CHIEF ENVIRONMENTAL CONDITIONS. 129 



marked rise in temperature is prevented by outward conduction and 

 radiation, and the body-temperature remains automatically at about 

 the same point as that of the material surroundings. On the other 

 hand, if the summed result of physiological changes be a disappearance 

 of heat within the cells, then any considerable fall in tissue-temperature 

 is automatically adjusted at an incipient stage by intake of heat from 

 without. The only exceptions to this rule that are worthy of mention 

 here are the cooling effect of transpiration, whereby the temperature of 

 foliage is sometimes as much as a few centigrade degrees below that 

 of the air, and the heating effect of sunshine, whereby leaves are some- 

 times a few degrees warmer than their surroundings. 



From thermodynamics we may be sure that the outward and inward 

 conduction of heat are automatically self-controlling, heat not being 

 conducted from a cooler to a warmer body. Thus an atmosphere at 

 a given temperature will never, by heat conduction, render a plant 

 either warmer or cooler than the air itself. But it is possible for 

 a plant, or other body, to receive heat by radiation from an imme- 

 diate environment the temperature of which is much lower than its 

 own. Conversely, for a time at least, it may radiate heat into an 

 immediate environment having a higher temperature. The first case 

 is very important in many instances, as in the absorption of sunlight 

 by green leaves, which will be considered under the topic "Light." 

 The second is of much less frequent occurrence, but may sometimes 

 be important in the cooling of leaves on clear nights when radiation 

 is rapid. 



As in the case of the entrance and exit of water, the nature and 

 condition of the plant surfaces exert a considerable influence upon the 

 possible rate of entrance or exit of heat, either by radiation or conduc- 

 tion. The effect of a more or less thorough insulation of the plant- 

 body would, of course, be the introduction of a correspondingly pro- 

 nounced lag in the temperature changes of the plant, as far as these 

 are due to radiation or to conduction to or from the exterior. Thus, 

 after a temperature change in the surroundings, some time may elapse 

 before uniform temperatures within and without again prevail. In 

 the case of roots this feature is probably of but little importance, 

 although the results of secondary growth often produce on old organs 

 of this sort a layer of cork and other modified cells of sufficient thick- 

 ness, so that a considerable retardation of heat conduction no doubt 

 ensues. Heat radiation appears to be of little or no importance in 

 subterranean organs. 



The aerial portions of the plant exhibit more considerable effects of 

 the retardation of heat transmission. This is especially notable in 

 many buds, in the stems of the larger plants, such as trees, and in 

 densely hairy leaves. The bark of the cork oak is familiar to every- 



