548 



sections showed more accumulated starch in the paHsade cells 

 under the black coat than where the coat was absent. 



Since such a coating of lam])black as was used in these 

 tests absorbs and intercepts practically all the luminous rays 

 of sunlight which fall upon the upper surface of the leaf, and 

 since it was found that carbon assimilation progressed normally 

 in spite of the black coat, only two possible explanations were 

 apparent. Either enough diffuse' light entered the leaf from 

 beneath, for this surface was not coated, or else the light rays 

 falling upon the black surface were absorbed, transformed and 

 transmitted as another form of energy into the leaf cells 

 beneath where this energy became effective in photosynthesis. 



Upon the under surface of some orange leaves, which several 

 days before had been coated heavily with lampblack on the 

 upper surface, were attached some lightproof boxes. Some of 

 these were Ganong's partial leaf form light screen, and others 

 were a somewhat different home-made type which were even 

 more effective than the Ganong type for excluding light while 

 admitting the passage of air to the stomata on the lower leaf 

 surface. These light screens were attached to the partially 

 coated leaves at evening time and allowed to remain until the 

 next afternoon. The leaves were then removed from the tree 

 and narrow strips cut from three areas, as follows : ( 1 ) From 

 an area which had been uncoated above and not screened 

 beneath, (2) from an area which had been coated with lamp- 

 black above and not screened beneath, and (3) from an area 

 which had been coated with lampblack above and protected from 

 the entrance of light to the lower surface by a light screen. In 

 some instances the portion clean above but screened beneath 

 was also cut for sectioning. These strips were then prepared 

 for microtome sectioning, sectioned ten microns thick and 

 stained with iodine and photographed. 



Careful examination of hundreds of these sections has con- 

 vinced me that as much carbon was fixed in that portion black- 

 ened above and screened beneath as in the portion entirely 

 clean and not screened beneath. This, then, leads us to the 

 conclusion that the energy which effects the carbon assimilation 

 is entering through the surface coating, no matter what that 

 material may be nor what its color or opacity may be. 



