DO PLANTS SEE?

King Country Chronicle, Volume V, Issue 342, 4 March 1911, Page 7

DO PLANTS SEE?

Singular Sensitiveness Discoveries. The answer to this question ;-c:ms to depend, like the answers to so m..ny oi' : .:cr questions. on a definition . What desre.-' of sensitiveness to iig'.it. merits the name of vision ? If to respond to lightstimulation by appropriate movements is to "see," plants certainly do so : wiiiie if nothinsr short of the formation and apprehension of a dei'mit? image of outside objects may he dignified by the name of sight, then the plant world is still l-li.id. :'ays Dr. D. T. Maedougal, director of the department of botanical res'arch of the Carnegie Institution, in the course of a report on some investigations in which he is engaged., "Light is, perhaps, the most important factor in the existence of plants, since energy is absorbed directly from its rays, and is used in b ;iiding up complex foods from eimrio substances obtained from the soil and air. If the plant is to obtain energy from light, the supposition would lie near that it must present its surfaces to the rays in such a. manner as to enable it to do this advantageously. for the amount of benefit to be derived from the rays would depend directly upon their intensity, and upon th<- angle at which they strike the surfaces. With this fact in hand one would at once suspect that the plant might have developed some power of measuring the intensity and direction of the rays.

"Any group of window plants may l>e seen bending towards the glass in such manner as to present the broad surfaces of the leaves a* right angles to the strongest illumination. The whole shoot appears to be concerned in the reaction, and we must use the blind-folding method to ascertain what parts are sensitive to light. If sheets of tinfoil are bound around the stem, and it is turned away from the window. This shows at once that the individual under treatment perceives light without the aid of the stems, although the swathed stems curve in the reaction. "Next turn attention to the flowers if present, and when these are blackcapped, the plant still turns unerringly to the proper quarter to receive its daily dole of sunshine. The leaves are now to be considered as a seat of the light-perceiving faculty. In most cases those organs have a distinct stalk or petiole and a broader blade, the chief purpose of the latter being to spread out an expanse of green tissue and entrap the rays and make their energy available for the chlotophyl (the green colouring matter of plants) processes. "If prepared sections of the blades of some of the more delicately reacting plants arc placed under the microscope it will be found that the outer walls of the epidermal cells are curved outward, making lenses which converge the rays upon the inner walls and allowing them to be transmitted to the cells beneath where they play upon the green collour bodies in which the construction of food material takes place.

"Imagine one of these epidermal cells to be a room with a convex skylight roof and a glass floor. When the rays come through and fall upon the floor they pass through to the room below, and drive the chlorophyl mills making sugar and other substances. The lateral walls of the skv-lighted room are lined with a living layer sensitive to light and if the leaf or the building is moved so that the rays strike the sensitive layer a signal is sent to a distant shifting mechanism. Slowly, but with unerring precision, this gets in motion and brings the leaf to a position where the rays once more come through the condensing skylight and pass through the floor to the food-making cells below. "The exactness with which the plant can measure intensity of illumination is so great," we are told in the report, "that if a small, rapidly trowing shoot, such as that of yorng mustard, is placed in the dar'.x for a few hours and then two standard candles are placed on opposite sides, the leaves will feel the unequal stimulation when one candle ii an inch marer than the other and the shoot will begin to curve towards it as towards a window. Some plants can appreciate a difference so small as one thrcc-hundrcd-Llv ::sandth of the intensity of a cr.ndle at a distance of a yard."— "Popular Science Sittings."