PECAN ROSETTE. 27 



which later recedes from the cell wall (PL XII, fig. J), and at last 

 the whole cell collapses and shrivels. 



The deposition of crystal aggregates of calcium oxalate is char- 

 acteristic of pecan leaves (PL VII, fig, B). These crystals begin 

 to form in giant binucleate cells of the palisade just after the young 

 leaf emerges from the bud. Finally the protoplasmic contents dis- 

 appear, and the crystal aggregate nearly fills the cell. These crystal 

 aggregates are distributed with considerable regularity in the healthy 

 leaf and in the majority of cases one to each vein islet (PL X, fig. E). 

 In the yellow spots of the secondary stage of the disease, on the other 

 hand, their formation and distribution vary widely. At the focal 

 centers in severe cases few or no crystals at all are formed, whereas 

 in the surrounding green parts they are often far more numerous 

 and sometimes larger than in the comparable healthy leaf. 



Averaging a large number of counts in cross sections of green 

 parts of diseased and normal leaves of the Frotscher variety col- 

 lected at Thomasville, Ga., it was found that in the normal leaves 

 20 crystal aggregates occurred to every 100 vein islets observed in 

 section, while in the diseased leaves 60 crystal aggregates were found 

 to every 100 vein islets. In Van Deman leaves collected at Baconton, 

 Ga., there were 16 crystal aggregates to every 100 vein islets in the 

 healthy leaves as compared with 54 in the diseased leaves. Further- 

 more, after all due allowance for differences in cortical area of the 

 diseased and healthy leaves compared, a much larger number of 

 these crystals was found in the cortex of petioles and midveins in 

 rosetted leaves. In view of the fact that waste organic acids usually 

 accompany carbohydrate formation (57, p. 173) these results are 

 significant. Since growth at the periphery of the yellow spots 

 in the secondary stage is often abnormally active, as is shown by 

 the size of the cells, an unusually large accumulation of such or- 

 ganic acids would naturally be expected to take place in that region. 

 Conversely, with the reduction of chlorophyll formation and assimi- 

 lation in the centers of the spots a smaller production of such acids 

 would occur. 



Development of the shield-shaped resin glands occurring mostly 

 on the lower surface of the leaves is also profoundly affected by the 

 disease. On the normal leaf these glands are rather regularly and 

 sparsely distributed over the surface of the blade and contiguous 

 to the veins and veinlets. In diseased leaves of the secondary stage, 

 on the contrary, the focal centers of the yellow spots are often 

 thickly covered with these resin glands both contiguous to the vein- 

 lets and well within the vein islets themselves. The more severe 

 the general effects of the disease the more numerous were these glands 

 found to be, until in places where the tissues were practically de- 



