84 ESAU 



concerned with photosynthesis or with the movement of water or food. The 

 viruses may, for example, delay starch removal from leaves or affect the 

 enzymes concerned with carbohydrate metabolism. However, viruses may also 

 prevent or delay starch accumulation. The recognition of the primary symp- 

 toms of a virus disease by microscopic observations should be helpful in re- 

 vealing the significance of the biochemical disturbances caused by a given 

 disease. 



Anatomic studies of virus-diseased plants give us some clues to the possible 

 causes of the external symptoms. The breakdown of the food-conducting tis- 

 sue in curly top, aster yellows, and other diseases in the yellows group is, 

 no doubt, related to the general stunting of the plant and its frequently pre- 

 mature death. The gum deposition and the closing of xylem vessels by tyloses 

 in the Pierce's disease of the grapevine are evidences of a disturbance in the 

 water-conducting system; this disturbance, in its turn, explains such external 

 symptoms as the sudden wilting of vigorously growing young vines and the 

 scalding and drying of leaves on vines of different ages. The translucency of 

 leaf veins in young leaves of plants affected with curly top finds its explana- 

 tion in the hypertrophic enlargement of parenchyma cells and the obliteration 

 of intercellular spaces by the crowding of the enlarging cells along the veins. 

 Continued hypertrophy of these cells and the hyperplasia that follows result 

 in an excessive thickening and distortion of the vein ribs, a symptom common 

 in later stages of development of the disease (Esau, 1933). The yellow patches 

 on leaves of mosaic-diseased plants are areas of the most pronounced under- 

 development and degeneration of the carriers of the green pigment, the chloro- 

 plasts (fig. 3 and 5). These are some of the examples of how an anatomic 

 study may serve as a background for the interpretation of the symptoms 

 visible to the naked eye. 



Anatomic aspects of virus translocation in plants. The specificity of 

 the virus-host relation is revealed not only by the nature and location of the 

 primary symptoms in the tissues of the host, but also — perhaps even more 

 convincingly — by the manner in which the virus is introduced into and trans- 

 ported within the plant. 



The viruses of the mosaic group, which induce symptoms generally through- 

 out the plant, are readily introduced into the host by mechanical means, for 

 example, by rubbing the leaf surface with an object previously contaminated 

 with juice from an infected plant. The virus apparently first comes in con- 

 tact with the cytoplasm of the surface cells, mostly those of the epidermal 

 hairs, that are broken by the rubbing. It then moves into the uninjured sur- 

 face cells, which may be other hair cells or epidermal cells, and farther into 

 the inner tissues. This is a cell-to-cell progression, and it causes a disturbance 

 in the protoplasts successively invaded by the virus (Zech, 1952). 



The movement of the virus from cell to cell, that is, across the walls separat- 

 ing the adjacent cells, is not completely understood. Commonly the plas- 



