422 SCIENCE PROGRESS 



after infection by adults than by young. Several other condi- 

 tions influence the length of the incubation period ; it is about 

 twice as long, for instance, when the virus is taken from plants 

 in an early stage of the disease than when taken from the later 

 stages, though the percentage of successes on inoculation is 

 about the same. After the aphid becomes infective it can carry 

 the disease to several successive plants by feeding on each for a 

 time. The progeny of the infected aphids kept from feeding 

 on infected plants can transmit the disease up to at least the 

 fourth generation. The infection is therefore " hereditary " 

 in the aphid. The possibility that there is an incubation period 

 in the insect before it can transmit the disease is not excluded. 

 Non-infected insects that remained on blighted plants for 

 different periods of time, from ten minutes to forty-eight hours, 

 and were then transferred to healthy plants, caused symptoms 

 of the disease to appear in the latter in from seventeen to 

 twenty-four days, the incubation period being about the same 

 in the case of those that fed fourteen, twenty-four, and forty- 

 eight hours, but being longer in those that fed for two hours 

 and ten minutes respectively. 



The study of insect transmission has therefore shown that 

 in curly top of the beet the insect is not a mere mechanical 

 carrier of the infection, but is an alternate host in which the 

 virus undergoes some change, and it has also shown that the 

 infection in spinach blight is hereditary in the insect. These 

 are important advances. 



Transmission by the juices of infected plants has been most 

 fully investigated in the mosaic disease of tobacco.^ This is 

 perhaps the most highly infectious virus disease of plants known, 

 though it does not kill. The virus is present in all parts of the 

 plant that have been found so far capable of separate examina- 

 tion — leaves, stem, roots, flowers, and seeds. In the latter 

 it is present in the seed coats, but evidently not in the embryo, 

 since it is not transmitted through the seed to the seedling. 

 Direct testing of the embryo for the presence of the virus after 

 removal of the enclosing tissues would probably not be possible 

 without causing contamination. Even the hairs of the leaf 

 contain it. Infection can be produced by inoculating any part 

 of the plant, the slightest abrasion, even broken or cut leaf hairs, 

 permitting entry. The leaf hairs of infected plants have been 

 carefully cut with sterile scissors so as to avoid injuring the 

 tissues below, and the scissors thus contaminated with the 

 contents of the hairs have been used to cut the hairs of healthy 

 plants and have caused these plants to become infected. 



1 Allard, H. A., The Mosaic Disease of Tobacco, U.S. Dept. of Agric. Bull., 

 40, 1914, and subsequent papers in Journ. of Agric. Res., v, p. 251, 1915 : vi, 

 p. 649, 1916 ; vii, p. 481, 1916 ; x, p. 615, 1917 ; and xiii, p. 619, 1918. 



