396 THE MICROSCOPE. 



elements. The oxygen derived from the atmosphere, by whatever 

 means it is introduced into the animal system, is expended in the pro- 

 duction of carbonic acid and water, both of which are thrown off as 

 excretions. It is true that water is exhaled in great quantities from 

 the surfaces of plants ; but it is that fluid which has been taken into 

 the system of the plant, and has not undergone decomposition ; it is, 

 therefore, not actually found in the body of the vegetable, as it is in 

 that of the animal. During the process of vegetation, protein is formed 

 from the constituents of water with carbonic acid and ammonia ; pro- 

 tein is formed in the animal body, and enters largely into the blood 

 and muscle. 



There is the closest affinity in the chemical nature of the products 

 between plants and animals. Vegetable albumen is identical in com- 

 position with that in blood and in eggs; casein does not materially 

 differ in milk and the juices of some plants: we have many other 

 equally striking characteristics, which modern chemical investigations 

 have unfolded. Plants in some characteristics differ most strikingly, 

 in being almost destitute of voluntary sensation and motion : here we 

 would not have sensibility confounded with irritability, a principle 

 which plants, in common with animals, possess. The simplest forms 

 of animal life manifest both sensation and volition, even those that 

 are fixed to rocks and other bodies presenting a ramified and vegeta- 

 tive form ; for instance, in the compound polyps, each individual 

 polyp displays both sensation and voluntary motion. It is, neverthe- 

 less, difficult to attribute satisfactorily the movement of some plants to 

 irritability alone. Thus we find plants, in an apartment with light 

 admitted on one side, not only turn the upper surface of their leaves 

 to the light, but bend their sterns and branches towards it. Many 

 other instances might be cited; but none of them, excepting the move- 

 ments of the OsciUatoria, more closely resemble volition. Plants, 

 again, differ from animals in having no nervous system. All animals, 

 without distinction, have a nervous system. Ehrenberg traced and 

 described "vessels and nerves in the Rotatoria and some Infusoria" 



Another great distinction is connected with the function of diges- 

 tion, which the simplest form of animals possess : those even which 

 turn inside out, the hydra, &c. have an internal cavity, into which 

 their food is taken at intervals ; but vegetables are nourished from the 

 surface, and by continual imbibition. 



It has been supposed, because the sap rises in plants, and in the 

 interior of some internodia and cells of some simple plants a rotatory 

 motion of fluid can be perceived, that plants, like animals, have a cir- 



