 

 mination in the laboratory on such days is obtained with difficulty, 

 because the necessary conditions are less easily secured and viable pollen 

 is much harder to obtain. 



FERTILIZATION IN THE FIELD 



In order to correlate the laboratory experiments with fertilization under 

 field conditions, extensive experiments were made by the junior author 

 in 1917 and 1918. Both the period of receptivity of the stigma and the 

 duration of viability of the pollen were studied. In the study of the 

 receptivity of the stigma the results were strikingly uniform. Two 

 hundred and eighty flowers were emasculated in one afternoon. For 

 this purpose spikes were chosen which, by the length of the protruding 

 awn and the first suggestion of the parting of the leaf sheath to release 

 the side of the spike, indicated that fertilization would have occurred the 

 following day. Forty flowers were pollinated immediately upon emascu- 

 lation. Forty were pollinated on each succeeding day for six days. 

 The results are shown in figure i. 





Fig. I. — Period of receptivity of the stigma under field conditions as shown by the number of seed pro- 

 duced ■when 40 flowers were pollinated on emasculation and the same number on six successive days after 

 emasculation. 



The percentage of successful pollinations increased for two days. Of 

 those pollinated two days after emasculation, 100 per cent of the ovaries 

 set seed. From this time there was a gradual decrease, until on the 

 sixth day no pollinations were successful. It is obvious that failures in 



