SUMMARY1Tests were made of the spreading power of suspensions of plant juice, finely divided carbon, a commercial spreader, and water on leaves ofNicotiana glutinosaand paraffin wax surfaces.2The contact angle of the suspensions of the plant juice and the “spreader” was lower than that of water and suspensions of carbon.3When rubbed over the surface the plant juice and “spreader” gave good contact compared with the water, and the particles of carbon adhered to the surface carrying with them a film of water.4The effect of carbon and plant juice in raising the number of lesions when present in the inoculum may be explained by the better contact of the inoculum with the leaf surface.5Carbon in the form of lamp‐black caused an equal rise in number of lesions with purified suspensions of three viruses.6The same virus sample at a number of dilutions showed approximately equal rises in number of lesions in the presence of a uniform amount of lamp‐black. When the lamp‐black was diluted in the same ratio as the virus, the effect diminished.7The extent of the rise given by carbon was shown to be largely a function of the leaf tissues, probably of the nature of the leaf surface. Differences were observed between plants and between leaves of t