Nov. 1. 1920 Composition of Normal and Mottled Citrus Leaves 189 



becomes materially increased as a result of the decomposition of such 

 materials (8). On the other hand, the occurrence of heavily compacted 

 layers of soil (plowsole) around the roots, especially when present im- 

 mediately below the depth of cultivation, and of soils of low organic 

 content {3) and low natural solubility afford conditions that are con- 

 ducive to mottle-leaf. Where such conditions occur, it is possible that 

 the supplies of those nutrients which are normally absorbed at relatively 

 high rates may become inadequate. The nature and extent of the root 

 system of citrus trees must also be considered in this connection. It is 

 interesting that the absorbing roots of citrus trees are not provided with 

 the usual root hairs. Consequently, they may possess less absorbing 

 surface than is afforded by other plants that normally absorb relatively 

 large amounts of nutrients. These and other related questions will be 

 more fully discussed elsewhere. 



The fact that mottle-leaf sometimes appears on trees that have been 

 injured by alkali suggests the possibility that alterations in permeability 

 occasioned by the presence of excessive concentrations of salts, or pos- 

 sibly toxic substances of other kinds in the soil moisture, may prevent 

 the roots from taking up normal amounts of calcium. 1 



If we may judge from the composition of normal leaves, the calcium 

 requirements during the period when mottle-leaf develops most pro- 

 nouncedly are extremely heavy. The leaves at that stage normally absorb 

 calcium at a high rate. 



Just why subnormal concentrations of calcium accompanied by super- 

 normal concentrations of potassium and phosphorus in the leaves should 

 afford conditions that tend to limit chlorophyl production is not known, 

 if indeed further investigations prove that such is the case. There may, 

 of course, be no causal relationship between these facts, but rather each 

 may be the result of causes not yet suggested. 



It is recognized that calcium is not a normal constituent of chlorophyl. 

 In addition, while iron is essential to the formation of chlorophyl yet 

 does not enter into its final composition, we are not aware that a similar 

 relationship exists between calcium and chlorophyl formation. Conse- 

 quently, even though further study should prove that mottle-leaf can be 

 produced as a result of an inadequate supply of available calcium, it is 

 probable that the lack of chlorophyl and its disappearance from the 

 localized areas of the leaves would be found to be indirect rather than 

 direct effects of a shortage of calcium. In any event, whether the 

 shortage of calcium or some other factor conditions the deficiency of 

 chlorophyl, photosynthesis is doubtless reduced by the lack of chlorophyl. 



With an adequate supply of nitrogen, phosphorus, and potassium pres- 

 ent in the soil moisture, osmosis might bring about the absorption of 



1 As is well known, the occurrence of mottle-leaf is sometimes correlated with the species of root stock, 

 but this phase of the subject has not been systematically investigated in California. Mr. H. Atherton 

 Lee has called the writer's attention to his studies on this phase of mottle-leaf in the Philippine Islands. 



