Page:Life Movements in Plants.djvu/280

250 cular movement in a growing organ from ascertained facts relating to the corresponding movement in a pulvinated organ. The investigations on motile and growing organs that have been described fully establish the two important facts that, Direct stimulus induces contraction and Indirect stimulus induces the opposite expansive reaction. These facts will be found to offer full explanation of various tropic curvatures to be described in the subsequent series of Papers,

There is no breach of continuity in the increasing contractile leaction in a growing organ under increasing intensity of stimulus ; the incipient contraction seen in retardation of rate of growth culminates in a marked shortening of the length of the organ.

Time relations of response, the latent period, the apex time, and the period of recovery are of similar order in pulvinated and in growing organs.

In condition of sub-tonicity the pulvinus of Mimosa responds to stimulus by an abnormal positive or erectile response. Under continued stimulation the abnormal positive is converted into normal negative. Growing organs in sub- tonic condition responds to stimulus by abnormal accelera- tion of rate of growth, which is converted into normal retardation under continuous stimulation.

Direct stimulus induces in Mimosa a negative response, with the fall of leaf. But Indirect stimulus induces the positive or erectile response. Similarly, Direct stimulus induces in a growing organ a negative variation, or retarda- tion of rate of growth, and Indirect stimulus a positive variation or acceleration of rate of growth.

The electric response to Direct stimulus is by, galva- nometric negativity, that to Indirect stimulus by galva- nometric positivity.