686 



aj^ainst the light, and, by feeling the leaf, it is noticeable that the dark green 

 places are somewhat thicker than the pale ones. Before Delacroix, Iwan- 

 ovvski^ had already emphasized the fact that the lateral shoots, developing 

 from the axes of diseased leaves, have the mosaic disease. This circum- 

 stance is very important and characteristic of the disease in which the loss 

 of color occurs in the young leaves ; as a rule, mature leaves do not be- 

 come diseased. Often the dark green places become somewhat convex so 

 that the surface of the leaves is somewhat wrinkled; in other, and rarer 

 cases, a reduction of the leaf surface sets in which can increase to such an 

 extent that, on the whole plant, only the mid ribs are present but no blades. 

 This latter characteristic has been mentioned by Heintzel- and Iwanowski, 

 but, according to Hunger' it is not typical of the disease, for he had also 

 observed it in Deli in healthy plants on open ground. 



Therefore, in the mosaic disease, we find the same characteristics as in 

 albinism; a sharp delimitation of the spots, a greater thickness of the green 

 places, and, at times, a reduction of the leaf surfaces, which, in the varie- 

 gated parts, remain small. This can also be transmitted artificially and 

 probably follows the same paths, i. e., the leptome. The only difference is 

 that the mosaic disease can be transmitted considerably more easily. Ever)' 

 particle of sap which falls from a diseased plant into an injury in a healthy 

 one is enough, under certain circumstances, to cause infection. We will 

 cite, as example, the description of an infection experiment made by 

 Koning*. On the 5th of July he cut the stem of a perfectly healthy plant 

 as far as the vascular bundles and inserted in the cut a small piece of the 

 spotted leaf from a diseased plant. On the 20th of July a dark fleck could 

 be seen near the edge of a young leaf, between the veins. In the course of 

 the next few days, specks appeared also on the other young leaves while 

 the leaf itself took on "an uneven, irregular appearance due to the increase 

 of the palisade tissue." The edge of the leaf appeared in places to be 

 strangulated, or slightly lobed. Later these spots dried up, after having 

 assumed a reddish brow^n color. Koning perceived concentric zones in the 

 larger spots, of which the outermost zones were the darkest. Not infre- 

 quently he found that whole pieces had fallen out of the leaf. The latter 

 characteristics are not mentioned by other observers, which fact supports 

 our theory that the disease can present different aspects in dififerent places 

 and in different varieties of tobacco. 



Koning gives only scanty notes on the anatomy of the diseased leaves. 

 In the very youngest stage of the spots, where no differentiation of palisade 

 and spong}^ parenchyma has set in. dark stripes appear between the cells 

 which represent strikingly large, air-filled intercellular spaces. These are 



1 Iwanowski, D., tJber die Mosaikkrankheit der Tabakspflanzen. Zeitschr. f. 

 Pflanzenkrankh. 1901, p. 1 ff. 



2 Heintzel, Kurt, Kontagiose Pflanzenkrankheiten ohne Mikroben mit beson- 

 derer Beriicksichtigung der Mosaikkrankheit der Tabaksblatter. Inaug- -Dissert. 

 Erlangen 1900. 



n Tvoc. cit, D. 274. 



4 Koning, C. J., Die Flecken- oder Mosaikkrankheit des hoUandischen Tabaks 

 Zeitschr. f. Pflanzenkrankh. 1899, p. 65. 



