308 Late "Difcoveries in Science. 



diftinguifh the arterial from the venal blood, is not the real 

 caufe which gives play to the irritability of the heart, but 

 that its action is particularly owing to the effecl of beat com- 

 bined with humidity. 



Van Marum, Hildebrant, and other philofophers think 

 that there is a real irritability in plants, and particularly in 

 thofe which have fpontaneous kinds of movement. It has 

 thence been afferted, that oxygen has the fame effect upon 

 thofe plants called fenfitive, fuch as the humble plant (««'- 

 mofa pudica), the hedyfarum gyrans, &c. as upon animals. 



Pefchier has made feveral experiments to afcertain whe- 

 ther this opinion was well founded ; but oxygen never ap- 

 peared to him to produce any effeel: upon thefe plants. He 

 afterwards examined whether thefe movements of thofe 

 plants called fenfitive are owing to real irritability-, like that 

 of animals. The numerous experiments which he made 

 on this fubjett gave him reafon to conclude, that no real 

 irritability could be afcribed to thefe plants^ and that all the 

 movements they experience, by the Ample touch or other- 

 wife, are merely mechanical ; and indeed plants have nei- 

 ther nerves nor mufcles, nor any organs analogous to thofe 

 which, in animals, appear to be the feat of irritability. 



This difference in the organization of animals and vege- 

 tables cannot be denied ; but on the other hand it is equally 

 certain, that what is called irritability in animals is owing to 

 mechanical caufes. The movement of the mufcular fibre in 

 animals is altogether as mechanical as the fhrinking of a 

 iprig of the fenfitive plant when it is touched. That of the 

 mufcular fibre is owing to an efflux of the nervous fluid ; 

 that of the fenfitive to the efflux of another fluid, which is as 

 little known. But it may here perhaps be faid, The animal 

 fibre, fuch as the heart, is flill irritable a long time after it 

 has been feparated from the reft of the body, which is not 

 the cafe with the vegetable fibre. This is true : but it does 

 not prove that it is not always a mechanical caufe which 

 a&s in thefe circumftances y its action only is of longer 



duration j 



