PECAN ROSETTE. 9 
complete disorganization follows. Woods (84, 85) found that in the 
lighter areas of badly diseased leaves the palisade parenchyma had 
not developed at all, but the tissue consisted entirely of a respiratory 
parenchyma with cells packed together rather more closely than nor- 
mal. In healthy leaves the palisade cells were four to six times as 
long as broad, whereas in the moderately diseased leaves these cells 
were almost as broad as long. The leaf surface becomes depressed 
in the light areas and raised in the green areas, thus giving a rough- 
ened appearance to the lamina. 
As first shown by Woods (84), the oxidizing enzyms are greatly 
increased in the diseased areas. He also found more starch in the 
form of granules in the yellow areas than in the green areas of the 
same leaf. The cells were often completely gorged with starch. Ex- 
amination in the early morning showed only a slight decrease, while 
healthy tissue at the same time was empty or contained only a trace. 
Starch translocation in the diseased leaf is greatly delayed in spite 
of the fact that diastase is present often in larger amount than in the 
normal leaf, and Hunger (41) from experiments in vitro concluded 
that the retarding effect upon diastase action is caused not by the 
oxidizing enzyms, but by reducing substances including tannin. 
Mayer (52) first showed that transmitsion of tobacco mosaic 
could take place through the expressed juices of diseased plants. 
Iwanowsky (42), and Beijerinck (17) independently demonstrated 
that the infective principle would pass through the pores of a Cham- 
berland filter, though such a filtrate was less infective than the un- 
filtered juice. Allard (4) proved that infection fails to result after 
the juice from diseased plants has been passed through a Livingston 
porous-clay cup filter. Transmission of the disease by an infective 
principle in the expressed juice was thoroughly demonstrated, and it 
was shown by Allard and others that oxidizing enzyms do not con- 
stitute this infective principle (4). Such plant juices diluted to 1 to 
1,000 in water were quite as infective as the undiluted juice; attenua- 
tion was indicated at 1 to 10,000, while at greater dilutions infection 
was found unlikely to take place (2). The virus is infectious to all 
susceptible plants, but such plants never develop mosaic so long as 
chances for infection are excluded, and this regardless of soil and 
climatic conditions. The infective principle may be present in all 
parts of a diseased plant except within the seed and has been demon- 
strated even in the trichomes (3, 6). Furthermore, infection may 
occur through inoculation of the trichomes alone. Cutting the mid- 
rib at the base or severing the larger veins on one or both sides does 
not prevent the final dissemination of the infective principle to all 
parts of the leaf and to other leaves of the plant. Environmental 
conditions may partly or even wholly mask the external signs for a 
time, but can neither cause nor cure the disease. 
76289°— 22- 2 
