A. F. Blakbslee 25 



infection. Since Q. pollen infects the seed, it doubtless would infect 

 the tissues of the plant when carried to flowers of normal plants. 

 Insect pollination might be one means of the spread of the disease 

 from lightly infected individuals with functional pollen. 



V. Inoculation and Grafting Experiments. 



The spontaneous appearance of Q. branches on field-grown plants 

 and the spreading of the character to new branches of affected in- 

 dividuals suggested that the peculiarity was in the nature of a disease. 

 In testing this supposition a series of inoculation and grafting experi- 

 ments were undertaken. 



(a) Inoculation Experiments. 



One of the commonest diseases of tobacco, another member of the 

 Solcmaceae, is " mosaic " or " calico " as it is called from the patches of 

 lighter colour on the leaves. The causal organism is not known, but 

 the infection is readily communicated from one plant to another by 

 mere contact of healthy plants with infected leaves. Mosaic diseases 

 of unknown nature have been discovered in other groups. It was 

 thought that the Q. complex might be due to such a mosaic type of 

 disease. Accordingly attempts were made to transmit the infection to 

 healthy plants by rubbing their leaves with those of Q. plants, and by 

 needle inoculations with the expressed juice of Q. leaves, but without 

 success. In one experiment two cions from normal plants were left in 

 the expressed juice from Q. leaves for 24 hours, and two other cions 

 for 48 hours, before grafting them on to normal stock. Neither of these 

 four cions took the disease nor caused the normal stock to turn Q. 

 Transmission of mosaic diseases by contact in other species is often 

 difficult of accomplishment and it is possible that more extensive trials 

 would have given occasional success with the Q. disease of the Jimson 

 Weed. However, mere contact of leaves cannot be a usual means of 

 infection in nature, since in the field normal plants, adjacent to Q.'s and 

 with their leaves touching, seem no more likely to be infected than 

 those adjacent only to normals. The disease is easily transmitted, 

 however, by grafting. 



(b) Grafting ExpeHments. 



Grafting of Q. branches on normal stock or the reverse union in- 

 variably has given infection. This has often resulted even when the 

 Q. cion has died and dropped off before a perfect union has been 

 established. PL V, fig. 8 is a photograph of a green-stemmed normal 



