PECAN ROSETTE. 19 



dwarfed, mottled, and roughened appearance typical of the second- 

 ary phase. Dormant and axial buds of one to several series may and 

 usually do prematurely develop' into depauperate shoots, and toward 

 the end of the season clusters of dwarfed branches are usually put 

 out from dormant and adventitious buds farther back on the branches 

 or main trunk. With each repeated sequence of premature abnormal 

 growth and subsequent dying back of the branches, the new twigs 

 and leaves tend to become more and more depauperate, so that a well- 

 marked case of several years standing presents a characteristically 

 gnarled appearance. 



HISTOLOGICAL AND CYTOLOGICAL STUDIES. 



No less striking than the external changes brought about by 

 rosette are the internal abnormalities of structure and metabolism 

 in the leaf. The abnormal histological characters develop with the 

 development of the leaf, and the most far-reaching internal derange- 

 ments are found in cases of the greatest external malformation. 



Within the yellow thin areas between the larger side veins (second- 

 ary stage) the tissue is usually less developed than normal, and the 

 cells are more closely packed together (PL V, figs. A and B; PL VI, 

 figs. E and F). In less severe cases the palisade tissue is well de- 

 veloped, but the intercellular spaces become almost obliterated (PL 

 VI, figs. A, B, and D). In other instances the palisade cells are dif- 

 ferentiated, but the individual cells are greatly shortened vertically 

 (PL V, figs. B, C, and D) and may be only two or three times as 

 long as broad, instead of seven to ten times, as in the normal leaf. 

 In the most severely affected leaves with extreme variations in leaf 

 diameter there is no differentiation into palisade and sponge tissue 

 at all in the center of these yellow spots (PL VI, figs. E and F), but 

 the tissue within these parts of the leaf consists entirely of closely 

 packed, more or less isodiametric cells without conspicuous air spaces. 

 Under these conditions the number of cell divisions may be somewhat 

 increased, the cells remaining smaller than normal (PL VI, figs. E 

 and F). Usually, however, the number of divisions is reduced, and 

 in some instances a parenchyma tissue only three cells deep has been 

 found. (PL V, fig. A.) Occasionally the entire tissue between the 

 margin and the midrib consists of undifferentiated cells. That this 

 lack of differentiation is not due to immaturity was shown by ex- 

 amination of young healthy leaves just after their emergence from 

 the bud. Even at this young and only partially expanded stage the 

 palisade tissue of healthy leaves was found to be well differentiated 

 (PL VII, fig. C). 



In the thickened portions along the veins, dr sometimes throughout 

 the laininse in the aborted nonmottled leaves of depauperate rosettes, 

 the size of cells may become increased in all three dimensions without 



