[Vol. 10 



208 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



been known as the Quercina form. It is not artificially trans- 

 ferable except by grafting, but certain other species of Solanaceae 

 are susceptible through grafting. It is transmitted by seed to 

 about 79 per cent of the offspring when pollinated with normal 



plants. 



The case of the curly-top of sugar-beets, which is generally 

 assumed to be related to mosaic disease, is peculiar in that no 

 infection by diseased juice can occur until the juice has passed 

 into the body of Eutettix tenella, in which it must remain a definite 

 time interval, or incubation period, before being infectious to 



beets . 



In this case not even grafting has been successful according 



the 



more recent reports. In still another category with resp< 

 2ctiousness may be included the case of mosaic in sug: 

 poke-weed, and other plants in which insect transfer 



the more effective method yet found. 



The well-known cases in tobacco, bean, cucurbit, and other 

 plants wherein the transfer of juice from diseased to healthy 

 plants, whether by aphids or by needle prick, is sufficient to re- 

 produce the disease, — these are more closely related, as to infec- 

 tiousness, to ordinary bacterial or other parasitic diseases. De- 

 tailed experiments by us confirm the view that the virus of tobacco 

 and of related mosaic diseases do not pass readily, if at all, through 

 uninjured surfaces. We have tested this by spraying the diseased 

 juice on the leaves, also by placing the diseased juice in glass 

 cells sealed to leaf surfaces for 24 hours or more. Under such 

 conditions the virus is practically inert. 



It is suggestive that in the tobacco mosaic, the tomato mosaic, 

 and many others, the gametes do not seem to possess the virus; 

 at least the embryo arising from the fused gametes is not dis- 

 eased, while the seed-coats are. It is conceivable that the reduc- 

 tion division is concerned in the elimination of the disease, 

 a possibility which, if established, would be significant. The case 

 of the bean is, however, an apparent exception, though the pos- 

 sibility of infection after early embryonic development is not 



excluded. 



Time prevents a more complete discussion of the bearing of 

 these facts, but the trend of the evidence seems to indicate that 



