exposed plants. Also, we may be reasonably sure that the tem- 

 perature under the glass w-s somewhat higher than the tempe- 

 rature outside, especially on quiet days when circulation of 

 air would be slight with a consequent slight tendency toward 

 equalization of temperatures under the glass and temperature 

 outside. In considering the behavior of the covered cultures, 

 it will be borne in mind, then, that the evaporation is known 

 to be higher and the light intensity lower for these than for 

 the exposed stations while the temperatur'^^ is probably higher 

 for the covered than for the exposed. 



The effect of the glass was shown by the plants in two 

 ways: (1) growth was always greater for the covered stations 

 than for the exposed, and (2) the plants of the covered sta- 

 tions showed a noarked difference in m.-inner of grorxth from the 

 plants of the exposed stations. The greater growth of the cov- 

 ered plants was shown In some cases by one, in some cases by 

 two or by all three of the growth measureraents taken. Not 

 only do the plants show greater growth, but the maxima in the 

 graphs of the various growth measureraents for the covered plants 

 do not usually coine at the same times as the maxima in the cor~ 

 responding graphs for the plants grown in the open. The prin- 

 cipal effect of the covering on the way in which the plants 

 grow is shown by a disturbance of the relation between dry 

 weight and leaf area. In previous discussion of this relation 

 for the exposed plants it was noted that the relative dry weight 

 and leaf area numbers are approximately the same for the four- 

 week plants. In the case of the covered stations, on the other 

 hand, every culture shows relative leaf area higher, usually very 

 much higher, than relative dry veight. The stem height in the 



