IS THE DOOR OF UTRICULARIA AN IRRITABLE MECHANISM?

Abstract

The present paper deals with the question whether the door mechanism in Utricularia is an irritable one (as e.g., Mimosa is irritable) or is mechanical, and in the opening paragraphs the two views are set forth together with mention of the protagonists and their factual contributions. In particular the recently published views of Kruck have been examined and found to be inadequate.(a) The evidence for the conclusions herein presented have been drawn from a study of the posture of the door, and its exact positional relations to the threshold. The correlated anatomical facts have been studied and set forth, and it has been shown that the physical properties of the door do not change during action and that its action depends wholly on the pressure of the external medium (water) induced by the out-pumping capacity of the walls of the trap, first observed by Brocher, on the water-tightness of the door procured by the velum, as shown by Lloyd, and upon the actuation of a tripping mechanism also earlier described by the same author. There are no changes in turgor in the door cells and no reception or transmission of stimuli by the bristles, which are simply a part of the tripping mechanism.(b) Evidence has also been obtained from a study of the flexures and movements of the door, and it is shown that such could not occur if there were changes in turgor. It is further shown that the histology of the door and threshold are correlated with the position, degree and direction of the flexures.(c) It has been shown that during the flexures and movements of the door, there is no change in the position or form of the air contained in the intercellular spaces of the door, such as have been said to occur as a result of the movement of sap into these intercellular spaces.(d) Killing of the bristles does not prevent the action of the door, although it has been postulated that the bristles receive and transmit stimuli to the tissues of the door.(e) The rate of movement of the door during action is so rapid that, while it does not preclude the interpretation that it is an irritable response, it at least exposes it to grave doubt. Since the action can be repeated as rapidly and as often as the experimental conditions permit there can be no period of recovery. The action is so rapid that, if the mechanism is an irritable one, the latent period, or the period of transmission, must be very much shorter than any other case in the plant kingdom.The conclusion is therefore reached that the mechanism of the door is purely mechanical, though this is not to say that the trap is purely passive, since the physiological activity of the walls has to play its part in exhausting water from the interior, as a reduced water pressure within is necessary for the action of the door.A comparative study of 75 species of Utricularia supports this conclusion.