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 transmission 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 



