PECAN ROSETTE. 25 



A comparison of the size and shape of the vein islets in large and 

 small mature leaflets on the pecan leaves and in large (mature) 

 juvenile (undivided) and type leaves on the same normal seedlings 

 showed very little variation. Ensign (32) found in healthy citrus 

 leaves that the shape varied in different parts of the leaf, but the size 

 was independent of the shape or location on the leaf. The size and 

 shape of the vein islets in citrus were approximately the same in 

 normal leaves and in chlorotic leaves of plants dwarfed from starva- 

 tion. A comparison of the vein islets in large and small leaves on the 

 same plant showed that the size was constant irrespective of the size 

 of the leaf. A comparison of the leaves of all ages on the same plant 

 showed that the youngest leaf had the smallest vein islets and with in- 

 creasing maturity a corresponding increase in the size of the islets 

 took place. Ensign concluded that the main skeleton of the vascular 

 system is laid down very early in the history of the leaf and that but 

 little differentiation later takes place. It is clear that in pecan rosette 

 the size, shape, and arrangement of the vein islets are considerably 

 altered from the normal. In fact, there is in the mottling of the 

 leaves in pecan rosette much that appears comparable in origin to 

 Liesegang's diffusion patterns, for example, those obtained with 

 drops of silver-nitrate solution on a layer of solidified gelatin in 

 which potassium bichromate had been dissolved. These " Liesegang 

 phenomena " have also been compared by Kiister (48) to the formation 

 of growth rings by Armillaria, Penicillium, and other fungi, to the 

 alternative lighter and darker areas of some leaf-spot diseases of 

 fungous or bacterial origin, and to a great variety of pattern phe- 

 nomena in both plant and animal kingdoms. The end results in all 

 these cases of organic origin are similiar in appearance to those 

 obtained by diffusion experiments carried out in vitro. Furthermore, 

 while great caution should be exercised in the interpretation of such 

 experiments the results are extremelj^ suggestive that diffusion in 

 colloidal systems plays a prominent role in the development of the 

 pattern phenomena in both health and disease. 



Profound derangements take place in starch assimilation and trans- 

 location. In healthy leaves collected at sundown the starch content 

 of the chloroplasts is uniform for each tissue with usually the 

 greatest accumulation in the palisade cells (PL XI, fig. A). Stained 

 with iodin the starch is seen in a more or less irregular mass at the 

 center of the chloroplasts but in no case completely filling the plastid. 

 Starch also occurs in the guard cells and in occasional plastids in the 

 bundle sheaths. Healthy leaves collected before sunrise the following 

 morning from similar situations on the same trees in general showed 

 no starch at all (PI. XI, fig. B). In these healthy leaves the iodin 

 gave merely a yellow stain to the cell contents. An extremely rare 



