1918] DISEASES OF PLANTS. 49 



elements being present in about the same proportion tn diseased as In healthy 

 tissue. Oarbodydrates are more abundant in dark green than in light areas. 

 Proteins are apparently present more abundantly in the lighter than in the 

 darker areas. 



The physiological phase of the disease Is thought to be a very important one. 

 Preliminary observations appear to show the existence of minimum, optimum, 

 and maximum points as regards temperatures favoring the disease, the check 

 from unfavorable temperatures suggesting the recovery of the plant. 



The infective principle is thought to be an enzym different in character from 

 that giving the guaiacum reaction. Like all colloidal compounds, including 

 enzyms, it is greatly absorbed by talc. There is a specificity of reaction between 

 the infective principle or mosaic enzym and formaldehyde (if not aldehydes 

 in general) which is not observable in case of concentrated antiseptics of other 

 kinds. The infectious properties are, however, destroyed by alcohol In concen- 

 trations destructive to enzyms, and by temperatures which inactivate enzyms 

 or hydrolyze some organic compounds, cooling having no greater effect than is 

 exerted on any chemical compound, including enzyms. The reproduction of the 

 mosaic enzym can be accounted for on physiological grounds, but the original 

 factors are unknown. 



The above facts and the fact that formaldehyde is one of the first products 

 of photosynthesis suggest a basis for the possible explanation of the physiologi- 

 cal aspects of mosaic diseases. The continued production of the mosaic enzym 

 is considered to be in accordance with the fundamental principles of pathology 

 and physiology. 



Ftirther studies of the mosaic disease of tobacco, H. A. Allard ( U. S. Dept. 

 Agr., Jour. Agr. Research, 10 (1917), No. 12, pp. 615-6S2, pi. 1). — In continua- 

 tion of the author's studies of the mosaic disease of tobacco (E, S. R., 36, p. 

 647), an account is given of an investigation of the susceptibility of tobacco 

 plants to infection through the trlchomes, resistance to mosaic disease in 

 tobacco plants, and the distribution of the virus in inoculated leaves. 



It has been found that the virus of the mosaic disease of tobacco is present 

 in the trichomes of the leaves, and the disease may be communicated to 

 healthy plants by inoculating the virus into the trichomes alone. The infective 

 principle does not readily invade uninjured trichomes or leaf tissues, but in- 

 fection readily follows when the virus is sprayed upon the leaves and subse- 

 quently rubbed in. Cutting across the midrib at the base of the leaf or sever- 

 ing all the larger veins on one or both sides of the midrib did not prevent the 

 final dissemination of the virus to other leaves and other portions of the plant. 



Thorough steam sterilization of the soil of the seed bed was found to com- 

 pletely destroy any virus which may be present in the soil. Infection from 

 diseased material in the soil appears to depend upon injury to some portion of 

 the root system. 



Nicotiana glauca has been found susceptible to the mosaic disease of to- 

 bacco, although visible symptoms of the disease may be very slight. The sap 

 of such plants proved highly infectious to healthy plants of N. tabacum. 



As carriers of the disease the author has found that the green peach aphis 

 (Myzus persicce) may become active in the greenhouse after feeding upon 

 diseased plants, and the large plant louse (Macrosiphum tabaci) is an efficient 

 carrier of the infective principle of the disease under field conditions. Red 

 spiders gave negative results, and white flies did not appear actively concerned 

 in the transmission of the disease. 



The control of tobacco wilt in the flue-cured district, W. W, Gabneb, F, A, 

 Wolf, and E. G. Moss (U. 8. Dept. Agr. Bui. .562 (1917), pp. 20, Jigs. 5),— Aji 

 account is given of investigations conducted coojwratively by the North Caro- 



