TEMPERATURE AND ITS CONDITIONS — 329 
processes of conduction is particularly noticeable in the 
case of ayuatic plants. Indeed, any alteration of the tem- 
perature of either the plant or the surrounding water 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, however, 
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 expen- 
diture 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 eva- 
poration 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 
