Many plants have poisonous seeds. Indeed in many species there is a tendency for the poison to be collected from other parts of the plant and concentrated in the seed. These poisonous seeds are of great importance in Canada, where they form a considerable proportion of the screenings, millions of bushels of which are cleaned from grain annually. It has been customary for millers to mix the screenings from the wheat they grind, with their bran and shorts. In the early days the screenings obtained at the large grain elevators of the west were destroyed, but of late years they also have been used in the manufacture of feeds. This practice, which has been fairly successful in the United States, has caused much trouble in Canada, where the screenings on the average contain a much larger proportion of poisonous seeds. The loss from this source has been confined, for the most part, to swine. Complaints have, however, come from men in other branches of the livestock business, especially those engaged in dairying, who claim that a great deal of sporadic abortion has been caused by the use of feed containing screenings.

Much investigation into the character of our native weed seeds is still needed, and it is probable that species not now known to be harmful will be definitely proved so in the future. This section gives a resume of the facts so far ascertained.