28 BALD 



mates in terms of proportionality. Sometimes one sample is said to be, 

 say, twice as strong as another: what is meant is that one sample has 

 produced twice as many lesions as the other. This is obviously not the 

 same as having twice the number of infectious virus particles per unit 

 volume of inoculum. 



The possibilities of using other units than local lesions for estimat- 

 ing the concentration of plant viruses have been somewhat neglected. 

 Many viruses readily infect inoculated plants but do not produce local 

 lesions. Others produce diffuse lesions evident on inoculated leaves, but 

 not discrete and countable. It is thus possible to use whole plants or 

 single leaves as the infection units, and the value of TV in equation i 

 (or 2) is then known. The plants used for such inoculation tests need 

 to be very uniform, as there are some practical and theoretical objec- 

 tions to the comparison of infection counts made with variable units 

 such as these may be. However, the Poisson distribution is not very 

 sensitive to distortion from heterogeneity of the units entering into the 

 estimate of the mean {pn or pv). Some of the theoretical objections do 

 not seriously interfere with the use of leaves and plants as units for 

 infection, provided the usual precautions necessary in quantitative work 

 are taken. One serious disadvantage is the amount of material and space 

 needed for trials, particularly with whole plants as units. 



The adaptibility of infection methods for measuring the concentra- 

 tion of plant viruses is shown by the following example taken from a 

 paper by Bennett on sugar beet curly top virus. Insect vectors of curly 

 top were fed on three lots of partially purified curly-top virus. One 

 sample of virus was the exudate from vascular tissues (phloem) of dis- 

 eased beet plants, one from expressed juice of beet plants, and one from 

 ground up bodies of insect vectors carrying the virus. Batches of non- 

 infectious vectors were fed through membranes on serial dilutions of 

 each of the three virus samples sweetened with sugar. The insects were 

 then placed singly on healthy test plants. In each instance about 42 

 per cent of the insects appeared to be the maximum number causing 

 infection on the test plants under the conditions of the experiment. 

 A^, the maximum number of plants infected in each trial, was taken at 

 42 per cent of the mmiber inoculated from each dilution by single in- 

 sects. The numbers of infected plants were fitted in two instances by 

 the simple equation ( 1 ) on the hypothesis there was no aggregation in 

 the original virus suspensions fed to the originally noninfectious in- 

 sects. The third (the phloem exudate) appeared to be aggregated, as it 

 was fitted by the more complex equation. The results of fitting these 

 dilution series are shown in Table II. 



