retained in the advancing development of the tissue. No change can be 

 observed at first in the epidermis. It shrivels later, becomes brown and 

 dry when the chlorophyll has disorganized in the underlying tissue and the 

 cells dry up. 



In extensive plantations the infection of the plants usually takes place 

 through contact with the hands of laborers who produce wounds when 

 thinning out the plants and otherwise working among them. The touching 

 of such places with fingers covered with sap from diseased plants is enough 

 to inoculate the majority of the healthy plants. The process has often been 

 tested experimentally. In an experiment made especially for this purpose 

 in Holland, Koning determined 80 per cent, of disease. 



The disease, moreover, is not restricted to tobacco, for Woods^ had 

 already reported that he could call forth similar phenomena when pruning 

 tomato plants. Hunger^ showed as an example that, in the same plant 

 species, different varieties behaved differently according to their origin. 

 He found in direct experiments with the heads of plants in Buitenzorg that 

 all the shoots (lateral shoots) of 50 examples raised from American seeds 

 had the mosaic disease. Df 25 plants grown at the same time from German 

 seed 9 were diseased. On the other hand, the shoots of the 25 specimens 

 raised from Indian seed showed no change. 



In speaking of the cause of this disease, we have already mentioned 

 that part of the observers assume the presence of micro-organisms without 

 having seen them. Iwanowski, in fact, describes a specific bacterium, but 

 Hunger found, in subsequent investigations, that the alleged organism dis- 

 appeared from the cell with the use of the chloral hydrate phenol mixture. 

 We can, therefore, say that no parasitic organism is known, as yet, for the 

 typical mosaic disease, or, rather, the majority of exact observations lead 

 to the theory that a physiological disease is concerned here, the transmission 

 of which takes place by means of carriers which, advancing in the infected 

 organism, cause, in the existing normal group of substances, the same 

 changes in the arrangement which produce the disease and in this way the 

 spread of the disease. The different degrees of susceptibility of the differ- 

 ent varieties — those with thick leaves being much more resistant than those 

 with thin leaves — prove that some predisposition must exist. The highly 

 prized Deli tobaccos (those with the tenderest leaves) suffer most. The 

 influence of cultivation is shown by the fact that virgin soils give decidedly 

 smaller percentages of sick plants than those already used repeatedly for 

 the cultivation of tobacco (cf. Hunger's field experiments)^. 



Two points of view are now held by the investigators who do not rec- 

 ognize microbes as the cause of the mosaic disease. One group believes 

 that the plant produces a poison, a virus, which is capable of producing the 

 same poisonous substances in the cell content of an inoculated plant, thereby 



1 Woods, A. F., Observations on the Mosaic disease of Tobacco. U. S. Dept. of 

 Agrriculture, Bull. No. 18, May, 1902. 



2 Loc. cit., p. 287. 



3 Zeitschr. f. Pflanzenkrankh. 1905, p. 289. 



