I now tried the effect of artificial lowering of temperature on various plants. A plant which is very easily affected by cold is a certain species of Eucharis lily. I first obtained responses with the leaf-stalk of this lily at the ordinary temperature of the room (17° C.). I then placed it for fifteen minutes in a cooling chamber, temperature -2° C., for only ten minutes, after which, on trying to obtain response, it was found to have practically disappeared. I now warmed the plant by immersing it for awhile in water at 20° C., and this produced a revival of the response (fig. 35). If the plant be subjected to low temperature for too long a time, there is then no subsequent revival.
[Illustration: FIG. 35.--DIMINUTION OF RESPONSE IN EUCHARIS BY LOWERING OF TEMPERATURE (_a_) Normal response at 17° C. (_b_) The response almost disappears when plant is subjected to -2° C. for fifteen minutes. (_c_) Revival of response on warming to 20° C.]
I obtained a similar marked diminution of response with the flower-stalk of Arum lily, on lowering the temperature to zero.
My next attempt was to compare the sensibility of different plants to the effect of lowered temperatures. For this purpose I chose three specimens: (1) Eucharis lily; (2) Ivy; and (3) Holly. I took their normal response at 17° C., and found that, generally speaking, they attained a fairly constant value after the third or fourth response. After taking these records of normal response, I placed the specimens in an ice-chamber, temperature 0° C., for twenty-four hours, and afterwards took their records once more at the ordinary temperature of the room. From these it will be seen that while the responsiveness of Eucharis lily, known to be susceptible to the effect of cold, had entirely disappeared, that of the hardier plants, Holly and Ivy, showed very little change (fig. 36).
Another very curious effect that I have noticed is that when a plant approaches its death-point by reason of excessively high or low temperature, not only is its general responsiveness diminished almost to zero, but even the slight response occasionally becomes reversed.
[Illustration: FIG. 36.--AFTER-EFFECT OF COLD ON IVY, HOLLY, AND EUCHARIS LILY _a._ The normal response; _b._ Response after subjection to freezing temperature for twenty-four hours.]