622 PLANT GROWTH AND PLANT COMMUNITIES 



and size of chloroplasts, particularly in the mesophyll tissue (Esau, 

 1944; Porter, 1954; Esau, 1956). Porter has shown that the yellow areas 

 of cucumber leaves aflFected with cucumber mosaic virus are hypo- 

 plastic, whereas the dark-green raised islands are hyperplastic and con- 

 tain abnormally long palisade cells. In some mosaic diseases, di£Eeren- 

 tiation into palisade and spongy parenchyma is suppressed (Esau, 

 1944), while in others, normal differentiation occurs (Porter, 1954). 

 Suppression of normal morphologic differentiation may be so acute as 

 to cause extreme leaf deformation, a good example of which is the 

 "shoestring" disease of tomato ( Figure 1 ) . 



Many viruses do not produce a mosaic pattern on the leaves but a 

 syndrome of dwarfing, leaf curling, adventitious roots or shoots, or yel- 

 lowing. These are characteristic symptoms of the so-called yellows 

 diseases (Figure 2). Many of the obvious effects of these viruses are 

 due to deranged tissues, especially conductive tissues. The viruses of 

 barley yellow dwarf, potato leafroll, beet yellows, beet curlytop, and 

 aster yellows all incite a degeneration of phloem tissue as a primary 

 effect (Bennett and Esau, 1936; Girolami, 1955; Esau, 1957). The 

 potato leafroll virus and the barley yellow-dwarf virus characteristically 

 incite a necrosis of the sieve tubes ( Figure 3 ) . The beet curlytop virus 



"^^^f^^f^f^ 



A 



Figure 1. Extreme leaf de- 

 formation ("shoestringing") in 

 tomato caused by the to- 

 bacco-mosaic virus. 



