﻿CURRENT LITERATURE 



Willis suggests that all Ceylon species have arisen as single mutations, 

 and that subgenera and genera may arise similarly. Some of the endemics of 

 Ceylon, which are and always have been very restricted, would indicate 

 that the same mutation need not go on appearing in order to become established. 

 However well a species may be locally adapted, it will be in great danger of 

 extermination until it has gotten beyond the degree of commonness repre- 

 sented by "very rare." "Having reached the maximum height [in the scale 

 of commonness] that it is going to reach, a species will ultimately descend, and 

 will sooner or later become extinct, though there is no evidence that as yet many 

 or any species are on the downward road." The statistical facts brought out 

 "support very strongly the hypothesis that the whole tree of descent of a 

 family may exist on the earth at the present moment, and that the area occupied 

 is in general an indication of the age of a species or a genus, if it has not already 

 attained its maximum." — Merle C. Coulter. 



The mosaic disease of tobacco. — Allard 10 has given an account of the 

 mosaic disease of tobacco that will be of much interest to all who desire to 

 become acquainted with the chief features of this interesting and destructive 

 disease. The most characteristic external feature of the disease is a mottling 

 of the leaf, due to partial chlorosis. The disease is communicable to a number 

 of the Solanaceae, but appears to be distinct from the very similar mosaic 

 disease of Phytolacca. The mosaic virus permeates all parts of the plant, 

 but does not infect the embryo; hence seeds of diseased plants produce healthy 

 individuals. The disease seems to be incurable and no plants of susceptible 

 species seem to be immune. The origin and distribution of the disease are 

 mysterious, and the author's experiments lead him to oppose the more com- 

 monly current theory that the disease is associated with unbalanced enzy- 

 matic activities and physiological toxins. Allard thinks that the disease is 

 parasitic and that it is communicated from plant to plant by certain aphids. 

 No organisms that might be responsible for the disease have yet been isolated. 

 This disease seems to have the characteristics of infectious chlorosis, as 

 described by Baur. 



Further studies of the mosaic disease, also by Allard, 11 . » have to do with 

 interesting special features. It is found that the dilution of the virus to i part 

 in iooo results in no impairment of infection, though dilution to i part in 

 10,000 results in an effective attenuation of the virus activity. While the 



10 Allard, H. A., The mosaic disease of tobacco. U.S. Bureau of Plant Industry, 

 Bull. 40. pp. 33. ph. 7 . 1914. 



» , Effect of dilution upon the infectivity of the virus of the mosaic disease 



of tobacco. Jour. Agric. Research 3:295-299. 1915. 



" , Distribution of the virus of the 



anthers, and pistils of affected tobacco plants. Jour. Agric. 



