Page:Life Movements in Plants.djvu/203



Accurate determination of the effect of temperature on growth presents many serious dilliculties on account. of numerous complicating factors. In nature, the upper part of the plant is exposed to the temperature of the air, while the root underground is at a very different temperature. Growth, we shall find, is modified to a certain extent by the ascent of sap. (See p. 139, lair/2t. 6.9.) The activity of this latter process is determian by the temperature to which the roots are subjected. The difficulty may he removed to a certain extent by placing the plant in a thermal chamber, with arrangement for regulating the temperature of the air. The air is a bad conductor of heat, and there is some uncertainty of the interior of the plant attaining the temperature of the surrounding air, unless the plant is long exposed to the definite and constant temperature of the plant chamber. Observation of the effects of different temperatures then becomes a prolonged process, with the possibility of vitiation of results by autonomous variation of growth. Reduction of the period of experiment by rapidly raising the temperature of the chamber introduces fresh difficulties; for a sudden variation of temperature often acts like an excitatory shock. 'l‘his drawback may