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 extremely 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 (PI. XI, 1a.g. 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 (PL XI, fig. B). In these healthy leaves the iodin 
gave merely a yellow stain to the cell contents. An extremely rare 
