56 VITAL FUNCTIONS OF PLANTS. [LECT. II. 



only serves to hasten the process of decomposition. 

 When a plant has attained the natural term of its 

 life and dies, or when its death is occasioned by 

 lightning or any other accident, the absorbing 

 power of the roots and leaves is lost ; and the sap, 

 which is contained in the vessels, is evaporated and 

 dissipated ; which would not be the case did these 

 functions not depend on vitality. 



To the living powers of vegetables we must 

 also revert to account for the changes of the sap 

 into the solid components, and the peculiar juices 

 of the plant. No mechanical principles can pro- 

 , duce these effects ; they are opposed to the che- 

 mical affinities which exist between the materials 

 composing the substance of the plant : nor can 

 they result from any cause, except from that prin- 

 ciple which, whilst it is present, gives life and mo- 

 tion to every being that is endowed with it ; and, 

 on being withdrawn, leaves the substance to the 

 control of those laws that regulate the combina- 

 tions of dead, inert, unorganized matter. It was 

 the opinion of Linnaeus and of many others, that, 

 although the greatest variety of plants may grow 

 at one time in the same field, yet that each of 

 them selects and takes up from the soil those par- 

 ticles of it only which are adapted to the peculiar 

 nature of their secretions; or that the soil con- 

 tained ready-formed all that is found in plants. 

 But there is no need of this supposition to explain 

 the great variety of secretions of plants retired 



