INTRODUCTORY REMARKS. 3 



Thirdlj, in respiration, or more properly assimilation, plants 

 decompose carbonic acid, fix the carbon which is the result of 

 that decomposition in their tissues, and restore the oxygen to 

 the atmosphere. The respiration of animals, on the contrary, 

 consists in the expiration of carbonic acid, which is formed by 

 the combination of the carbon which the animal system wants 

 to throw off with the oxygen absorbed from the atmosphere. 

 Plants therefore, in respiration, absorb carbonic acid and elimi- 

 nate oxygen; while animals absorb oxygen and eliminate 

 carbonic acid. 



Fourthly, there is a difference in the ultimate elements of the 

 permanent tissues of plants and animals ; for while those of the 

 former consist only of three elements, namely, carbon, oxygen, 

 and hydrogen ; those of the latter are composed of four, namely, 

 carbon, oxygen, hydrogen, and nitrogen. 



In reference to the above distinctive characters, it must be 

 particularly remarked that they are only general, namely, those 

 derived from comparing together, as a whole, the members of the 

 animal and vegetable kingdoms. To all of the above charac- 

 ters there may be found some exceptions, when we compare 

 particular individuals. For their elucidation, however, we must 

 refer to the general contents of this volume. 



It was formerly beheved that an absolute distinctive character 

 existed between plants and animals, in the cells of the former 

 consisting essentially of cellulose, while those of the latter were 

 formed of gelatine. The recent researches of Schmidt, Lowig, 

 Kolliker, Schacht, Virchow, Huxley, and others, have, however, 

 shown that cellulose also exists as a constituent of several ani- 

 mals. The presence of starch was also formerly considered as a 

 certain characteristic of a plant, but recent investigations have 

 also shown that that substance, or at least one isomeric -ndth it, is 

 also to be found in some animals. Neither the presence of 

 cellulose or starch can be now considered, therefore, as present- 

 ing any absolute marks of distinction between plants and animals. 

 We arrive accordingly at the conclusion, that in the present state 

 of our knowledge, it is impossible to give a complete and perfect 

 definition of a plant, in contradistinction to what is to be regarded 

 as an animal 



B 2 



