The Growth of Higher Plants in Soils Free of Microorganisms

In a study of the action of bacteria in soil it is frequently desirable to determine the relation that exists between the microorganisms and higher plants. The procedure is comparatively simple if small plants or seedlings are used. For this purpose test-tubes, especially the large Pyrex tribes described by Knudson, 1 give excellent results. Where it is necessary to grow large plants and to carry these to maturity, the procedure is not so simple. On account of the size of the apparatus and the length of time required for the development of large, plants, it is difficult to prevent contamination. Some of the points which deserve special care are: (1) sterilization of the apparatus, of the medium, and of the seed; (2) planting of the bacteriafree seed or seedling; (3) adding the sterilized water required for plant growth. The nature of the medium used for the plants determines the time of sterilization. If a synthetic medium, liquid or solid, is used it rarely requires a long period for sterilization. Soil, however, is extremely difficult to sterilize and in order to kill all the bacterial spores that may be present it is necessary to heat it for a 'tong time and at a high temperature. The destruction of bacteria on the seed-coats is in the majority of cases readily effected by means of mercuric chloride solution in a partial vacuum. Although a great number of methods have been recommended for the sterilization of seed, I have uniformly secured