82 ESAU 



Pierce's disease in the grapevine and the dwarf disease in the alfalfa affects 

 primarily the xylem tissue (Esau, 1948a). The water-conducting cells are 

 occluded by products of disintegration of cell contents (gums) and balloon- 

 like cellular outgrowths (tyloses), both derived from adjacent parenchyma 

 cells. The primary nature of these symptoms has been graphically demon- 

 strated by inoculating the virus into grape seedlings, each plant through a 

 single leaf, and checking the kind, location, and time of appearance of symp- 

 toms. The first discernible symptoms were tylose development and gummosis 

 in the water-conducting cells; and their location and spread were related to 

 the place of inoculation. 



Since viruses show differences in their specific relation to the tissues of the 

 host, the recognition of the primary internal symptoms should prove useful 

 in assigning the viruses to the appropriate groups. In other words, the in- 

 ternal symptoms may be utilized in the classification of viruses, at least with 

 reference to the larger groupings of them. As was just elaborated, some viruses 

 induce the primary symptoms in the phloem, others in the xylem, and still 

 others throughout the plant. In addition, the primary symptoms themselves 

 may vary in details of their development. Among the phloem-limited viruses 

 some induce a collapse and death of phloem cells that pass through an ap- 

 parently normal course of development (potato leaf roll, cereal yellow dwarf) ; 

 others bring about abnormal growth phenomena and a profound disturbance 

 of the pattern of differentiation of the phloem before necrosis occurs (curly 

 top, aster yellows). Thus one should be able to separate the phloem-limited 

 viruses into smaller groups by using the character of degeneration of the 

 phloem tissue. We need, however, many more data on anatomic symptoms of 

 viral diseases before their value for classifying viruses may be properly 

 estimated. 



The growth changes in the phloem of plants affected by curly top and aster 

 yellows, just alluded to, involve hyperplasia that results in the development 

 of a large number of abnormal sieve elements (fig. 7-10). As is well known, 

 sieve elements are regarded as the principal conducting cells in the phloem, 

 and their protoplasts and walls show certain peculiar characteristics. The ab- 

 normal sieve elements develop most, though not necessarily all, of the fea- 

 tures found in the normal sieve elements of a given plant; and they may be 

 associated with companion cells. However, their size and shape are abnormal, 

 their arrangement unorderly (fig. 10), and they soon degenerate and collapse 

 (Esau, 1941; Girolami, 1955). The overproduction of sieve elements suggests 

 a disturbance of a fundamental process of differentiation, a process that de- 

 termines the position and relative numbers of xylem and phloem cells in a 

 given position in the plant. The degree to which this process may be upset is 

 strikingly illustrated by the observations that the abnormal sieve elements 

 may differentiate within the xylem in curly top-diseased plants. This abnor- 

 mality indicates that, if biochemical gradients are associated with the diver- 



