87 



Low temperatures and light Intensities probably aeooxmt for the 

 low values of dry weight fol* the last two periods of the sea- 

 son. The temperature record for this station stops at the 

 period beginning Sept. 2 and the light record at the period 

 beginning Aug. 5, however. For Ooleman, as for College, a 

 comparison l:etween the plants for the four-we^k and the tv.'0- 

 week periods brings out several interesting relations between 

 evaporatioh during the last two weeks of growth and the be- 

 havior of the plants. An inspection of the two week graph 

 for Coleman will show that the values of leaf-produot for 

 the periods beginning June 24, July 8 and July S2 are very high, 

 while after four weeks of growth the graph of dry weight in- 

 stead of sho?7ing high points for these neriods is would be ex- 

 pected from the two-week values, shows relatively lew points. 

 Also, the culture started June 11 -hich shows a relatively low 

 value for the txo-week growth period has the highest dry weight 

 of the season for this station after four weeks of growth. 

 This behavior of the plants may be explained by the evapora- 

 tion Values during the last two weeks of the four-week growth 

 periods mentioned. It will be noted that for the four-week 

 culture beginning Jxine 11 the evaporation during thejlast two 

 . weeks is relatively low while for the cultures beginning 

 June 24 and July 8 the opposite condition obtains. The plants 

 of the first-named culture were thus exposed to a lower aver- 

 age evaporation rate during the last two weeks of their growhh 

 than those of the two last-named cultures which probably accounts 

 for the high dry weight of the first as compared to the last 

 two. The culture started on July 22 shows a dry weight above 

 those beginning June 24 and July 8 and it will be noted from 



