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Pathogenicity Studies. Although the cause of the disease is not known, 
it is very easy to inoculate and obtain infections with the virus. A series 
of infection-experiments will be undertaken by the class as follows: 
General instructions. Students will work in pairs, A, B and C, as 
assigned by the instructor. There will be four experiments, each involv- 
ing the cooperative action of three pairs of students. Two pairs in each 
experiment will inoculate the healthy plants provided as directed, while 
the third pair will prepare check-plants. 
Healthy tobacco, tomato and petunia plants will be provided. Do 
not touch these plants except when directed and then exactly as directed. 
Experiment I. Inoculation by touching. PROCEED as follows :— 
17. Thoroughly wash the hands and disinfect in alcohol pro- 
vided. 
18. Pairs A and B, squeeze and rub but do not crush, between 
thumb and finger, a diseased tobacco leaf; then immediately squeeze 
and rub with the same thumb and finger, leaves on the healthy plants. 
LABEL. 
19. Pair C, do not touch diseased plants but squeeze and rub 
with thumb and finger (do not crush), the leaves on the healthy plants 
provided for checks. LABEL. 
Experiment IIT. Incculation with sap from diseased planis. PRO- 
CEED as follows:— 
20. Pairs A and B. Each pair will thoroughly clean and rinse 
a mortar in sterilized distilled water. Grind three diseased tobacco leaves 
in a little sterilized distilled water in the mortar. The virus is thus freed 
from the leaf-tissue. Then very thoroughly wash the hands and disinfect 
in alcohol provided. Flame and cool a needle. Dip the point into the 
virus and prick the midrib of several young leaves of the healthy plants 
provided. Dip the needle each time before pricking. LABEL. 
21. Pair C, thoroughly wash the hands and disinfect with 
alcohol. Do not touch diseased plants. Flame and cool a needle. Dip 
it in sterilized distilled water and prick the midrib of several young leaves 
of the healthy plants provided. LABEL. 
Experiment III. Root tnoculatton. PROCEED as follows:— 
22. Pairs A and B, thoroughly wash the hands and disinfect with: 
alcohol provided. Do not touch the tops of the plants. Sterilize a 
scalpel by dipping in alcohol and flaming. Thrust the scalpel into the soil 
about the healthy plants provided, so as to cut some of the roots. Pour 
into the soil, where the roots were cut, some of the diseased-leaf extract 
prepared by pairs A and B in experiment II. LaBE . 
23. Pair C, thoroughly wash the hands and disinfect with alcohol. 
Proceed as in number 22 but use sterilized distilled water to pour on the 
roots, instead of the diseased-leaf extract. LABEL. 
Experiment IV. Inoculation through wounds. PROCEED as follows:— 
24. Pairs A and B, thoroughly wash the hands and disinfect in 
alcohol provided. Break a young leaf from each of the healthy plants 
provided. Rub a diseased leaf between thumb and finger and touch the 
broken leaf-stub on the healthy plants but do not touch them elsewhere. 
LABEL. 
25. Pair C, thoroughly wash the hands and disinfect with 
alcohol. Do not touch diseased plants. Break a young leaf from each 
