TEMPERATURE AND ITS CONDITIONS 323 
is readily transmitted from the one to the other. Similar 
transmission of heat from the soil to the roots can take 
place and no doubt has a considerable effect in promoting 
the well-being of the latter, which as we have seen con- 
tinually lose heat by the evaporation of water from their 
cells into their intercellular spaces. Here, however, as in 
the last case, the conduction of heat varies in direction 
according to the relative temperatures of soil and root. 
The absorption of heat from the air in contact with the 
general surface must play a part very frequently in the 
heat interchanges. As in the other cases mentioned, how- 
ever, the direction of this interchange is not constant. 
While we can thus recognise these ultimate sources of 
heat supply, we find, no less evident, certain ways in which 
heat is given off by the plant in greater or less amount. 
Of these losses the first and most important is the ex- 
penditure which is necessary in order to evaporate the water 
of transpiration. There can be no doubt that the amount 
of transpiration is very largely determined by the amount 
of the sun’s rays which the plant receives. Not only are 
its stomata open widely in bright light, so that the vapour 
can readily diffuse into the air, but the actual evaporation 
from the cells into the intercellular passages is enormously 
accelerated during the absorption of the radiant energy. 
The amount of the latter which is taken up by a leaf has 
been computed to be nearly fifty times the amount which 
can be utilised in the process of photosynthesis; if the 
heat were allowed to accumulate in the leaf unchecked, 
it has been calculated that its temperature would rise during 
bright sunshine at the rate of more than 12° C. per minute, 
with of course very rapidly fatal results. What is not used 
for photosynthesis is employed in the evaporation of the 
water of transpiration, the leaf being thus kept cool. _ It 
is noteworthy that whether the leaf is brightly or only 
moderately illuminated the same relative proportions of the 
total energy absorbed are devoted to the purposes of trans- 
piration and photosynthesis. 
