LECT. II.] VITAL FUNCTIONS OF PLANTS. 55 



sits in its body those secretions which render it 

 useful to mankind for economical and medicinal 

 purposes. The simple absorption of fluids by the 

 roots of plants might, perhaps, be explained on 

 mechanical principles ; but the circulation, or, 

 rather, progressive motion, of the sap, can only 

 be accounted for on the supposition that plants 

 are living beings. Various theories have been of- 

 fered to account for the ascent of the sap in plants; 

 but this is not the moment for criticising these dif- 

 ferent opinions ; it is only necessary to observe, 

 that the sap continues to rise during the life of the 

 plant, and ceases when it dies ; and that it ob- 

 viously does not depend on capillary attraction, or 

 any mechanical impetus independent of vitality. 

 If a plant in a flower-pot be kept for some time 

 perfectly dry, till vitality ceases, no supply of 

 water can again be taken up by its vessels, how- 

 ever favourable the temperature may be; for, in 

 this case, the plant having been deprived of one 

 essential agent, its vessels no longer act, and it 

 dies. Did the ascent of the sap, however, depend 

 on any other circumstance than the living action 

 of the vessels (which, I trust, we shall afterwards 

 be able to prove is not the case), it would nly be 

 necessary, in order to restore any decayed plant, 

 to supply it with moisture in a proper tempera- 

 ture. Dead plants imbibe fluids, it is true, but 

 they are not nourished by them; and the moisture 



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