MOSAIC DISEASE OF TOBACCO. 83 



Pathological Anatomy. 

 Leaves. 



As might be supposed, there are great differences in structure between 

 normal, healthy leaves and leaves affected with the mosaic disease. These 

 differences are greatest, naturally, in badly diseased leaves. Woods ^ was 

 one of the first to point out this fact, and his statements have been re- 

 peatedly verified by the UTiter. He stated that the hght colored areas 

 were not normal, and that "this difference consists in the fact that in 

 badly diseased plants the palisade parenchyma of the light colored areas 

 is not developed at all. All the tissue between the upper and lower epider- 

 mis consists of a spongy or respiratory parenchyma rather more closely 

 packed than normal. In moderately diseased plants the palisade paren- 

 chyma of the light area is greatly modified. Normally the palisade 

 parenchyma cells of a healthy plant are from four to six times as long as 

 broad. In a moderately diseased plant, however, the cells are nearly as 

 broad as they are long, or at most, not more than twice as long as broad. 

 As a rule, the modified cells of the leaf pass abruptly into the normal cells 

 of the green area." 



From the above it can be seen that Woods was of the opinion that the 

 light green areas were abnormal or diseased, and that the dark green 

 areas were normal and healthy. The writer in his observations found 

 this to be true in general, but occasionally the dark green areas showed a 

 more closely packed parenchyma than in normal leaves, and always the 

 palisade layer was well developed and approached the normal in character. 

 The development or non-development of the palisade layer, as Woods 

 hinted, is dependent on the degree of severity of the disease. The lighter 

 the attack the less are the palisade cells and parenchyma tissue altered, 

 and vice-versa. This the writer found to be true in so far as anatomical 

 differences were concerned, but as will be noted later, the dark green, 

 apparently normal, healthy tissue contained some of the infective agent of 

 the disease. 



The structure of the dark green areas varies only slightly from that of 

 the normal leaf, with the few exceptions above noted, and may be con- 

 sidered normal in character. The writer has sectioned many leaves in all 

 stages of disease, and these structural differences have always been found 

 to occur in the manner above indicated. These differences in structure 

 have been taken up more or less in detail, as some investigators have held, 

 and still hold, that the dark green areas are the part diseased, and that 

 the light green areas are normal, inasmuch as they approach the normal 

 leaf in color in many cases, most probably basing their assumption on 

 the fact that the dark areas form blister-like growths and are sometimes 

 darker in color than normal leaves. No one recently appears to have 



> Woods, A. F.: Inhibiting Action of Oxidase on Diastase. Science, n. s., XI., No. 262, 17-19 

 (1900). 



