LII'K OF I'LAXTS. 



m 



to solid parts, by which means heat must always be disen- 

 gaged ; but this would be to avail ourselves of a process of 

 nourishment, which is speedily accomplished, and of which 

 we can scarcely take advantage in winter. We must there- 

 fore recollect, that the roots of trees stand in a layer of earth, 

 which is not easily freezed, especially when it is covered with 

 snow ; that the sleeping life of plants in winter indures the 

 removal of stimuli as easily as that of animals, who, during 

 their winter sleep, are surrounded by very cold air or by ice. 

 We must recollect, that when the thermometer, held in 9, 

 hole bored in the stem of a tree, rises during winter, this is a 

 natural consequence of the slow transmission of heat through 

 the rind filled with resinous sap, and that, besides, snow around 

 living trees does not dissolve sooner than around dry stakes. 



The production of an internal heat in plants is therefore at 

 least very doubtful ; and we must as yet limit it to the rare 

 cases in which a considerable heat is generated, namely, in 

 the spadix of the Arum species, and, according to one report, 

 also in the Pandanus when it flowers ; although this pheno- 

 menon proceeds less from any preponderance of vital activity, 

 than from a process which is truly chemical, and which re- 

 lates to the evolution of elementary bodies in the blossoms; 

 (Sennebier, Phys. Veget. ; Hall in Bradley''s and Adams' 

 Med. and Phys. Jour. ; Bory St Vincent in Ann. Gener. des 

 Sciences Phys.) 



378. 



We come now to. another vital phenomenon in plants, 

 namely, visible motions. As, however, there are a multi- 

 tude of brisk and often of wonderful movements, which may 

 be explained by mechanical or physical arrangements, we are 

 led to inquire to what class we must refer these peculiar vi- 

 tal motions. Now, when we attend, in the first place, to the 

 absence of mechanical arrangements, — ^next to the fixed order 

 which occurs in these movements, — and, lastly, to the excit- 

 ing cause, which operates by irritation,— we shall find it dif- 

 ficult to consider this kind of mption as of any other th^n a 

 dynamical nature. 



