Most of the evidence for the production of toxic substances, by one plant, that affect the growth and survival of another, is circumstantial, according to a review by Bonner (5). There is direct evidence that the leaves of Artemisia absinthium L. (3), Encelia farinosa A. Gray (6), and Thamnosma montana Torr. & Frem. (2), produce substances which are toxic to other plants. A substance has been isolated from the roots of guayule which is toxic to guayule but which is unimportant agriculturally because this substance remains only a short time in the soil (4). Also dead brome grass roots have been found to be toxic to brome seedlings (1); however the evidence here is indirect (5). Le Tourneau et al. (13) have shown also that water extracts from tissues of many crops and weeds inhibit germination of wheat.
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Kommedahl et al. (1959) studied this question.