64 



aud smell, preserve a great delicacy, when their sensibility has not been 

 impaired by injudicious qr too frequent impressions. The sensitive cen- 

 tre is in no one better developed, and fitter to direct safely the use of the 

 organs of motion. No other animal can articulate vocal sounds, so as to 

 acquire speech. 



This greaier extension of life in man, from the number and perfection 

 of his organs, makes him liable to many more diseases than the other 

 animals. It is with the human body, as with those machines which be- 

 come more liable to be deranged, by increasing the number of their wheels, 

 with a view of obtaining more extensive or more varied effects. 



All organized bodies are possessed of assimilating functions ; but as 

 assimilation requires means varying in number and power, according to 

 the nature of the being which performs it, the series of assimilating phe- 

 nomena commences in the plant by absorption, since it draws immediate- 

 ly from the earth, the juices which it is to appropriate to itself. Its ab- 

 sorbing system, at the same time, performs the functions of a circulatory 

 organ, or rather, the circulation does not exist in plants, and the direct 

 and progressive motion of the sap which ascends from the root towards 

 the branches, and sometimes in a retrograde course, from the branches, 

 towards the 'roots, cannot be compared to the circulation of the fluids 

 which takes place in. man, and in the animals which most resemble him, 

 by means of a system of vessels which every moment bring back the flu- 

 ids to the same spot, and convey them over the whole body, by making 

 them describe a complete circle, frequently, even, a double rotation (ani- 

 mals with a single or double circulation, that is, whose heart has one or 

 tv/o ventricles.) Plants breathe after their own manner, and produce a 

 change in the atmospherical air. by depriving it of its carbonic acid gas, 

 the result of combustion and of animal respiration, so that by a truly ad- 

 mirable reciprocity, plants, which decompose carbonic acid, and allow 

 oxygen to exhale, continually purify the air, which combustion and animal 

 respiration are incessantly contaminating*. 



The functions preservative of the species are common to animals and 

 plants. The organs by which these functions are performed, when com- 

 pared in these two kingdoms of nature, offer a resemblance which has 

 struck all naturalists, and has led them to observe, that of ail these acts 

 of vegetable life, no one is more analogous to the animal oeconomy, than 

 that by which fecundatio'n is effected. 



* This opinion originated with PHIESTLEY, and was generally adopted, in opposition 

 to the experience of his cotemporary, the celebrated SCHEELE. Subsequent physiolo- 

 gists, especially ELLIS, GILBT, and T. DE SAUSSURE, have shown, by well conducted ex- 

 periments, that all plants, whether growing- in absolute darkness, in the shade, or when 

 not exposed to the direct rays of the sun, " are constantly removing a quantity of oxygen 

 from the atmosphere, and substituting- an equal volume of carbonic acid." Thus far 

 these philosophers nearly agree. They differ, however, very widely respecting the 

 manner in which this change is effected. ELLIS supposes that the 'leaves, flowers, 

 fruits, stems, and roots of plants, emit carbonaceous matter, which combines with the 

 oxygen of the surrounding air. GILBT and SAUSSUHE are of opinion, that the oxygen is 

 absorbed by the respiratory organs, and that the carbonic acid is formed within the 

 plant. 



Although vegetables, under the ordinary circumstances of their growth, consume oxy- 

 gen during respiration, and disengage carbonic acid, yet, according as their situation and 

 particular condition may require, they partially absorb the carbonic acid from the air, 

 convert it to their use, decompose it, and emit the oxygen which results from the de- 

 composition, especially when they are exposed to the sun's rays. The illustration of 

 this subject belongs to vegetable physiology. Copland, 



