and a suitable temperature are necessary conditions. Repeated stimulation at short intervals fatigues the organ, making it less and less responsive, until finally all signs of sensitiveness disappear, to return only after a period of rest. Certain chemical substances which are known to abolish or suspend the contractility of animal protoplasm have been found to affect in a corresponding manner the movements of barberry stamens. Thus nicotine, alcohol, and the mineral acids destroy all power of movement. A one-per-cent solution of morphine is similarly active, while curare, the powerful nerve poison which leaves the contractility of muscle unaffected, is found to exert no influence upon the stamens of Berberis. The effect of arsenic and corrosive sublimate is to render the filaments rigid and brittle, while if poisoned with prussic acid or belladonna they become relaxed and flaccid. By exposure for a short time to the vapor of chloroform or ether the
Fig. 13.—Series of anthers connecting; the primitive form with that having valvular dehiscence: A, Podophyllum emodi; B, Podophyllum peltatum; C, hypothetical transition form; D, the barberry form. All somewhat diagrammatic.
power of movement is suspended, but may return after removal from the influence of the anæsthetic.
Moreover, experiment shows that the part of the filament which contracts is not necessarily the part touched that is to say, there is a transmission of stimulus from cell to cell. So long as it was believed that the contents of neighboring plant cells were always completely separated by an imperforate wall, no satisfactory explanation could be given of such a transference of impulse, but now that modern microscopy has revealed the presence of protoplasmic threads passing through the cellulose walls of sensitive tissue, making the living matter continuous, the phenomenon in question may be understood as a manifestation of that fundamental property of protoplasm, irritability, to which we also refer the sensitiveness of animals, even though it be exhibited in a highly differentiated nervous system.
Since the irritability of the stamens is found so commonly