Page:Collected Physical Papers.djvu/374

354 record is given of the effect of carbonic acid gas on growth (fig. 112). A jar is filled with this gas, and emptied over the plant; the invisible gas, on account of its heavier weight, falls in a stream and surrounds the plant. The record shows that this gave rise to an immediate acceleration of growth, which continued for two and-a-half minutes; this preliminary acceleration was followed by retardation of growth as shown by the down curve. The Balanced Crescograph thus not only exhibits the beneficial effect of an agent, but also indicates the dose which prolongs the beneficial effect.

Growth is modified by the action of visible light; two different effects are produced depending on the intensity. Strong stimulus of light induces a diminution while feeble stimulus induces an acceleration of the rate of growth. The effectiveness of light in modifying growth depends moreover on the quality of light; the effect is very strong in the ultra-violet region of the spectrum with its extremely short wave-length of light; it declines almost to zero as we move towards the less refrangible rays, the yellow and the red, with their comparatively long wave-length. As we proceed further to the infra-red region we come across the vast range of electric radiation, the wave-lengths of which vary from the shortest wave I have been able to produce (0.6 cm.) to others which may be miles in length. There thus arises the important question whether plants perceive and respond to the long ether-waves, including those employed in signalling through space.