203 NATURAL HISTORY. 



called sap, rises into the stem ; from thence it is carried 

 into the leaves, which are the special organs of exhala- 

 tion and respiration, and it is through their agency that 

 the superfluous oxygen is thrown off. The roots, there- 

 fore, performing the digestive process of extracting nutri- 

 tion, have been considered as the stomach of the plant, the 

 stem its body, and the leaves its lungs. 



All plants, however, are not rooted in the earth ; there 

 are many which float on the water, sending their fila- 

 mentous roots below the surface ; some, the lichens, for 

 instance, spreading themselves over the naked rock, seem 

 to derive no aid from the earth, and others, like the mis- 

 tletoe, and the family of parasites, springing from other 

 trees, obtain from them the living principle. 



The multiplication of plants takes place, most com- 

 monly, through the seeds which are formed in the flower ; 

 sometimes by means of special organs destined to produce 

 the germ of a new individual ; and sometimes by the 

 simple division of their tissue, or driving their roots 

 through the earth, shoot up at a distance from the 

 mother plant, and maintain an independent growth. 

 The common strawberry, as is well known, sends out 

 its runners, and forces its way over every obstacle, until 

 a favorable spot is found for putting forth a bud ; this 

 being effected, the thread-like roots fix themselves firmly 

 in the ground, and a new progeny is formed. Every 

 part of a plant, at least from those of the most perfect 

 development, is capable of sending forth new roots; 

 branches turned downward, and covered with earth, take 

 root, and reproduce a new growth of the same species ; 

 the same is also true of roots, which have the faculty to 

 give rise to leaves and stems. 



Plants are distinguished as simple or compound. In 



