856 DEVELOPMENT OF HEAT BY PLANTS. 
several functions, we might conclude that their temperature 
would rarely or ever, under natural circumstances, correspond 
with that of the atmosphere around them. 
We have already noticed, that during the germination of seeds 
a considerable development of heat takes place (page 851). This 
is more especially evident when a number of seeds germinate 
together, as in the process of malting. The development of heat 
in flowering has also been alluded to (page 827). The rise of 
temperature which thus occurs in the processes 
of germination and flowering is due, without 
doubt, essentially to the production of carbon 
dioxide. To estimate the degree of heat deve- 
loped during germination and in flowering, 
Sachs devised the apparatus (jig. 1172), where 
the flask f contains a strong solution of potash 
or soda, 1, which absorbs the carbon dioxide set 
free. Inthe neck of the flask is placed a funnel, 
v, with a small filter perforated by a needle. 
Into this are placed soaked seeds or freshly cut 
flower-buds in the act of opening, and a bell- 
glass, g, used to cover the whole. Through 
the opening of the bell-glass, which is plugged 
with cotton wool, w, is passed a thermometer, 
t, whose bulb is so placed as to be surrounded 
Fig.1172. Apparatus 00 all sides by the seeds or flower-buds. 
for estimating the We have still to inquire, whether the ordi- 
seu ate ceerat! nary vital actions which are going on in plants 
nation of seedsand are calculated to raise or diminish their tem- 
the eabening of perature. The experiments of Hunter, Schoepf, 
Sache. ape “ Bierkander, Maurice, Pictet, and more especi- 
ally of Schiibeler, lead to the conclusion that 
the trees of our climate with thick trunks exhibit a variable 
internal temperature, being higher in the winter and at sunrise 
than the surrounding atmosphere—that is, at periods of great 
cold, or of moderate temperature ; and lower in the summer or 
at mid-day—that is, at periods of great heat. In no observed 
cases were such trees noticed to possess exactly the temperature 
of the atmosphere around them. The experiments of Réaumur 
on trees with slender trunks exposed directly to the sun’s rays 
showed a considerable increase of temperature in them over the 
external air. These experiments of Réaumur are, however, by 
no means satisfactory. 
The temperature of trees under the above conditions depends 
upon various causes, such as the sun’s rays, the amount of eva- 
poration, chemical changes which take place during assimilation, 
&c., the conducting powers of the wood, and particularly upon 
the temperature of the soil in which the plants are grown. In 
the active periods of the growth of plants, when evaporation is 
constantly going on, and the fixation of carbon taking place, 
