THE ACTIONS OP FORCES ON ORGANIC MATTER. 29 



or a raised state of molecular vibration, enables incident forces 

 more easily to produce changes of molecular arrangement in 

 organic matter. But besides this, it conduces to certain vital 

 changes in so direct a way as to become their chief cause. 



The power of the organic colloids to imbibe water, and to 

 bring along with it into their substance the materials which 

 work transformations, would not be continuously operative 

 if the water imbibed were to remain. It is because it 

 escapes, and is replaced by more water containing more 

 materials, that the succession of changes is maintained. 

 Among the higher animals and higher plants its escape is 

 facilitated by evaporation. And the rate of evaporation is, 

 other things equal, determined by heat. > Though the 



current of sap in a tree is partly dependent on some action, 

 probably osmotic, that goes on in the roots; yet the loss of 

 water from the surfaces of the leaves, and the consequent 

 absorption of more sap into the leaves by capillary attraction, 

 must be a chief cause of the circulation. The drooping of a 

 plant when exposed to the sunshine while the earth round 

 its roots is dry, shows us how evaporation empties the sap- 

 vessels; and the quickness with which a withered slip re- 

 vives on being placed in water, shows us the part which 

 capillary action plays. In so far, then, as the evaporation 

 from a plant's surface helps to produce currents of sap 

 through the plant, we must regard the heat which produces 

 this evaporation as a part-cause of those re-distributions of 

 matter which these currents effect. In terrestrial 



animals, heat, by its indirect action as well as by its direct 

 action, similarly aids the changes that are going on. The 

 exhalation of vapour from the lungs and the surface of the 

 skin, forming the chief escape of the water that is swallowed, 

 conduces to the maintenance of those currents through the 

 tissues without which the functions would cease. For 

 though the vascular system distributes nutritive liquids in 

 ramified channels through the body; yet the absorption of 

 these liquids into tissues, partly depends on the escape of 



