Nov. i, 1930 Composition of Normal and Mottled Citrus Leaves 177 



maturity. In addition, mottle-leaf is usually most pronounced from 

 September to February, when it becomes very noticeable on the leaves 

 of the previous spring and summer cycles of growth. 1 



Some light may be thrown on mottle-leaf by comparing the compo- 

 sition of mottled leaves with that of normal leaves at different stages 

 of growth. By reference to Tables V and VIII it will be seen that the 

 composition of the ash of the former is quite similar to that of normal 

 leaves approximately 6 weeks of age, although the total ash content of 

 mottled leaves is considerably higher (compare Tables VI and IX). 

 It is especially interesting to note that the nitrogen content of mottled 

 leaves is somewhat higher than that of normal leaves at the age of 1 

 week and much greater than that of normal leaves at the age of 6 weeks. 



The data indicate, therefore, that the essential nutrients are depos- 

 ited in mottled orange leaves at abnormal rates. A satisfactory expla- 

 nation of this fact can not now be given. The rising sap is itself probably 

 abnormal in composition. 



By calculating the weights of the several constituents contained in 

 a unit number of mottled leaves, it is found (Table X) that notwith- 

 standing the fact that the average size of the mottled leaves was less 

 than one-half that of normal leaves they contained as great amounts of 

 potassium and approximately as much phosphorus per leaf (compare 

 Tables VII and X). On the other hand, the content of calcium was 

 less than one-third as great as normally occurs, while the magnesium, 

 sulphur, and nitrogen were intermediate in amount. 2 



The preceding analyses represent extreme cases of mottling. Sam- 

 ples of Valencia orange leaves at a less advanced stage have also been 

 studied. These latter were of an intermediate size, showing the typical 

 yellowish spots between the veins. They were selected from trees a 

 considerable portion of whose foliage was normal and some of which 

 bore a fair crop of fruit. The results are recorded in Table XI 



The percentages of calcium and potassium closely approach those of 

 severely mottled Valencia leaves (Tables VIII and IX), but the phos- 

 phorus content is more nearly normal. The percentage of nitrogen was 

 found to be no greater than occurs in normal Valencia leaves. 



Thus, it appears that the early stages of mottling are first attended by 

 the absorption of subnormal amounts of calcium 3 and supernormal 

 amounts of potassium and phosphorus, and that modifications in the 

 absorption of nitrogen occur later. 



'Mottled leaves fall off in large numbers during the latter part of the -winter and early spring. New 

 shoots developing at this season give the trees the appearance of having recovered from the disease. These 

 latter, however, may become mottled the following fall. It is never safe to pass judgment on the state 

 of the disease in the spring or early summer. We have never known of a leaf once severely mottled which 

 became normal later. New leaves grown later, however, may be entirely normal. 



2 These data were calculated for only a portion of the samples of mottled leaves, because the average 

 weight of the leaves was not determined for all the samples. 



» Jensen (7) found that the yellow spots of mottled orange leaves, similar to those discussed here, contain 

 less calcium than the remaining portion of the leaf. 



9507°— 20 2 



