Tumor formation on stem fragments in vitro.

The formation of tumors in tissues of higher plants by the crown-gall organism, (Phytomonas tumefaciens), is influenced by several factors. First, the bacteria themselves must be virulent. Avirulent strains of the crown-gall organism will not produce a gall in susceptible tissue unless that tissue is also stimulated by a growth hormone (1, 10) or by products from a gall produced by fully virulent organisms (6). The tumefacient power of a strain of crown-gall organisms may be eliminated by the presence of certain amino-acids in the culture medium, the most potent of which is glycine (7, 11). The organism, even when fully virulent, has not been found capable of producing a gall unless introduced into wounded tissue. Very small wounds, involving only a few cells, were found to be sufficient to bring about gall formation, but when the wound was small, the gall was also small (Riker 1923). Introduction of bacteria directly into a plant cell was not followed by gall formation, at least in the tomato trichome (5). Not all higher plants respond with gall formation to inoculation with Ph. tumefaciens. Elliott (3) lists plants in 33 different botanical families, all dicotyledons, in which galls are formed in response to inoculation with this organism. There are, however, many species of plants, even among the dicotyledons, which appear to be immune to this organism, while, among monocotyledons, immunity is general. Gall formation seems capable of occurring on practically every tissue of a susceptible plant from the apical growing point to the roots. The anatomical studies of Noel (1946) and other workers indicate that the epidermis, cortex, cambium and pith may all be stimulated to abnormal growth by the crown-gall organism. That the formation of galls in susceptible plants is strongly influenced by temperature, has been demonstrated first by Riker (12) and later by Braun (2). Gall formation on tomato plants is completely inhibited if the plants are kept at a tem-