ioi 



the other stations which include 11 or 12 tv,-o-v/eek periods. 



When the two-7.-eek averages are compared with the four- 

 week averages, it is seen that the graph of leaf area and dry 

 weight for fout^^weeks do no+- parallel the grar)h of leaf-pro- 



duc t for two weeks. That is, the climate, registered in one 



tv/o-week 

 way by ^leaf-product fwhich is approximately proportional to 



leaf area and dry weight) is registered by the four-week leaf 

 area and dry vreight in quite another ay. It should be remem- 

 bered that the two-week and the four-week cultures were carried 

 on over practically the same interval cf time at each station 

 so th.at the plants of both series are registering the same 

 total climatic effect. It will be noted tha"^ the two week 

 stem height averages parallel the four-week stem height aver- 

 ages rather closely, however. This behavior of the various 

 growth measurements as it bears on the use of plants for 

 m,easuring environmental conditions will be discussed below. 



As has been noted above, if plants are to be used for the 

 measuremant of environmental conditions, it would seem neces- 

 sary to standardize the plants. It is known that a plant is 

 always changing in its power to rescind to external conditions. 

 Also, +^here is a change in *-h.e internal condition of the plant 

 resulting in a decrease in the ra-^e of growth as the nlant be- 

 comes older. Both of these points have been emphasized in 

 previous discussion. The relatively great effect of evapo- 

 ration in the second two weeks of growth on the dry weight pro- 

 duced is an illustration of th"^ first type of variation whJle 

 the gradual falling off of the rate of growth of stems as the 

 plants become older is an illustration o+ the second type. 

 By starting all the plants from the seed, as vas done in the 



