of some interference with the vital functions of the roots caused in the first instance by the deleterious influence of the agent employed. [1] This idea of a selectivity of the roots has been recently revived by Colin and Lavison (1910) who found that when peas were grown in the presence of barium, strontium or calcium salts no trace of barium could be found in the stem, strontium only occurred in small quantities, while calcium was present in abundance. They concluded that apparently salts of the two latter alkaline metals could be absorbed by the roots and transferred to the stem and other organs, but that this is not the case with salts of barium. They obtained similar results with other plants, beans, lentils, lupins, maize, wheat, hyacinth. Their proof is not rigid, and exception could be taken to it on chemical grounds. [2] Vide Daubeny, _Journ. Chem. Soc._ (1862), p. 210. In spite of the enormous amount of work that has been done on this subject of toxic action and stimulation it is yet too early to discuss the matter in any real detail. A voluminous literature has arisen around the subject, and in the present discussion some selection has been made with a view to presenting ascertained facts as succinctly as possible. No attempt has been made to notice all the papers; many have been omitted perforce; it would have been impossible to deal with the matter within reasonable length otherwise. A full and complete account would have demanded a ponderous treatise. This widespread interest on the part of investigators is fully justified, as the problems under discussion are not only of the highest possible interest to the plant physiologist, but hold out considerable promise for the practical agriculturist. CHAPTER II METHODS OF WORKING I. DISCUSSION OF METHODS. In the course of the scattered investigations on plant poisons and stimulants, various experimental methods have been brought into use, but these all fall into the two main categories of water and soil cultures, with the exception of a few sand cultures which hold a kind of intermediate position, combining certain characteristics of each of the main groups. The conditions of plant life appertaining to soil and water cultures are totally different, so different that it is impossible to assume that a result obtained by one of the experimental methods must of necessity hold good in respect of the other method. A certain similarity does exist, and where parallel investigations have been carried out this becomes evident, but it seems to be more or less individual, the plant, the poison and the cultural conditions each playing a part in determining the matter. 1. _Water cultures._