22 THE ELEMENTARY STRUCTURE OF PLANTS. 



cases occur on the lower confines of the two kingdoms, which 

 exliibit forms of the greatest possible simplicity of structure, and 

 of a minuteness of size that baffles observation. Even here the 

 uncertainty may be attributable rather to the imperfection of 

 our knowledge, than to any confusion of the essential character- 

 istics of the two kinds of beings. If we cannot absolutely define 

 them, or, at least, cannot always apply the definition to the actual 

 and certain discrimination of the lowest plants from the lowest 

 animals, Ave may indicate the special functions and characters of 

 each. The essential characteristics of vegetables doubtless depend 

 upon the position which the vegetable kingdom occupies between 

 tfie mineral and the animal, and upon the general ofUce it fulfils. 

 Plants, as stated at the outset (1), are those organized beings that 

 live directly upon the mineral kingdom, — upon the surrounding 

 earth and air. Tliey alone convert inorganic, or mineral, into 

 organic matter ; while animals originate none, but draw their Avhole 

 sustenance from the organized matter which plants have thus elab- 

 orated. Plants, having thus the most intimate relations with the 

 mineral world, are generally fixed to the earth, or other substance 

 upon which they grow, and the mineral matter on which they feed 

 is taken directly into their system by absor^ition from without, and 

 is assimilated under the influence of light in organs exposed to the 

 air ; while animals, endoAved A\dth volition and capable, of respond- 

 ing promptly to external impressions, have the poAver of selecting 

 the food ready prepared for their nourishment, Avhich is received 

 into an internal reservoir or stomach. The permanent fabric of 

 plants is composed of only three elements. Carbon, Hydrogen, and 

 Oxygen. The tissue of animals contains an additional element, 

 viz. Nitrogen. Plants, as a necessary result of assimilating their 

 inorganic food, decom})ose carbonic acid and restore its oxygen to 

 the atmosphere. Animals in respiration continually recompose car- 

 bonic acid, at the expense of the oxygen of the atmosphere and the 

 carbon of plants. These peculiarities will be explained and illus- 

 trated in the progress of this work. 



Sect. II. Op the Cells and Cellular Tissue of Plants. 



17. The question recurs. What is the organized fabric or tissue 

 of plants, and how is vegetable groAVth effected ? The stem, leaves, 



