541 



The Effect ox Piiotosvxtiifsis of X'arioi's Leaf Coatings. 



The luminous portion of radiant energy appears to be indis- 

 pensable in the photosynthetic work of green leaves, and it is 

 a prevailing idea that this energy must fall upon the leaf 

 surface as illumination to be effective in carbon* assimilation. 

 A screen reduces the rate of carbon fixation in |)roportion to 

 the amount of illumination intercepted, seeming therefore to 

 bear out these statements. It is not at all clear in what manner 

 the illumination falling upon a leaf cell transfers its energy to 

 the chlorophyll to enable it to carry on its photosynthetic 

 function. 



Temperature is recognized as a ven- important factor in 

 photosynthesis, perhaps even more im])ortant than illumination, 

 for these are chemical reactions and subject to the \'an t'Hoff- 

 Arrhenious law of increasing rate of reactions in a rising tem- 

 perature, even though the coefficient of increase may dift'er in 

 the several chemical processes which comprise photosynthesis. 

 Much of the radiant energy falling u]ion and absorbed by a 

 leaf surface appears to be utilized by the cells as heat, only 

 about 0.5 to 3.0 per cent being used as I'r^^ht energy, according 

 to the views of many physiologists. 



Citrus, especially sweet orange, leaves are very favorable 

 for these studies because there are no stomata on the upper 

 surface, and also because during the night practically all the 

 starch stored during the day is used up. Microtome sections 

 of leaves removed from the tree just before dawn and stained 

 in iodine show that only a few chloroplastids here and there 

 in the mesophyll cells have any starch remaining in them. 



Structim^e of ax Ok.\X(-,e Le.\f. 



An orange leaf has a rather thick epidermis, especially on 

 the side exposed to the sunlight directly. Beneath this there 

 are two layers of palisade cells, and a partial third layer, very 

 closely crowded together. Leaves which grow near the trunk 

 or heart of the tree and are thereby shaded by the outer foliage 

 are verv much thinner than the outer leaves, from one-half to 

 four-fifths as thick. The i)alisadc cells, moreover, are in only 



