The pectic substances which constitute the middle lamellae of fleshy fruits, and thus serve as the cementing material between cells, are of great interest to the plant pathologist because of the fact that if an invading organism is to make its way into a tissue, it must either pass along the line of the middle lamella between the cells, or it must bore through the cells. In the first case a pectic enzyme at the hands of the parasite may be presupposed, and in the second case a cellulose enzyme.The mechanism of infection by Sclerotinia cinerea has received particular attention recently by Cooley (4) and by Valleau (8).Cooley maintains that its hyphae in plum tissue are mainly intracellular; while Valleau finds them to be entirely intercellular, and produces photomicrographs to prove it. Cooley could not demonstrate the presence of an enzyme which would dissolve the calcium pectate of the middle lamella. He also failed to find