GENERAL 367 



Although metabolism may involve exothermal as well as endothermal 

 chemical changes, these appear to balance approximately, the heat produced 

 being derived almost solely from respiration. The swelling of dry seeds 

 does, however, produce a distinct temporary rise of temperature, and the 

 rapid commencement of respiration in the moistened seeds produces a 

 secondary rise *, 



Even when the living cells respire actively, organs containing a large 

 amount of dead tissue can never be much warmed. In the cell itself only 

 the protoplasm is active, and the production of heat in it must often be 

 great enough to produce an injurious or even fatal rise of temperature, were 

 it not for the rapid removal of heat by the surrounding water 2 . Similarly 

 the temperature of the most actively respiring Bacteria cannot be appreciably 

 higher than that of a fluid medium in which it is growing. 



The curves of respiration and heat-production are approximately parallel 

 when plants are exposed to varying conditions in saturated air. For instance, 

 after an injury respiration and heat-production attain a maximum at about 

 the same time. Changes of temperature affect respiration and heat-production 

 in corresponding degree, and as far as is known continued rises of tempera- 

 ture produce increasing differences between the temperature of the plant 

 and that of the surrounding medium until the fatal limit is reached. Other- 

 wise the temperature of the plant closely follows that of the surrounding 

 medium, and hence plants appear to have no power of regulating their 

 temperature like mammals by either increasing the production of heat or 

 diminishing the loss of it. The cooling effect of transpiration may prevent 

 the plant from being excessively heated by insolation, but this is a purely 

 accessory physical effect, and as far as any physiological regulation comes 

 into play this is concerned solely in preventing a fatal loss of water. Hence 

 the transpiration from an exposed leaf may be checked just when its cooling 

 effect is most needed, the plant sacrificing the exposed organ rather than 

 risk its whole existence. The protective movements of certain leaves do 

 actually involve a temporary increase of transpiration, but usually the 

 movement is such as to reduce not only the transpiration but also the 

 exposure to the radiant energy of the sun 3 . 



Poikilotherms have this advantage over homoiotherms, that their body 

 temperature may vary within wide limits without danger to life. On the 

 other hand, homoiotherms, if well nourished, may remain active at low 

 temperatures which suppress the activity of poikilotherms more or less 

 completely. The latter, however, avoid the waste of energy involved in 

 maintaining a high body-temperature. 



1 Wiesner, Versuchsstationen, 1872, Bd. xv, p. 138. 

 8 Engelmann, Bot. Ztg., 1888, p. 713. 



3 Cf. Ewart, The Effects of Tropical Insolation, Annals of Botany, Vol. n, 1897, pp. 450, 

 457, 459- 



