8;i 



it will be observed that after four weeks of growth these plants 

 sho'.^ a lower dry v/eight value than the four-v/eek plants of 

 the culture starte 1 July 31 in spite of the fact that the two- 

 week plants :f the latter culture kave much lo-'er growth values 

 than the tv-'O week plants of the former culture, (see the two- 

 week graphs for the two cultures in question). It is therefore 

 evident that some influence operated during the second two 

 weeks of growth 6f ea^h of these cultures which tended to depress 

 the rate of growth of the plants started on July 17 and accele- 

 rate the rate of growth of those started July 31. The two- 

 week climatic graphs show that the rate of evaporation was 

 higher during the second two weeks of the four-week growth 

 period beginning July 17 than during the first two weeks, and 

 that the evaporation during th© second two weeks of the four- 

 week period beginning July 31 was lower than during the first 

 two weeks. The shifting of the growth maximiun in th© two sets 

 of cultures from the culture started July 17 for the two-week 

 plants to the culture started July 31 for the four week plants 

 thus receives a possible explanation in variations in the evapo- 

 ration rate during the second half of the longer culture periods. 

 While it is true that the changes in ev poration rate upon 

 which the above conclusion is based are slight, it is probable 

 that with high rates such as are found in these three periods, 

 evaporation may be at a critical point for the plants and 

 slight variati-^ns in this condition may very well produce rela- 

 tively large plant effects. The decreasing walues of dry 

 weight at this station for the last four periods of the season 

 seems to be most reasonably interpreted as related to the cor- 

 responding decrease in temperature index values. 



