MOSAIC DISEASE OF TOBACCO. 95 



The above results show that when blue light is used there is a suppres- 

 sion of leaf color variation more or less permanent in character, the 

 treated plants, with one exception, showing no typical symptoms of the 

 disease for at least two weeks subsequent to the removal of the hoods. 

 It cannot be said, however, that the disease was controlled, as inoculation 

 of healthy plants with the juice from these leaves produced the disease 

 in nearly every case. 



The causal agent of the disease was still very active in the apparently 

 normal fully recovered leaves, and was highly infectious. 



Discussion of Results. — The results of these experiments do not agree 

 entirely with those obtained by Lodewijks, particularly in the case of 

 action of the blue light, inasmuch as the plants covered with the blue 

 hoods, although showing an apparent recovery from the mosaic, still 

 contained the causal agent of the disease, and by inoculation with the 

 juice expressed from these plants into healthy plants the disease was 

 again produced in practically all cases. It should be noted that the 

 visible symptoms of the disease were suppressed, the reason for which 

 may be as AUard (loc. cit.) suggests in his work on the mosaic disease of 

 tobacco. He states, with respect to Lodewijks' observations, "If the 

 malady in question was true infectious mosaic disease, one is inclined to 

 believe that covering the young plants temporarily reduced the color 

 contrasts of the mottled areas. These changes may have led Lodewijks 

 to conclude that a partial or a complete cure had been effected in his 

 experiments." 



It might be inferred from the above that on the removal of the hoods 

 exposing the plants to normal daylight, they would soon regain the color 

 contrast, but this is not entirely so in the case of the blue light, as has 

 been shown. The apparent recovery, therefore, is not entirely the result 

 of a suppression of color contrast due to the action of blue light on the 

 leaves as suggested by Allard, but is undoubtedly so in part. 



It is evident that the treatment of plants as above recorded does not 

 destroy the causal agent of the mosaic disease, whatever may be its char- 

 acter, the treated leaves apparently still containing the causal agent, very 

 probably in the same manner as do the parts of a plant which do not show 

 visible symptoms of the disease, as the stem, lower leaves, roots, etc., the 

 juice of which is often highly infectious. It would appear from the re- 

 sults that the new terminal growth subsequent to the removal of the 

 hoods would develop the trouble, and this was the case in plant C-2, but 

 not apparently so with plants A-2 and B-2. Lodewijks' opinion, therefore, 

 that in the plant a "virus" and "anti- virus" are present, and that certain 

 abnormal conditions cause the "virus" to be produced in excess, bringing 

 about a mosaicked appearance, while if the "anti- virus" is produced in 

 excess, immunity is secured, will hardly hold, as it is clearly shown that 

 even after apparent cure the causal agent is present and active. 



It is significant to note that under the influence of blue light both 

 assimilation and starch formation are decreased, thus bringing about a 



