DESCRIPTION AND PLANS OF A KANGE OF NEW PINE-STOVES. 187 



previously produced. In the thermometer the mercury falls and 

 rises; the plant, on the contrary, only advances. The mean 

 variations of the thermometer, to which vegetable phenomena are 

 always referred, have nothing corresponding to them in vegetable 

 life ; for the young plants do not withdraw into the seed, nor the 

 leaves into the bud, if cold succeeds heat. To keep within reality, 

 the plant must be compared to a machine which does its work in 

 proportion to the impulse given by heat and by the chemical 

 action of light. If the impulsive force is insufficient to set the 

 machine in motion, all is stopped, but the produceof its previous 

 work is so much gained, and when the impulse recommences, a 

 new produce is added to the former. Thence the necessity I 

 have alluded to of registering only temperatures above 0°, for 

 we are certain that below that point the vegetable machine is 

 stopped. Thence also the need of ascertaining whether certain 

 plants do not cease their functions at + 1°, + 2°, &c, as the 

 limits of plants towards the north,'* and the daily observation of 

 various facts, seem to indicate. In following up this idea, the 

 action of several agents, such as humidity, of which the effect is 

 immense on plants, may be compared to the numerous causes 

 which influence the working of a machine. Let us take the 

 steam-engine for instance. It is without doubt set in motion by 

 caloric, but also it is necessary that there be no deficiency of water, 

 that all its parts be in a good state, that friction be diminished 

 by oil, &c. The definitive amount of work is in proportion to 

 all these causes. Organised beings are not less complicated. 

 Calculations which are applied to them are always approximative, 

 like those of the powers of a machine ; and the same process must 

 be followed in making them, that is to say, we must take into 

 account the impulsive forces, time and all accessory circumstances, 

 not to fall into serious errors, or to come to numerical products 

 without any useful results. 



XXIV .—Description and Plans of a Range of New Pine- Stoves, 

 lately put up in the Gardens at Bicton. By James Barnes, 

 Gardener to Lady Rolle, Bicton, Sidmouth, Devonshire. 



(Communicated June 6, 1850.) 



The following is some account of a new Pine structure which 

 was erected here about a twelvemonth ago, and, having had 

 both a winter's and summer's experience of it, I am now able to 



* See my memoir on the Polar Limits of Species in the Bibliotheque Uni- 

 versale, 1848 ; Archives des Sciences, vol. vii., p. 5 ; or in the Annales des 

 Sciences Naturelles de Paris, 3rd series, vol. ix. 



