MISCELLANEOUS 409 



The t^^Tll-leaf method — as do the other three methods, as we shall see 

 later — shows that the mam changes m mature leaves with night and day- 

 are changes in assimilable carbohydrates, soluble sugars, and starch (deter- 

 mined by the acid hydrolysis method). The acid hydrolysis undoubtedly 

 breaks dowm some other polysaccharides besides starch. During the day 

 photos;yTitliesis leads to the accumulation of starch and sugars in the 

 leaves, which increases the dry weight of the leaves. At night the starch is 

 partly or wholly digested and the sugars and starch in part used up by 

 respiration of the leaf and in part transported to other regions of the plant. 

 Dermy's data show that the total nitrogen of the leaf did not change measur- 

 ably from day to night. 



He finds that a modification of Sachs' half-leaf method gives results that 

 check ^^•ith the twin-leaf method. Sachs' method consisted in cutting a 

 given area from one-half of many leaves for the first sample and a like sur- 

 face from the other half of the leaves for the later comparable sample. 

 Two main criticisms of this method have been offered: the mutilation of 

 the leaf due to removal of the first sample modifies the processes going on 

 in the other half of the leaf, and the two halves of the leaves are not sym- 

 metrical. Consequently, equivalent tissue is not used in the two cases. 

 Another error m this method has been pointed out: the water content of the 

 leaf generally increases during the night and with it the area of the leaf; 

 consequently, the morning sample, while it has the same area, has less 

 tissue. Dermy's modified half-leaf method consists in selecting plants 

 that have symmetrical leaves and cutting off one-half of the several leaves 

 close to the midrib for the first sample and using the other half, discarding 

 the midrib, for the later comparable sample. In sampling, the right and 

 left halves of the leaves were taken alternately for the first and, of course, 

 the remaining halves for the later sample. Here, as in the twin-leaf method, 

 the several constituents were determined on the total amounts in the 

 samples and need not be related to surface or to wet or dry weight, all of 

 which are variables. Unlike the twin-leaf method, tliis method can be 

 used on plants with alternate as well as opposite leaves. Denny concludes 

 that when the half-leaf method is used with proper precautions it gives 

 reliable results. Apparently the disturbance due to mutilation is not as 

 great as has been assumed. 



As stated above, the dry and wet w^eights, the assimilable carbohydrates, 

 and the total leaf surface in mature leaves vary with day and night. There 

 are, however, tw^o fractions that remain constant in mature leaves during 

 day and night, according to Denny's findings: residual dry weight (the 

 total dry weight minus the assimilable carbohydrates) and the total nitro- 

 gen. Mason and Maskell ^^ had already used the residual dry weight as a 

 basis for calculating changes; and Denny confirms the constancy of this 

 fraction and justifies its use as a basis for calculating leaf changes. While 

 Denny finds in the leaves that he studied that the total nitrogen is a con- 

 stant, he warns that w^orkers ought to determine that it is a constant in 



