PECAN ROSETTE. . 21 



cells than rounded leaves of the same plant, but the narrower form 

 of such leaves is due to a larger number of cell divisions with spin- 

 dles parallel to the long axis. 



In pecan rosette the general shape of the leaf seems to follow this 

 rule, depending upon the orientation of the cell divisions rather than 

 upon differences in the size or the shape of the cells. That is, the 

 linear shape is not due to the development of cells elongated parallel 

 to the midrib, but to a difference in the number of cell divisions in 

 the two axes; nor are reduction in both length and breadth of leaf 

 caused by a decrease in the size of the cells, but rather to a decrease 

 in the number of cell divisions in both axes. On the other hand, 

 there is often a large and localized change in both the size and the 

 shape of the diseased cells, but not as related to leaf shape nor neces- 

 sarily to leaf diameter. In the linear type of leaf the cells are as 

 likely to be enlarged parallel to the short as to the long axis of the 

 blade, and in portions of leaves profoundly reduced in both length 

 and breadth the palisade and the spongy cells are at the same time 

 often considerably enlarged in all three dimensions. 



As a result of a considerable number of measurements of the 

 thickness of leaves from healthy and rosetted trees of like age and 

 variety, striking differences were found associated with the disease. 

 (Table II.) Comparisons were made between leaves collected from 

 the north and from the south sides of trees, but no constant differ- 

 ences were found which could be referred to situation, since all 

 leaves were taken from the lower, outstanding branches, and under 

 these conditions those on the north received nearly or quite as much 

 light as those on the south periphery. 



In each case the figures are based on 10 to 15 measurements of 

 the thickness of each of several sections from comparable parts of 

 each of 10 or more leaves. From such mesfsurements it was found 

 that the average variation from the greatest thickness in normal, 

 individual leaves was 18 per cent, with extremes varying between 

 10 and 22 per cent. In the various types of rosetted leaves the ex- 

 treme differences in thickness varied between 11 and 62 per cent 

 of the greatest thickness. The least variation was found in the 

 nonmottled linear or aborted leaves, while the greatest differences 

 occurred in mottled leaves. Extreme variations in thickness of 

 normal leaves of the Frotscher variety were 131 to 187 microns, 

 while in diseased leaves of the same variety the range was from 70 

 to 234 microns. The smallest measurements were taken at the thin 

 places in the leaves where tissue differentiation was lacking. The 

 Van Deman specimens examined had slightly thicker leaves, but the 

 same relations in thickness were found to hold between the healthy 

 and the rosetted leaves of this and several other varieties. 

 76289"— 22 4 



