10 BLTLLETIN 1038, U. S. DEPAETMENT OF AGRICULTURE. 



Xisliimiira (55) found that tlie bladder clierry {Physalis alkekengi 

 Linn.), after inoculation with tobacco-mosaic virus is capable of act- 

 ing as a carrier without itself showing external signs of the disease. 

 Cross-inoculation experiments (5) have shown that mosaics in to- 

 bacco, tomato, pepper, and petunia are caused by the same infective 

 principle. On the other hand, the mosaic of pokeweed {Phytolacca 

 clecandra Linn.) , though readily transmitted by expressed plant juices 

 within the species, was not found to be cross inoculable on tobacco or 

 vice versa (8). Tobacco mosaic is transmissible by insects. 



Another type of infectious mosaic is that carefully worked out by 

 Baur (11-16) in the Malvaceae. He showed that the Abutilon 

 mosaic is transmitted only by grafting and not by inoculation with 

 the expressed juices, as occurs in tobacco mosaic. As in the latter 

 disease, however, seed transmission does not occur. He found that 

 when scions of the immune Abutilon arboreum Sweet are grafted 

 on the variegated A. thompsoni Hort. they gTow vigorously and re- 

 main apparently normal. However, if scions of the green but sus- 

 ceptible A. indicum Sweet are now grafted upon the immune A^ 

 arboreum they become infected and develop the typical mottling. On 

 the other hand, the contagium passing through the immune A. arbo- 

 reum is not capable of remaining there and giving infection if this 

 portion of the shoot is subsequently grafted into a susceptible stock. 

 In the case of the immune Lavatet'ia arborea Linn., however, there 

 is no transmission at all when double grafted between the mottled 

 Abutilon thompsoni and the green susceptible A. indicum. Baur 

 succeeded in transmitting this mosaic by grafting to about 50 species 

 and varieties of Abutilon and related plants. 



It was found that if the leaves of variegated plants were removed, 

 or if the shoots were cut back so that no leaves remained and the 

 new shoots developed in darkness, only the first two or three leaves 

 were mottled. If these mottled leaves were then removed the plants 

 remained permanently green in the light unless they were grafted 

 with scions from other variegated plants. However, if axillary buds 

 on old parts were forced into growth these produced shoots with 

 mottled leaves which in turn infected all the newly formed leaves 

 on the plant. Furthermore, when scions of a green but susceptible 

 variety were grafted upon defoliated, mottled plants the scions re- 

 mained green ; but here again, if a mottled shoot was allowed to de- ■ 

 velop from the sto.ck it rapidly infected the whole plant. The con- 

 tagium is,^ therefore, capable of infecting only the embryonic leaves, 

 and in the buds it may be stored up for months in inactive form. 



3 The term " contagium" has been suggested by Dr. H. M. Quanjer as synonymous with 

 any infective principle, whether of known or unknown origin. Its use in the place of 

 " virus " with reference to the so-called " filterable-virus " diseases does away with ob- 

 jectionable connotations and leaves nothing to be taken back. 



