F. W. WENT 293 



cases causing necrosis of tissues. In the second place growth defects 

 result which can be traced to deficient photosynthesis. 



2. Phloem necrosis, which prevents further translocation of food and 

 growth substances, resulting in stunted growth of all organs, high carbo- 

 hydrate content of leaves, etc. 



3. Developmental deficiencies, in which one or more growth processes 

 are affected. This third group is the most interesting from our standpoint, 

 and within it we can recognize different types. 



A. Retarded growth of stem, accompanied in the later stages 

 by increased lateral bud development, leading to witches'- 

 brooms. This is typical for deficient auxin supply. In the first 

 place the lack of auxin prevents normal elongation of the 

 main shoot, and secondly the lateral buds can grow out since 

 they are no longer inhibited by auxin. Similar effects may be 

 observed in dwarf varieties of plants, where dwarfing is due to 

 excessive auxin destruction (31). Thus we are led to assume 

 that such virus effects are due to auxin deficiency caused by 

 the virus, and perhaps also to destruction of auxin {Primus 

 virus 3, Saccharum virus 3, and Delphitjium virus i are good 

 examples of this case). 



B. Retarded mesophyll growth. The early symptoms of this 

 type of virus are narrow leaves. In severe cases the mesophyll 

 may be practically lacking so that only the midrib remains. 

 This would be a typical case where the virus specifically 

 affects phyllocaline without influencing vein growth. As an 

 example may be quoted Cucumis virus i on Spinacia, tomato, 

 etc. and the shoestring virus of tobacco {Nicotiana virus i). 



C. Leaf curling due to retarded vein growth. In a few cases 

 this may be accompanied by unchecked stem growth, but 

 more often stem elongation is inhibited as well. In these cases 

 mesophyll growth is more or less normal so that the whole 

 leaf bulges and is corrugated. Examples are Rubus virus 3, 

 Gossypium virus i, and Nicotiana virus 10. 



Thus if we summarize the effects of viruses on growth phenomena, 

 we are led to assume that vein growth, mesophyll growth, and stem 

 growth are unrelated phenomena, which may be specifically affected 



