These experiments may possibly give some indication as to the action of copper salts on plant roots. So long as the solution of copper salt is dilute enough, the absorption layer of the root, acting as a semi-permeable membrane and upheld by the resistant protoplasm, is able to keep the copper out of the plant and to check its toxicity. As soon as a certain limit is reached the copper exercises a corrosive influence upon the outer layer of the root whereby its functions are impaired, so that it is no longer able efficiently to resist the entry of the poison. As the concentration increases it is easy to conceive that the harmful action should extend to the protoplasm itself, so that the vital activities of the plants are seriously interfered with and growth is entirely or partially checked, death ensuing in the presence of sufficiently high concentrations. 2. _Effect of copper on germination._ The action of copper on the germination of seeds, spores and pollen grains has attracted a certain amount of attention, and although the results are apparently contradictory this is probably due to the different plant organs with which the observers have worked. (_a_) _Seeds._ Miyajima (1897) showed that the germinating power of such seeds as _Vicia Faba_, _Pisum sativum_, and _Zea Mays_ was partly destroyed by a 1% solution of copper[4], _Zea Mays_ being the most resistant and _Vicia Faba_ the least resistant of the three. Micheels (1904–5) stated that water distilled in a tinned copper vessel was more favourable for germination than water from a non-tinned vessel. He suggests that this is due to copper being present in the water in a colloidal form in which the particles are exceedingly small and maintain themselves in the liquid by reason of a uniform disengagement of energy in all directions, to which energy the influence on germinating seeds must be attributed, the nature of the suspended substance determining whether the influence be favourable or not. It is questionable, however, whether Micheels was really dealing with a true colloidal solution of copper or with a dilute solution of some copper salt produced by oxidation of the copper vessel from which his distilled water was obtained. [4] The English translation in Just’s _Bot. Jahresber._ speaks only of a “solution of copper,” and in no case is the specific compound mentioned. (_b_) _Spores and pollen grains._