8 BULLETIN 193 



Griffon (7) studied the action of the light which had passed 

 through leaves. He placed a privet leaf in a chamber with a known 

 percent of carbon dioxid, usually about 9 percent, and placed over the 

 top one or more leaves of the plants undergoing observation. In this 

 way all the light reaching the privet leaf was forced to pass through the 

 experimental leaf. A gas analysis made at the close of the trial en- 

 abled him to determine the amount of carbon dioxid which the privet 

 leaf had used. He thus determined the fact that light which passed 

 through one leaf of many species was still strong enough to decom- 

 pose carbon dioxid, when present in amounts of 5 to 10 percent, 

 but that the rapidity of the process was greatly reduced by this pre- 

 liminary filtration. In the sycamore maple this was reduced 48 times, 

 but in the beech only seven times. When two leaves were interposed, 

 no decomposition of carbon dioxid whatsoever took place, save that 

 a very feeble action was observed under the two beech leaves. Such 

 data show that although some light passes unabsorbed through both 

 sun and shade leaves and that in some cases it is able to decompose 

 carbon dioxid, yet the amount which passes is so small that it cannot 

 play an important part in the nutrition of the trees or in forest suc- 

 cession. It would seem probable then that a weakened white light 

 rather than a filtered light is the controlling factor in the development 

 of all trees (14). 



WEAKENED WHITE LIGHT IN FORESTS 







The intensity of the light falling on the forest floor as compared 

 with the total light in the open has been studied by a number of work- 

 ers. It is sufficient for present purposes to cite only a few of the re- 

 sults thus far obtained. 



^ First among the students of the matter is Wiesner (14) who 

 adapted the work of Bunsen and Roscoe to botanical research. Using 

 photographic paper 1 he attempted to determine the light values in for- 

 ests in many parts of the world and also the minimum light require- 

 ments for many plants. The following table has been compiled by 

 Zon and Graves (16) from the works of Wiesner "for some of our 

 native trees and shrubs and European species commonly grown here." 



Critics of this method often fail to appreciate the fact that photosynthesis is 

 not the only function of the plant affected by light. 



