Page:Researches on Irritability of Plants.djvu/49

26 thus be applied. By means of a variable resistance included in the circuit, the intensity of the stimulus can be increased or diminished. Care must, however, be taken that the heat produced in the platinum loop shall not be such as to scorch or otherwise injure the tissue.

I find that injury from scorching may be avoided by adding a drop of water at the point of contact and afterwards removing excess of water by blotting-paper. This thin film of Water protects the tissue from a burn. It is, again, not absolutely necessary to place the platinum wire in contact with the plant. Excitation will take place if the heating-wire is in close proximity. How practicable this form of stimulus may be rendered will be observed from the record (fig. 7) of two successive excitations by this method, which are seen to be uniform. For certain electrical investigations it is essential that stimulus other than electrical should be employed. This requirement is admirably fulfilled by the thermal mode of stimulation.

Another mode of stimulation—namely, that of thermal radiation—can also be employed, though not so conveniently as the former. A certain area may be rendered radiant by the passage of an electrical current. A Nernst electrical lamp can be conveniently utilised for the purpose. This, when rendered incandescent, gives out radiation of constant intensity. This radiation consists not only of light rays but also of a large proportion of obscure heat-rays. The excitatory value of the latter is more efficient than the luminous rays. The radiant surface of the Nernst lamp is suitably placed in front of a concave metal mirror, by