130 ENVIRONMENTAL CONDITIONS. 



one as a substance with a low power of heat conduction, and the hairy- 

 buds and younger leaves of such forms as mullein (Verbascum thapsus) 

 are known to assume the temperature of their surroundings, after a 

 rapid temperature change in the latter, with a distinctly sensible lag. 

 One of the main internal conditions affecting the rate of outward and 

 inward heat conduction is that of moisture-content, which, as we have 

 seen, is also of great importance in the determination of the rates of 

 intake and outgo of water. 



The rate of absorption or elimination of radiant heat is affected 

 also by the nature of the plant periphery; rough surfaces radiate and 

 absorb a greater proportion of energy rays than do smooth ones; 

 plaited and folded surfaces radiate and absorb much less than plain 

 ones of equal area; the color of the tissue is highly important in this 

 connection, and the exposure of the surfaces considered, with reference 

 to the earth's surface and to the sky, is also of primary importance. 

 Most of the heat radiation and absorption by plants occurs in the 

 direction toward and from the sky, and by far the greater portion of 

 the radiant energy absorbed is from direct sunshine. This latter 

 feature has been emphasized in our discussion of transpiration and will 

 be touched upon again in connection with the treatment of light. 

 Of course, it is to be remembered that the effect of heat radiation in 

 producing a body-temperature higher or lower than that of the sur- 

 roundings is soon limited by the increased rates of both radiation and 

 conduction in the opposite direction, so that very great differences 

 between outside and inside temperature are not to be expected. The 

 heating effect of direct sunshine upon green, transpiring leaves is 

 limited, not only by outward conduction and radiation, but also by 

 the cooling effect of transpiration. Even in intense sunshine the 

 temperature of turgid, rapidly transpiring leaves is frequently or 

 usually below that of the surrounding air.^ 



The main generalization in connection with the temperature rela- 

 tion is simply that the temperature of the plant is never very different 

 from that of its immediate surroundings. The important effect of 

 the different rates of heat exchange between plant and environment 

 is practically confined to the determination of the temperatures of 

 leaves and other similarly exposed parts when under the direct rays 

 of the sun, and to the production of a more or less pronounced lag 

 in the tissue temperature changes brought about by great and rapid 

 alteration in the environmental temperature. 



iShreve, 1914. 



