88 MASS. EXPERIMENT STATION BULLETIN 175. 



moved and placed on the underside of the leaves of tobacco plants, en- 

 closed in a small cloth-covered cage, and were allowed to remain on the 

 tobacco leaves of the plants in these cages for four days. After this 

 length of time the plants were removed from the cages and placed on a 

 bench at some distance from those containing mosaicked plants badly 

 infested with white fly. On none of the plants did mosaic develop. The 

 plants were later placed in close juxtaposition to those in the original 

 benches, which, as indicated, were at this time heavily infested with 

 the white fly and badly mosaicked, but although the plants remained 

 until maturity, no cases of mosaic developed on them in spite of a heavy 

 infestation of white fly. 



The writer's observations on the activities of aphids as carriers of in- 

 fection have not been so extensive as in the case of the white fly, as only 

 minor infestations of the former occurred in the greenhouses; and the 

 indications pointed to the fact that, although there were a certain number 

 of aphids present on the leaves of both healthy and diseased plants, so far 

 as was observable no cases of infection from this source arose, as the 

 mosaic developed only on an average of 1 case out of 30, except on the 

 plants which were artificially inoculated with the juice from diseased 

 leaves. It should be stated, however, that aphids present in the green- 

 house were not of the same species as that under consideration by Allard, 

 and there is no reason to doubt the accuracy of his observations on the 

 species tabaci Perg. 



The question of insects as carriers of the mosaic disease as well as of 

 many other diseases is still open to discussion ; and it may be that in the 

 case of the mosaic a very heavy infestation of aphids is necessary to bring 

 about a successful infection of healthy plants, as the amount of active 

 infective material carried by such insects would in any case be very small, 

 and accumulative effects of the activities of several insects might be 

 necessary to introduce a sufficient amount of the active principle to trans- 

 mit the disease. 



Workmen. 



It has been shown that the disease is highly infectious and it has also 

 .been proved repeatedly by many investigators that it is very easy to 

 transmit the disease to healthy plants at the time of transplanting. A 

 workman handling diseased seedlings, and subsequently healthy ones, 

 will very often infect them. Several instances of tliis have come to the 

 writer's attention, every other plant for some distance in a row developing 

 mosaic within a month after transplanting. The same condition has also 

 been observed by Clinton {loc. cit.) in Cormecticut, and can only be ex- 

 plained by the fact that the workman's hands were infected through 

 handling a diseased plant, and the infection then transmitted to healthy 

 ones, the causal agent being introduced through broken tissue of the 

 leaves or roots of the seedlings. This method of transmission is particu- 

 larly striking in the above case, as the same individual plants every other 

 plant in a row when working the ordinary planter. Of course, there 



