176 THE MICROSCOPE. 



tinguish them from animals ; but this opinion is erroneous, 

 as we find that the formation of gems or buds is common 

 in animals belonging to the class Protozoa. In the hydra 

 we perceive the germs developed as small ovoid eleva- 

 tions upon the cylindrical body of the animal, and are, 

 when examined in this state, like the first formation of 

 the buds in plants, mere masses of cells ; but as their 

 growth proceeds, these cells undergo a special arrange- 

 ment, so as to produce the different tissues of the body, 

 and acquire the proper form of the polype: on the same 

 principle, the bud in the plant is gradually developed, until 

 it terminates and becomes a branch. 



Plants, like animals, possess excitability, or the faculty 

 of being acted upon by external stimuli, impelling them 

 to the exertion of their vegetable powers. Light acts on 

 plants, directing the growth of the stem, vigour, and 

 colour, the direction of the branches, position of leaves, 

 the opening and shutting of flowers. Heat influences the 

 protrusion of buds, and other stimulants affect vegetable 

 irritability; as an instance of which, cut plants, when 

 fading, revive if placed in water impregnated with certain 

 chemicals. 



Besides the physical and physiological distinctions gene- 

 rally pointed out as marking the line between animals and 

 plants, chemistry furnishes many others. Thus, one of the 

 great functions of a plant is to decompose water, and assi- 

 milate its components to the vegetable tissues ; and it is 

 equally a property of animal life constantly to reform itself 

 from the same elements. The oxygen derived from the 

 atmosphere, by whatever means it is introduced into the 

 animal system, is expended in the production 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 ammo- 

 nia; protein is formed in the animal body, and enters 

 largely into the blood and muscle. 



