INTRODUCTOKY REMARKS. 3 



lately recognised. We shall at present, therefore, only allude 

 to those characters by which plants may in a general sense be 

 distinguished from animals, leaving the more extended investi- 

 gation of the subject to the pages of this volume. 



In the first place, we find that plants hold an intermediate 

 station between minerals and animals, and derive their nourish- 

 ment from the earth and the air by which they are surrounded, 

 and that they alone have the power of converting this inorganic 

 or mineral matter into organic. Animals, on the contrary, live 

 on organic matter, and reconvert it into inorganic. In other 

 words, plants produce organic matter, and animals consume it. 



Secondly, plants are generally fixed to the soil, or to the sub- 

 stance upon which they grow, and derive their food immediately 

 by absorption through their external surface ; while animals, 

 being possessed of sensation and power of voluntary motion, can 

 wander about in search of the food which has been prepared for 

 them by plants and other animals, and which they receive into 

 an internal cavity or stomach. Plants are, therefore, to be re- 

 garded as destitute of sensation and power of voluntary motion, 

 and as being nourished from without ; while animals are pos- 

 sessed of these attributes, and are nourished from within. 



Thirdly, 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 characters 

 there may be found some exceptions when we compare particular 

 individuals. 



It was formerly believed that an absolute distinctive character 

 existed between plants and animals, in the cell-walls of the 

 former being composed essentially of cellulose, and those of the 

 latter of gelatine. The researches of Schmidt, Lowig, KoUiker, 

 Schacht, Virchow, Huxley, and others, of late years, have, how- 

 ever, shown that cellulose also exists as a constituent of several 



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