Curiously enough, while weaker solutions of vanillin alone produced stimulation, the stimulating effect of this toxic agent disappeared entirely on the addition of the inorganic substances. The results showed that the addition of certain inorganic salts to solutions of toxic organic compounds was decidedly beneficial to the plant. Another important problem has come to the front with regard to these toxic substances--How do these substances get into the plant? Are they all absorbed if they occur in the soil, or is there any discriminatory power on the part of the root? In other words, do the roots perforce take in everything that is presented to their surfaces, or have they the power of making a selection, absorbing the useful and rejecting the useless and harmful? Daubeny (1833) described experiments in which various plants, as radish, cabbage, _Vicia Faba_, hemp and barley were grown actually on sulphate of strontium or on soils watered with nitrate of strontium. No strontium could be detected in the ash of any of the plants save barley, and then only the merest trace was found. Daubeny concluded that the roots _were_ able to reject strontium even when presented in the form of a solution. “Upon the whole, then, I see nothing, so far as experiments have yet gone, to invalidate the conclusion ... that the roots of plants do, to a certain extent at least, possess a power of selection, and that the earthy constituents which form the basis of their solid parts are determined as to _quality_ by some primary law of nature, although their _amount_ may depend upon the more or less abundant supply of the principles presented to them from without.” Some years after, in 1862, Daubeny reverted to the idea, stating “I should be inclined to infer that the spongioles of the roots have residing in them some specific power of excluding those constituents of the soil that are abnormal and, therefore, unsuitable to the plant, but that they take up those which are normal in any proportions in which they may chance to present themselves[1].” This, however, was not held to apply to such corrosive substances as copper sulphate. De Saussure had found that _Polygonum Persecaria_ took up copper sulphate in large quantities, a circumstance which he attributed to the poisonous and corrosive quality of this substance, owing to which the texture of the cells became disorganised and the entrance of the solution into the vegetable texture took place as freely, perhaps, as if the plants had been actually severed asunder[2]. Daubeny concluded that a plant is unable to exclude poisons of a corrosive nature, as this quality of the substance destroys the vitality of the absorbing surface of the roots and thus reduces it to the condition of a simple membrane which by endosmosis absorbs whatever is presented to its external surfaces, so that whenever abnormal substances are taken up by a living plant it is in consequence