ents, but lies rather in the production of unfavorable soil conditions by the excretion of substances from the roots of preceding crops of plants.
The effect of root excretions is strikingly shown by some experiments in which wheat was grown in pure quartz sand. The sand contained a very insignificant amount of plant nutrients, but the young wheat seedlings were able to draw sufficient nourishment from the seed to maintain growth during the time of the experiment. Pots of fresh quartz sand were planted with wheat simultaneously with an equal number of pots containing quartz sand which had previously grown wheat for twenty-one days. The growth of the wheat in the "exhausted" sand was only about 45 per cent, of that in the fresh sand, in spite of the fact that both sets of plants were supplied with nutrients from the seeds. It would be obviously incorrect to ascribe the "exhaustion" of the sand to a depletion of plant nutrients at the outset. The harmful effects following a previous crop appear to be more probably due to the presence of deleterious substances arising in the sand during the growth of that crop.
Evidence from another experiment showed that the roots leave substances in the soil which may be removed by proper treatment. Three crops of wheat seedlings were grown in pots and showed decreasing yields. When the last crop was removed, an aqueous extract of the soil was prepared and used as a nutrient solution in which a fourth set of wheat plants was grown. A portion of the soil extract was shaken with carbon black, which acts as a strong absorbing agent, and filtered free from carbon black at the end of a half hour. When a soil extract possessing deleterious properties is given this treatment with an absorbing agent, it is almost invariably improved, on account of the removal of the harmful substances. In this "exhausted" soil the same results of the carbon black treatment were noticed as in the case of deleterious soil extracts. The extract of the "exhausted" soil produced as good plants, after treatment with carbon black, as the extract of fresh soil treated in the same way.
The antagonistic action of one plant upon another was shown by the harmful effect of trees upon wheat. Small trees were transplanted into paraffine wire pots and kept growing in a greenhouse where optimum conditions of light and moisture could be obtained. Wheat was also planted in each pot and allowed to grow about three weeks. As soon as the first set of wheat plants had been cut and weighed, a second set was planted and this was repeated at intervals of about three weeks, until nine successive crops had been grown, always growing control crops in pots without trees. The growth of the wheat in the pots containing trees was poorer than the controls during the summer while the trees were actively growing, "but when autumn came and the trees
