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 the 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 
