172 Journal of A gricultural Research vol. xx, no. 3 



It is interesting to note that in certain respects the composition of 

 orange leaves changes with growth, somewhat as is the case with the 

 vegetative portion of other plants. With certain cereals a considerable 

 portion of the potassium, magnesium, phosphorus, and nitrogen migrate 

 from the leaves into other parts of the plant as maturity approaches (9, 

 10). The potassium tends to accumulate in the straw of rice, while the 

 magnesium, phosphorus, and nitrogen are translocated to the grain. 



The composition of citrus leaves differs markedly from that of cereals 

 in certain other respects. The ash content of the former increases much 

 more rapidly and reaches a very high point in the old leaves. The cal- 

 cium content increases very rapidly during the most actively growing 

 period and continues to be deposited in the leaves, although at a some- 

 what slower rate, almost until the time the leaves fall off. 



While it is probable that the composition of normal orange leaves 

 varies to some extent when grown in different parts of the world or on 

 different soils in a given locality, careful study of the analyses of the 

 Florida-grown leaves published by Blair (2) and those reported from 

 Italy by Olivieri and Guerrieri (13) suggests that these were immature 

 leaves. From Jensen's results (7), it is evident that his samples were 

 not composed of mature leaves. Recognition of the relationships be- 

 tween the age and the composition of orange leaves is especially im- 

 portant in the study of the composition of mottled leaves, as will be 

 pointed out more fully later. 



It does not necessarily follow from the preceding discussion that a por- 

 tion of a given element .potassium, for example, migrates back into other 

 parts of the tree after the leaves reach a certain stage of development. 

 Increase in the size of a leaf, owing to the elaboration of carbonaceous 

 matter, may dilute the nutrients present and, therefore, lower the per- 

 centage without there being an actual loss. To establish this point, it is 

 necessary to determine the weights of the constituents present per leaf at 

 different periods. From the average weights of the individual leaves at 

 each period we have calculated the content of the different constitu- 

 ents, expressing the results in grams per 1,000 leaves. (Table VII.) 



The old Navel leaves were considerably smaller on the average than 

 either those representing maturity or 6 weeks of age, while the mature 

 Valencia leaves were larger than the old leaves of the same variety. In 

 addition, the leaves of each sample of the Valencia variety were consider- 

 ably larger than the corresponding Navel leaves. 



Despite these irregularities in the size of the leaves, the data show that 

 the content of calcium in a given orange leaf increases very rapidly during 

 the early part of the growth period. In the Navel leaves, approximately 

 a tenfold increase in calcium content took place between the first and the 

 sixth week of age. From the sixth week to maturity a further increase, 

 more than twofold, took place, and finally the calcium content increased 

 still further as the leaves approached the time of normal dropping. 



