84 MASS. EXPERIMENT STATION BULLETIN 175. 



investigated the structure of the dark and light areas carefully in the case 

 of the tobacco, except Woods. It was to verify Woods' statements that 

 the writer took up this phase of the matter, and mention will again be 

 made of it in connection with the biochemistry of the leaf. There can 

 be no doubt as to the correctness of Woods' contention that the light green 

 areas are abnormal and diseased; but that the dark green areas are not 

 diseased, at least in certain cases, cannot be so definitely stated. Their 

 structure may be somewhat modified by the increased functioning thrown 

 on the healthy cells. On the other hand, it is fallacious to state that the 

 Ught green are the healthy, and the dark green are the diseased, portions 

 of a leaf. 



Plates III. and IV. show three cross sections from leaves. III. showing 

 the cross section of a healthy leaf; IV., that of the light green area of a 

 diseased leaf and of a dark green area of the same leaf. It wiU be noted 

 that the palisade layer is practically suppressed in IV. (1), or the Ught 

 green portion, while in IV. (2) the palisade layer approaches the normal 

 in character except for a closer packing of cells in general. Milder 

 cases of diseased leaves vary between these limits. These figures are 

 from camera Incida drawings of material killed and fixed in medium chrom- 

 acetic acid. In the material used the normal leaf section is somewhat 

 thicker than those of the diseased leaf, but for comparative purposes is 

 perfectly satisfactory. 



Stems. 

 The anatomical differences in the leaves of healthy and diseased to- 

 bacco plants have been given in the preceding paragraphs, and as it was 

 desired to carry the investigations further to cover the entire plant, re- 

 peated examinations were made of both cross sections and longi-sections 

 of stems of plants in various stages of disease, and also of healthy, normal 

 plants grown both in the field and greenhouse. It should be stated at 

 this point that occasionally the writer has observed on the stems of some 

 badly mosaicked plants a mottling, or, rather, a streaking of the stem, a 

 portion of which would be darker green than the remainder, and this is 

 without question a manifestation of the mosaic disease. Sections of such 

 stems, however, showed absolutely no variation in structure from those of 

 normal plants, and in no case, although the examinations covered an 

 extended period of time, was it possible to show any structural difference 

 between the stems of badly diseased mosaic plants and those of healthy 

 plants of the same age. Examinations of the stem close to the terminal 

 apex of the plant revealed the same conditions as those of other parts of 

 the stem. No differences were observable except in the matter of size and 

 arrangement of cells, such as would naturally be expected when we take 

 into consideration the differences in size and development of the stem near 

 the terminal apex and progressively towards the base. 



