542 



one layer with sometimes a partial second layer. The mesophyll 

 cells are very spongy, with large cavities between them. The 

 stomata are all on the under surface of the leaf. 



Methods. 



Several methods were used to determine the effects of the 

 surface coatings on leaves. Two methods were found espe- 

 cially satisfactory. One of these was essentially Sach's method 

 for determining by weight the increase of dry matter in a leaf 

 during the daytime. JNIany leaves were prepared on the tree 

 with one-half of the upper surface coated with a given sub- 

 stance and the other half not coated and the lower surface 

 remaining normal. Usually the midrib was taken as the dividing 

 line between the two halves, but in some leaves the half near 

 the petiole or in others the apical half was coated. In all cases 

 a suf^ciently large number of leaves was employed to secure 

 more accuracy. Several days were allowed to elapse between 

 the time of coating the leaves and using them further in the 

 experiment so that the leaf might accommodate itself to the 

 changed conditions. In the morning, before daybreak, one 

 circular disc of one sq. cm. area was cut from each half of 

 a hundred or more leaves very carefully, and quickly killed in 

 an oven after having been carefully cleaned in water by rub- 

 bing with the fingers. In cutting these discs the larger veins 

 were avoided, although it may be said that orange leaves do 

 not have as prominent veins as many other plants often used 

 in such experiments as this. In the middle afternoon the same 

 leaves were cut from the tree, cleaned in the laboratory thor- 

 oughly and as quickly as possible, and then a number of discs 

 were cut, with the same instrument as used in the morning, 

 from each half. These four lots of discs were dried at 100 

 degrees C. for about two days, or until thoroughly dried, and 

 then weighed to .001 of a gram. The relative weights per 

 square meter area were computed, the greater weight of the 

 discs cut in the afternoon representing accumulated dry matter. 

 No attempt was made to determine what this dry matter con- 

 sisted of, but presumably it was mostly starch and sugars. 

 Some objections have been raised by certain physiologists 



