THE VIRUS DISEASES OF PLANTS 419 



comprising by far the greatest number of the diseases here 

 referred to, insect transmission is fully established in a number 

 of cases and is probably general. 



Less common is transfer by direct inoculation with the juice 

 of the infected plants. It is only certainly known to occur in the 

 mosaic group. All attempts to secure infection in this way 

 have failed in the peach yellows and phloem-necrosis groups, 

 nor does there appear to be any evidence of it in the infectious 

 chloroses. 



Transmission through the seed is rare. It is only certainly 

 known to occur in a few cases of mosaic diseases, e.g. in beans 

 and clover, and occasionally in cucumbers, and in the mosaic- 

 like disease of Datura mentioned in the next paragraph. It 

 does not occur in most of the mosaic diseases tested, but is 

 perhaps doubtful in one or more of the Solanaceae affected with 

 mosaic, especially one form of tomato mosaic.^ 



Amongst the most interesting cases of transmission by 

 grafting is the recently described " quercina " disease oi Datura 

 stramonium.*' It is fully infectious by grafting on healthy 

 plants of this species, and spreads also in the field in some way 

 unknown. Tested on a number of related species it was trans- 

 mitted by grafting to a few in a mild form, producing little effect 

 on the host. From the latter it could (in some cases at least) 

 be transmitted back to healthy Datura plants. On immune 

 species the virus failed to pass through as little as 3-6 cm. of the 

 stem to reach a normal Datura graft in the stem near by. In 

 the leaf roll of potato, also, allied species such as tomato can be 

 infected by grafting, but the symptoms are less marked than on 

 the potato or may even be entirely absent, though a graft is 

 capable of transmitting infection back to the potato. Several 

 such cases of " carriers " have been demonstrated in other 

 virus diseases of the Solanaceae. Even in the case of the 

 infectious chlorosis group a similar phenomenon has been 

 observed by Baur. When scions of an apparently immune 

 species of Abutilon are grafted on A. thompsoni plants affected 

 by chlorosis, they remain apparently healthy and show no 

 symptoms. But they are capable of transmitting the disease to 

 susceptible species grafted on them. On the whole, the examina- 

 tion of the experiments on transmission by grafting throws 

 little light on the nature of these diseases, except that it gives 

 room for speculation as to what the nature of a virus that 

 requires organic union of the tissues for its passage can be. 

 Possibly it is a parasite of the highly specialised type known in 



^ Westerdijk, Johanna, Die Mosaikkrankheiten der Tomaten, Meded 

 Phytopath. Labor. " Willie Commelin Scholten," i, igio. 



* Blakeslee, A. F., A Graft-infectious Disease of Datura resembling a 

 Vegetative Mutation, Journ. of Genetics, xi, p. 17, 192 1. 



