SUMMARY
1. Symptoms of infection on resistant and susceptible varieties of tomato (Lycopersicum esculentum Mill.) are briefly indicated, together with a numerical method of recording the progress of infection.
2. The histology of infection on tomato varieties is described. Penetration is stomatal and no appressoria or other modifications are formed. The mycelium is intercellular and without haustoria, and develops normally only for so long as the host cells are alive.
3. By the use of a controlled environment chamber, the frequency of penetration is shown to be far greater at a humidity fluctuating from saturation to 85 % than at constant saturation. The suggestion is made that penetration is controlled, in part at least, by a hydrotropic stimulus.
4. The initial rate of spread of the mycelium within the host, immediately subsequent to penetration, is lower in the variety “Mainerop” than in “Giant Red” and is lower also in the basal region of the plant than in the middle. This behaviour runs parallel with the subsequent differences observed in the external symptoms of infection.
5. Normal stomatal penetration occurs over a wide range of immune and “inappropriate” hosts.
6. Symptoms of infection have been recorded only in the varieties of L. esculentum, in L. Humboldtii Dun., and in two strains received as L. racemigerum which, appear to be more closely related to the cultivated varieties than, to the true currant tomato.
7. Confusion occurs as to the correct nomenclature of the currant tomato. The name Lycopersicum pimpinellifolium Mill, is preferred. L. racemigerum Lange (=Solanum racemigerum) and L. racemiforme Lange are synonyms.
8. The extent and distribution of the mycelium in the currant tomato appears to be determined largely by the age and relative maturity of the leaf. The mycelium is intercellular and without haustoria, and can apparently remain alive in the leaf for a considerable period of time. No conidiophores are produced. No reaction of the host cells is observed except where, on account of structural considerations, host and parasite are brought into more immediate contact. Here, individual host cells are frequently necrotic.
9. On many other Solanaceae, including species of Solatium, Hyos‐cyamus, Nicotiana, Schizanthus, and other genera, the course of events is essentially similar to that observed on the currant tomato.
10. In other Solanaceae, and in plants belonging to the Scrophu‐lariaceae, Compositae, and Cucurbitaceae, an extensive mycelium is never developed, even under apparently favourable conditions.
