AND THE STRUCTURE OF PLANTS^. 



87 



Plants are also possessed of cutaneous secernents or perspiratory ves« 

 sels ; and in many plants the quantity of fluid thrown off by this emunctory 

 is very considerable. Keil, by a very accurate set of experiments, ascer- 

 tained that in his own person he perspired 31 ounces in twenty-four hours. 

 Hales, by experiments equally accurate, determined that a sun-flower, of 

 the weight of three pounds only, throws off" 22 ounces in the same period 

 of time, or nearly half its own weight. To support this enormous expen- 

 diture it is necessary that plants should be supplied with a much larger 

 proportion of nutriment than animals ; and such is actually the fact. Kiel 

 ate and drank 4 lb. 10 oz. in the twenty-four hours. Seventeen times 

 more nourishment was taken in from the roots of the sun-flower than was 

 taken in by the man. 



Plants, nevertheless, do not appear to have the smallest basis for sensa° 

 tion, admitting that sensation is the result of a nervous system ; and we 

 are not acquainted with any other source from which it can proceed ; not- 

 withstanding that Percival and Darwin, as already observed, have not only 

 endowed them with sensation, but with consciousness aleo : and the latter, 

 indeed, with a brain, and the various passions and some of the senses to 

 which this organ gives birth.* 



Yet though the vessels of plants do not appear to possess any muscular 

 fibres, we have evident proofs of the existence of a contractile and irritable 

 power from some other principle ; and a variety of facts concur in making 

 it highly probable that it is by the exercise of such a principle that the di? 

 ferent fluids are propelled through their respective vessels : nor is there 

 any other method by which such propulsion can be reasonably accounted 

 for. Grew ascribed the ascent of the sap to its levity, as though acting 

 with the force of a vapour : Malpighi, to an alternate contraction and dila- 

 tation of the air contained in what he erroneously conceived to be air-ves- 

 sels : Perrault, to fermentation : Hales and Tournefort, to capillary attrac- 

 tion : not one of which theories, however, will better explain the fact than 

 another, as Dr. Thomson has ably estabhshed ; as he has also the proba- 

 bility of a contractile power in the different sets of vessels distributed so 

 wonderfully over the vegetable frame.f 



That a contractile power may exist independently of muscular fibres, we 

 have abundant proofs even in the animal system itself. We see it in the 

 human cutis or skin, which, though totally destitute of such fibres, is almost 

 for ever contracting or relaxing upon the application of a variety of other 

 powers ; powers external and internal, and totally different in their mode 

 of operation. Thus, austere preparations and severe degrees of cold con= 

 tract it very sensibly : heat, on the contrary, and oleaginous preparations, 

 as sensibly relax it. The passions of the mind exercise a still more pow^- 

 erful effect over it : for while it becomes corrugated by fear and horror, it 

 is smoothed and lubricated by pleasure, and violently agitated and con- 

 vulsed by rage or anger. 



Yet, could it even be proved that the vessels of plants are incapable of 

 being made to contract by any power whatever, still should we have no 

 great difficulty in conceiving a circulatory system in animals or vegetables 

 without any such cause, while we reflect that one half of the circulation of 

 the blood in man himself is accomplished without such a contrivance ; and 

 ^bis, too, the more difllicult half, since the veins, through the greater exten*^ 



* Wildenow, Princip. of Botany, ^ 226, 



* 3yst. of Chem. vol. v. p. 388. 1^07. 



