MOTION OF THE JUICES. . 331 



vessels (ducts) of the wood, to the leaves, there is concen- 

 trated by evaporation, "elaborated" by the processes 

 that go on in the foliage, and thence descends through the 

 vessels of the inner bark, nourishing these tissues in its 

 way down. The facts from which this theory of the sap 

 first arose, all admit of a very different interpretation : 

 while numerous considerations demonstrate the essential 

 falsity of the theory itself. 



Flow of sap in the plant not constant or necessary. 

 We speak of the Flow of Sap as if a rapid current 

 were incessantly streaming through the plant, as the blood 

 circulates in the arteries and veins of an animal. This is 

 an erroneous conception. 



A maple in early March, without foliage, with its whole 

 stem enveloped in a nearly impervious bark, its buds 

 wrapped up in horny scales, and its roots surrounded by 

 cold or frozen soil, cannot be supposed to have its sap in 

 motion. Its juices must be nearly or absolutely at rest, 

 and when sap runs copiously from an orifice made in the 

 trunk, it is simply because the tissues are charged with 

 water under pressure, which escapes at any outlet that 

 may be opened for it. The sap is at rest until motion is 

 caused by a perforation of the bark and new wood. So, 

 too, when a plant in early leaf is situated in an atmosphere 

 charged with moisture, as happens on a rainy day, there is 

 little motion of its sap, although, if wounded, motion will 

 be established, and water will stream more or less from all 

 parts of tlje plant towards the cut. 



Sap does move in the plant when evaporation of water 

 goes on from the surface of the foliage. This always hap- 

 pens whenever the air is not saturated with vapor. When 

 a wet cloth hung out, dries rapidly by giving up its 

 moisture to the air, then the leaves of plants lose their 

 water more or less readily, according to the nature of 

 the foliage. 



Mr. Lawes found that in the moist climate of England 



