420 SCIENCE PROGRESS 



some plant rusts and mildews that needs the infected host cell 

 to be alive and in normal health for its support and fails to 

 maintain itself in weakened or dying cells. Wound infections, 

 such as inoculation pricks, may fail in such cases by injuring the 

 host cells, and only the infections of uninjured cells, with the 

 establishment of symbiotic relations between living host and 

 parasite, succeed. 



The study of insect transmission is more helpful. Until it 

 was discovered, there were many who held that no living 

 organism need be invoked to account for the origin and infectious 

 nature of these diseases, and there are still some supporters of 

 the " enzymic " theory of mosaic disease who believe that the 

 whole disturbance is due to nutritional disorders consequent on 

 the introduction of some stimulus of a non-living nature that 

 upsets the balance of the enzymic activities of the plant. ^ But 

 the continued extension of some of these diseases to new areas, 

 the rapid multiplication of the virus in the plant, its capacity 

 to cause infection at very high dilutions sometimes exceeding 

 I to 10,000, its power of unlimited transmission to new healthy 

 individuals, its conveyance by insects and other points of 

 similarity to certain virus diseases of animals, and above all, 

 the discovery that the insect carrier may not become immedi- 

 ately infective after feeding on a diseased plant but requires 

 a period of time before it is capable of transmitting the disease, 

 have convinced most plant pathologists that, as Quanjer has 

 recently put it in surveying these diseases, " One can hardly 

 deny the existence of a whole world of ultra-microscopic 

 organisms, causing them " {Rept. Intern. Potato Conf. of 1921, 

 p. 138). 



Undoubtedly the most important case in this connection is 

 the " curly top " disease of beet, the first in which insect trans- 

 mission was established. 2, ^ The disease is only known to be 

 transmitted by grafting and by the beet leaf-hopper, Eutettix 

 tenella. After the latter has fed on diseased plants, it is not 

 infectious for a period of from four to forty-eight hours. The 

 shorter periods are obtained only at high temperatures. This 

 is a clear case in which the infective agent requires to undergo 

 some change or development in the insect transmitter before it 

 can reinfect its plant host. If fed for only a short time on a 



^ Chapman, G. H., Mosaic Disease of Tobacco, Massachusetts Agric. 

 Exper. Stat. Bull., 175, 1917. Several of the recent German workers on the 

 leaf roll and mosaic diseases of potato do not find it necessary to invoke the 

 intervention of a living parasite in these diseases. 



2 Rand, F. V., and Pierce, W. D., A Co-ordination of our Knowledge of 

 Insect Transmission in Plant and Animal Diseases, Phytopathology, x, p. 216, 

 1920. 



3 Severin, H. H. P., Minimum Incubation Periods of Causative Agent of 

 Curly Leaf in Beet Leaf-hopper and Sugar Beet, Phytopathology, xi, p. 424, 1921. 



