Page:Life Movements in Plants.djvu/171



In experiments with different pulvinated organs, great difference is noticed as regards their excitability. If elec- tric shock of increasing intensity from a secondary coil be passed through the pulvini of Mimosa, Neptiinia, and Erythrina arranged in series, it would be found that Mimosa would be the first to respond ; a nearer approach of the secondary coil to the primary would be necessary for Neptiinia to show sign of excitation. Erythrina would require a far greater intensity of electric shock to induce excitatory movement. Organs of different plants may thus be arranged, according to their excitability, in a vertical series, the one at the top being the most excitable. The specific excitability of a given organ is different in different species.

In addition to this characteristic difference, an identical organ may, on account of favourable or unfavourable conditions, exhibit wide variation in excitability. Thus under favourable conditions of light, warmth and other factors, the excitability of an organ is greatly enhanced. In the absence of these favourable tonic conditions the excitability is depressed or even abolished. I shall, for