PROCESS OF INFECTION AND VIRUS SYNTHESIS 



17 



semiaxes is not far from unity. The semiaxes have been averaged to produce 

 the curves sho^^^l in Fig. 6 for three different strains of TMV. While there is a 

 significant difference m the growth rate of the lesions produced by different 

 strains, the rate at which the radius of a lesion increases is constant for each 

 strain over the 5-day period that measurements could be made. Measure- 

 ments beyond a 5-day infective period for the fastest growing strains are not 

 practicable because of coalescence of lesions. 



1 2 3 4 5 6 



Days after inoculation 



Fig. 6. Lesion growth of three different strains of TMV on N. glutinosa leaves. From 

 llappaport and Wildman (1957). 



Key: squares, Ul; circles, U2; triangles. Holmes' rib grass. 



The conclusion is reached from these experiments that a perimeter of cells 

 becomes necrotic and is added to the circumferance of a lesion at a rate 

 that can be estimated as involving 6 cells per day for Ul, 4.5 for U2, and 3 

 per day for U8 (Holmes' rib grass). 



There is a fairly sharp line of demarcation which sets the necrotic area 

 apart from surrounding green tissue. However, microscopic exammation of 

 the green cells at the edge of the necrosis shows that obvious signs of degener- 

 ative changes are apparent in cells that are 3 to 4 cells in advance of the 

 encroaching necrosis, in the case of strains Ul and U2. Strain U8 presents a 

 different picture. The necrotic area is bounded by a layer of 5 to 6 cells 

 which become greatly swoUen in appearance. Often, the layers of swollen 

 cells can be detected as a yeUow halo surrounding the necrotic area of the 

 lesion. Extending beyond the swoUen cells, degenerative changes can be seen 

 in a zone extending 4 to 5 cells further into the green tissue. 



VOL. II — 2 



