Tests made at Rothamsted have carried this point still further. _Pisum sativum_, _Phaseolus vulgaris_, _Triticum vulgare_, _Zea japonica_, _Tropeolum Lobbianum_, sweet pea (American Queen), nasturtium, and cow pea--the first three of these being the species used by Otto--were grown in (1) ordinary distilled water, which was found to contain traces of copper, (2) glass distilled water, for about a month, till no more growth was possible owing to the lack of nutriment. In every single case the root growth was checked in some degree in the ordinary distilled water, the roots seeming to the eye to be less healthy and less well developed. In Pisum, Tropeolum and Zea, the shoot growth of the coppered plants appeared stronger than that of the controls, and this was borne out when the dry weights of the plants were obtained. In every other case the coppered plants were inferior, root and shoot, to those grown in the pure water. With the first three plants it appears that while the toxic water has a bad effect on the roots, yet the growth of the shoots is increased. The idea suggests itself that this apparent stimulation is in reality the result of a desperate struggle against adverse circumstances. The roots are the first to respond to the action of the poison, as they are in actual contact; their growth is checked, and hence the water absorption is decreased. No food is available in the water supply from the roots, so the plant is entirely dependent on the stores laid up in the seed and on the carbon it can derive from the air by photo-synthesis carried on by the green leaves. The result of the root checking in these particular cases seems to be so to stimulate the shoots by some physiological action or other, that this process of photo-synthesis is hastened, more carbon being converted into carbo-hydrates, so that the shoot development is increased, yielding a greater weight of dry matter. In each of the other cases observed the shoot was obviously not stimulated to increased energy by the poison, and so the whole plant fell below the normal. Other experiments showed that barley roots are peculiarly sensitive to the presence of minute traces of copper, as very little root growth took place in the copper distilled water, and root growth was also entirely checked by the presence of one part per million copper sulphate in the pure glass distilled water. Yet again, one litre of pure distilled water was allowed to stand on a small piece of pure metallic copper foil (about 1½″ × ½″) for an hour, and even such water exercised a very considerable retarding influence upon the root-growth, checking it entirely in some instances.