22 BULLETIN 40, U. S. DEPARTMENT OF AGRICULTURE. 



solutions of iron salts were applied to the leaves. Likewise, crystals 

 of ferrous sulphate applied to the roots were effective. In the 

 writer's experiments with tobacco, mosaic leaves in some instances 

 have been painted with a 1 to 2 per cent solution of ferrous sulphate. 

 This solution in different strengths has been introduced into the leaf 

 tissues and other parts of mosaic plants. In no instance has this 

 treatment produced any noticeable effect in checking the subsequent 

 course of the disease. Keeping mosaic plants in darkness serves to 

 induce marked chlorosis, which merely diminishes to some extent the 

 color contrasts in mottled leaves. 



Although it does not appear possible to free a plant from the 

 mosaic disease, the expression of obvious symptoms depends almost 

 entirely upon the treatment which affected plants receive. During the 

 progress of the malady in small, stunted plants the leaves, when 

 young, are sometimes so slightly changed in appearance that many 

 of these finally appear to outgrow all traces of the disease. In such 

 plants this feeble and transitory expression of the disease makes it 

 appear as if there were a gradual migration of obvious symptoms 

 toward the top of the plant. 



On the other hand, a rapid and extensive development of symptoms 

 is coincident with rapid growth. Likewise, conditions which retard 

 growth must of necessity check the further appearance of mosaic 

 leaves and branches. 



EXCISION EXPERIMENTS WITH DISEASED PLANTS. 



Various excision experiments have shown that the complete re- 

 moval by antiseptic methods of all portions of a plant showing 

 visible mosaic symptoms does not rid the plant of the disease. Pro- 

 nounced symptoms later appear in other growing portions of plants 

 thus treated. The excision of mosaic branches sometimes seems 

 actually to hasten the appearance of pronounced symptoms of the 

 disease in branches previously normal in appearance. Plants showing 

 mosaic symptoms only in the topmost leaves or even in the blossoms 

 alone may be cut completely to the ground, yet mosaic symptoms, 

 often of the most malignant character, again appear in the shoots 

 which arise from these stubs. 



SOME PROPERTIES OF THE VIRUS OF THE DISEASE. 



The sap of mosaic plants is highly infectious to all healthy sus- 

 ceptible plants. An extremely minute quantity of the virus in- 

 jected into the tissues is capable of initiating the disease. Tests 

 have also shown that simply pouring the sap of mosaic plants liber- 

 ally upon the leaves of healthy plants is oftentimes sufficient to 

 communicate the disease. 



