208 
Functions of Vegetables. 
have pinnated leaves. During the night, and some- 
times in dark cloudy weather, the leaves droop or fold 
over each other j this has been called the sleep of 
plants ; and it has been supposed that it may be a kind 
of necessary repose in some way useful to the vegetable 
constitution. A similar change takes place in other 
plants from mechanical impulse, as in the sensitive plants 
the leaves of which, b} r the slightest touch, close toge- 
ther, and exhibit the same appearance as those plants 
which are supposed to be under the influence of sleep. 
Heat of plants .- — During the chemical changes that 
take place in plants, it cannot be doubted that heat is 
evolved or abstracted ; and it is extremely probable 
that plants, as well as animals, have the power of regu- 
lating, although in a lower degree, the excesses of tem- 
perature to which they are exposed. The snow which 
falls on the leaves and stems of living plants melts 
sooner than on dead matter of the same kind, — an ob- 
vious proof that the temperature is higher ; but the 
heat of vegetables is so much superior to that of the 
atmosphere, as to be indicated by the thermometer. A 
remarkable fact is stated by Senebier with regard to 
the increased temperature of the white-veined variety of 
the arum maculatum in a certain period of its growth, 
when the flower was for a few hours very hot ; it was 
preceptible from three or four o’clock in the afternoon 
till 11 or 12 at night ; and when the temperature of the 
air was 14 or 15 degrees of Reaumur’s thermometer, 
the heat of the plant, when it was highest, was seven 
degrees above it. This curious fact, as is justly observ- 
ed by Dr Smith, is well worthy of attention, and may 
perhaps be observed in other plants. 
