VIRUSES, 1'1U)VIRUSI;S AND Till- CONTIICT OF SYSTEMS 



Other instances are due to the release and invasion of self-propa- 

 gating particles. All plants o( Lathynis tingitanns, in the experiments 

 of Johnson, produced infective symptoms when their sap was 

 injected into the bean Phascolus vulgaris. All plants of the potato 

 clone King Edward, in experiments of Salaman and LePellcy, 

 produced infective symptoms when grafted onto other clones. 

 Similarly the variety of apple, Lord Lambourne, when grafted on 

 different stocks, according to Crane, develops different characteristic 

 malformations, rubbery wood and stunted chat fruit, which spread 

 through the whole tree and, like the Ahutilon virus, will pass through 

 wood of another variety. 



The mode and degree of infection varies in these various plants. 

 The principle, however, is the same in all. Just as reciprocal crosses 

 enable us to test the reaction of a plasmagene in the cytoplasm of 

 one species with the nucleus of another, so grafting enables us to 

 test the reaction of something reproductive in the cytoplasm of one 

 species with the nucleus of another, indeed of many others and over 

 a range of many genera. A protein produced in the cells of one 

 organism can propagate itself injuriously in the cells of others. 

 Invasion, a characteristic of true viruses, is a necessary means of 

 demonstration of these agents. In the animal tumours, on the other 

 hand, the principle is slightly different. A protein produced in one 

 organism mutates so as to propagate itself injuriously in the same 

 organism. Here invasion is no longer necessary for demonstration 

 and does not always occur. 



We are bound to make the distinction between provirus and 

 plasmagene on the basis of capacity for invasion following grafting. 

 Yet this criterion is, of course, a purely practical one arising from 

 diverse conditions. In some instances the difference between invasion 

 and non-invasion may be simply one of the size of particles. In other 

 instances the difference may be simply one between presence and 

 absence, as it sometimes is with nuclear genes. 



The case of Yellows in ever-bearing strawberries demonstrates 

 the great theoretical and practical importance — and difficulty — o£ 

 the question of origin and transmission in making the distinction 

 between plasmagenes and viruses. Some 30 clonal varieties of these 

 strawberries are subject to this disease and many have been destroyed 

 by it. The variety Progressive, made in 1908, first showed symp- 



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