VITAL CHARACTERISTICS OF PLANTS. 177 



There is the closest affinity in the chemical nature of 

 the products between plants and animals. Vegetable 

 albumen is identical in composition 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 strik- 

 ingly, in being almost destitute of voluntary sensation 

 and motion : here we would not have sensibility confounded 

 with instability, a principle which plants, in common with 

 animals, possess. The simplest forms of animal life mani- 

 fest both sensation and volition, even those that are fixed 

 to rocks and other bodies presenting a ramified and vege- 

 tative form ; for instance, in the compound polypes, each 

 individual polype displays both sensation and voluntary 

 motion. It is, nevertheless, difficult to attribute satisfac- 

 torily 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 stems and branches towards it. 

 Many other instances might be cited ; but none of them, 

 excepting the movements of the Oscillator ia, 1 more closely 

 resemble volition. Plants, again, differ from animals in 

 having no nervous system. 



Another great distinction is connected with the function 

 of digestion, which the simplest form of animals possess : 

 those even which turn inside out, the hydra, 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 the internodia and cells of some 

 simple plants, a rotatory motion of fluid can be perceived, 

 that plants, like animals, have a circulation of fluids 

 This opinion is at least disputable, the sap of plants as- 



(1) Oscillatoria, a genus of confcrvoid algae, the filaments of which are en- 

 closed in tubular cellulose sheets, open at the ends, from which the fragments 

 emerge when they are broken across. It is the remarkable spontaneous move- 

 ments of the Oscillator iucece, which make them objects of so much interest 

 for the microscopist. They are found on damp ground, amongst mosses, rocks, 

 stones, and in fresh and salt water. Another of the same family frequently 

 covers over the surface of standing water, to which it imparts a green colour; it 

 is called Aphanizomenon Flos-aquce, by Morr and Dr. Hassall. 



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