550 



Martin * states that increase in transpiration in a spray-coated 

 leaf probably is not to be explained by the increased tempera- 

 ture. If, however, a leaf normally reflects from its surface a 

 large percentage, perhaps 50 per cent, of the incident radiant 

 energy, but a coated leaf perhaps not more than 20 per cent, 

 the increase in heat units within the coated leaf would be 

 considerable and might well explain the increased transpiration. 



In most of the stained sections of coated and uncoated 

 leaves it was found that more starch accumulated in the palisade 

 cells beneath a surface coating, whether the coating were opaque 

 or not, than accumulated in the palisade cells of a clean leaf. 

 This mig"ht indicate that some element or quality of direct 

 sunlight inhibits somewhat the photosynthetic activity in the 

 outer palisade cells, and that this inhibitive element or quality 

 is removed by the presence of a surface coating. Perhaps it is 

 merely the illumination itself which is the inhibitive factor, 

 and that the normal function of the outer densely chlorophyllous 

 palisade cells is to screen out the illumination and transform 

 its energy to another fomi useful to the leaf cells. In an 

 uncoated orange leaf the greatest amount of starch, by the 

 iodine-stained section test, is accumulated in the mesophyll 

 cells, but in a coated leaf as great an amount seems to be 

 accumulated in the palisade cells. 



It Is interesting to note, too, that in those leaves which 

 were coated above with lampblack and darkened beneath with 

 a light screen the mesophyll cells next to the lower epidermis 

 showed as much starch accumulation as did the corresponding 

 cells in a normally illuminated leaf. 



Conclusion. 



From the foregoing experimental data we may draw at 

 least one conclusion. While photosynthesis depends on illu- 

 mination for its energy, that illumination need not fall actually 

 upon the leaf surface. A more or less opaque surface coating 

 on the leaf does not exclude the energy of illumination falling 

 upon it, but transfers that energy in some form through to the 

 leaf cells. 



* Martin, W. H., Journal of Agr. Ees., Washington, 1916: 529-547. 

 See also, Duggar, The Effect of Surface Films on the Rate of Transpira- 

 tion, Ann. Missouri Bot. Garden. 



