SOME PROBLEMS IN LIGHT AS A FACTOR OF FOREST GROWTH 231 



proved with improvement in the soil conditions. My feeling 

 is that the seedlings in the third box which are 'to be watered 

 by nutritive solutions will show greater mortality, and is based 

 on the theory that trees, in order to assimilate larger quantities 

 of nutritive substances contained in the soil, must have propor- 

 tionately a greater amount of light to enable the plant to assimi- 

 late the available food. For a given amount of nutrition in the 

 soil there must be a proportionate amount of light above the 

 soil. Since the light under the third box is at the minimum essential 

 to the life of the seedlings, while the soil conditions probably at their 

 optimum, the consequent loss of the seedlings therefore must be greater. 

 In other words, this experiment, I expect, will prove not only the 

 falsity of the general belief that trees became contented with less light 

 when grown on fertile soil but that just the reverse is the truth, namely, 

 that with greater soil fertility the light requirements of trees increase. 



2. A Test to Determine the Effect of Heat upon the Tolerance of 

 Forest Trees. 



In order to determine whether heat can change the light require- 

 ments of forest trees the following experiment may be suggested: 



Rows of seedlings of several species with different light require- 

 ments, say the pine, spruce, and oak, should be covered with boxes in 

 the same way as in the test of the effect of soil fertility. To provide, 

 however, for a difference in the temperature conditions under the boxes, 

 each lot of seedlings should be covered with two kinds of boxes — one 

 painted a shiny white, the other a dull black. The lots of seedlings 

 under these two kinds of boxes will then have the same minimum of 

 light and similar other conditions except heat. The difference in heat 

 under the two kinds of boxes which can be secured will probably be 

 not very great during one day — at most one or two degrees — yet 

 the total sum of heat received during the entire vegetative period by 

 the plants under the black boxes will be considerably greater than 

 that under the white boxes. Both the temperature of the air and 

 the temperature of the soil under the boxes should be recorded in 

 the usual manner. If the plants under the black boxes show a larger 

 survival and better development than the plants under the white boxes, 

 then it is evident that heat, either because in this experiment it is, like 

 light, also at its minimum, and therefore its increase improves the 

 growth, or that heat in general increases the energy of assimilation 



