TEMPEEATUEE AND ITS CONDITIONS 327 



Terrestrial plants show less direct evidence of the loss 

 of heat by conduction. Their roots however no doubt 

 give up a certain amount to the soil at different times, just 

 as at others they absorb heat from the latter. 



When we compare approximately the amount of heat 

 absorbed by a green plant with that which is given off by 

 it, we find that in all cases there is a certain excess of the 

 former. Most plants thus show a certain gain of heat 

 from their environment. This does not however usually 

 manifest itself by a rise of temperature in the tissues. 

 There is no uniformity in the absorption either ; at times 

 when there is the greatest balance in favour of absorption 

 throughout the whole plant, parts of it may be giving off 

 considerable quantities and may be cooler than the average 

 temperature of the whole plant. 



The gain of heat which is secured in this way is to be 

 largely regarded, as we have already seen, as supplying 

 energy to the plant. This is devoted at first to constructive 

 processes, and thus much of it is rendered potential, being 

 afterwards reconverted into the kinetic form and made to 

 reappear, when it once more largely takes the shape of heat, 

 and is subsequently devoted to purposes of growth, meta- 

 bolism, repair of cell-substance, &c., as we have already 

 seen. But we may now lay a certain stress on the fact that 

 at any rate a part of this liberated heat is devoted to a 

 raising of the temperature of the cells which are the seat 

 of its liberation. 



We have thus an elementary though very incomplete 

 mechanism for the regulation of the temperature of the 

 plant. An excess of heat is absorbed : part is at once 

 applied to purposes of growth, metabolism, &c. ; part is 

 retained, and the store is as it were economised, being 

 liberated later with some reference to the temperature of 

 the parts concerned in the vital processes. 



This regulation of heat however is very rudimentary 

 and imperfect. We do not find that an increased loss of 

 heat stimulates metabolism in such a way as to set up 



