Popular Science Monthly/Volume 60/December 1901/Sensory Mechanism of Plants
| SENSORY MECHANISM OF PLANTS. |
By D. T. MACDOUGAL,
FIRST ASSISTANT AND DIRECTOR OF THE LABORATORIES. NEW YORK BOTANICAL GARDEN.
THE relation of the vegetal organism to its environment has demanded a much more generalized type of sensory action than that of the animal, Thus but few species of plants have developed special perceptive organs. The sensory functions are exercised by extended regions of the body, yet the delicacy of appreciation of differences in the intensities of external forces is not surpassed by that of the animal. Thus no plant has sensory organs for the reception of light-stimuli, yet, as a matter of regulation of their main function of food-building, leaves react to differences in intensity far beyond the range of the unaided human eye. Special tactile organs are differentiated in tendrils and in certain 'carnivorous' species and 'sensitive' plants,
Fig. 1. Surface View of Cells of Perceptive Region of the Columna of Stylidium graminifolium. After Haberlandt.
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| Fig. 2. Longitudinal Section through a Single Papilla of an Epidermal Cell of the Columna of Stylidium graminifolium which is Sensitive to Contact. | Fig. 3. Epidermal Cell of the Perceptive Layer of Tendril of Entada scandens. | |
in which members are adapted to a narrow and unusual non-typical purpose. Here also great delicacy and accuracy is obtained, and the contact or weight of a body inappreciable to the sense of touch of any known higher animal may act as a stimulus. This refinement of reaction in undifferentiated tissues is quite remarkable. As a further instance it may be cited that leaves of certain seedlings are capable of appreciating an intensity of light equal to .00033 of a standard candle.
Very naturally the first studies made in this subject attempted to discover an arrangement in plants comparable to a simplified neuro-muscular system of the animal. Expectations of this character were of course bound to meet with signal disappointment; a fact that should have been apparent if the history and widely different purpose of the animal and vegetal organism had been taken into consideration. Parallelisms between the reactions of plants and animals even to the same class of stimulus are to be accepted with great caution. Thus it has recently become apparent that the heliotropism of animals as investigated by Loeb is widely different from the heliotropism, or phototropism, of plants both as to the features of light acting as stimuli in the separate cases, and the general nature of the consequent reactions.
Recent papers by Nemec on the transmission of impulses in plants, and the discussions of geotropism and the organs of equilibrium of plants by Noll, Czapek and Haberlandt have awakened much interest in the mechanism of irrito-motility of plants.
Not fully appreciating the significance of the diffused and generalized forms of perception organs, much effort has been directed toward fixing on specialized protoplastic tracts, with functions analogous to nerves. The quest has not yet met with decided success in any single instance. We have, however, arrived at the general conclusion that the ectoplasmic layers of the protoplasts of peripheral cells function as sensory organs, and that impulses are transmitted between the motor and sensory zones by these layers and their interprotoplastic threads. As to the nature of the impulse, one can only hazard a meaningless guess that it may consist in a chain of chemical, catalytic or osmotic disturbances.
Two noteworthy attempts have been made to ascribe the function of transmission of impulses to specially differentiated structures. The first was by Haberlandt who dealt with the transmission of impulses in
Mimosa, the common 'sensitive plant' of the tropics, cultivated in conservatories. An impulse set up at the tip of a pinnule of one of these plants is conducted through petioles, stems and branches to a distance of a meter at a rate varying from 6 to 31 mm. per second. A study of the structure of the plant reveals the presence of a connected series of long tube-like cells in the fibro-vascular bundles, usually turgid, and containing relatively small protoplasts. It is argued that impulses take the form of hydrostatic disturbances communicated through the system of tubes. This conclusion, however, disregards many well-known adverse facts. Thus it is possible to secure the conduction of an impulse through a section of stem, one or even two centimeters in length, which has been killed by a steam jacket and allowed to desiccate. Then again, when excised stems have been placed in connection with the most powerful force pumps, or the action of the strongest osmotic solutions, and artificial disturbances set up, no reactions were induced in the pinnules, although great hydrostatic movements must have been initiated. The above mentioned hypothesis must be declared 'not proven,' although it is a puzzling matter to attempt any suggestion of a method by which transmission could be accomplished through 2 cm. of dead tissues, and a meter of living tissue.
Nemec finds a somewhat regular coincidence of fibrillar structures in the apical portions of roots, with the pathway which impulses should travel in passing from the perceptive region to the motor zones. The occurrence of these structures has been well known for some time, and the theory of their function as special transmitting organs has something in its favor, especially as these fibrillae have continuous intercellular communications. No facts are at hand to suggest the presence of these fibrillar organs in other members of the body.
The decentralized organization of the plant, the intimate and delicate morphogenic and physiologic correlations existing among all its members and its reflective system of irritability, make unnecessary, and preclude, the differentiation of transmitting tracts, except in certain narrowly specialized organs adapted to other than their typical vegetative purposes. The most recent hypothesis as to the geotropic action of the
plant is in accordance with these ideas. By this theory the maintenance of equilibrium is made possible by the appreciation of gravity as the result of the position of granules in sheath cells in every part of the body, these cells acting as statoliths and sending impulses to the motor zones of the organs in which they are found.