\Y. 1>. Brurlby 265 



itself not showing any signs of disease, is well known to animal patho- 

 logists but does not seem to have been previously applied to plants. 



In the course of some preliminary investigations on the mosaic 

 disease of tomato which are being carried out in the pathological 

 laboratory at Kew, the freedom from disease of new shoots springing 

 from a diseased stock was noted. The plant in question was one of 

 two which were received on September 9th, from a grower in whose 

 houses tomatoes have shown the disease for some years. The roots 

 were shaken partly free from soil, and were found to be well-developed. 

 The plants had been topped some time previously and were each bearing 

 small clusters of immature fruit. The foliage was good but calicoed 

 down to the lowest leaf. The plants were posted to the laboratory 

 and on receipt some two days later were immediately potted in light 

 loamy soil and. placed on the floor in a cool glasshouse. 



One plant shortly died and was destroyed. Of the other all the 

 leaves and the upper portion of the shoot withered and were cut away. 

 Three new shoots however developed, all originating on the stem well 

 above the lower leaves and these appeared perfectly normal, i.e. were 

 healthy shoots from a diseased stock. 



Now two criticisms may be made which are vital to this observation. 

 One concerns the condition of the original plant, for if the diagnosis 

 of the malady as mosaic disease be a mistaken one, the observation 

 has no interest. The second concerns the new shoots, for if the disease 

 is not really absent from them but present in a latent form, the apparent 

 recovery merely serves to confirm Allard's observation. 



But it is the lack of a final criterion which is the fundamental 

 difficulty in all investigations of mosaic disease, for where there is 

 apparently no causal organism, the rules of Koch are inapplicable. 



Concerning the first criticism — that the original plant possibly did 

 not suffer from mosaic disease — two criteria only are available ; one, 

 the symptoms of the disease ; and the other its infectious quality. 



Now there is no tomato disease of known parasitic origin, which 

 produces external symptoms conceivably to be mistaken for those of 

 calico or mosaic disease. On the other hand chlorotic or etiolated 

 plants, or those suffering from excess or deficiency of food and other 

 materials in the soil, or from soil sickness, whilst often assuming a 

 greyish or yellowish colour, never show the characteristic and peculiar 

 blotching or mottling of calico. 



The infectious nature of the malady from which the original plant 

 suffered is a question only to be determined by experiment. The 



