326 
PHYSIOLOGY. 
BOOK II. 
cuticle thicker and harder, perspire much less than other 
plants. 
That a motion of fluids really exists in plants is, therefore, 
undoubted. It is most rapid in the spring and early summer, 
and most languid in winter; but never actually suspended, 
unless under the influence of frost. This has been demonstrated 
by Biot, who, by means of an apparatus described in the 
Institut Newspaper, succeeded in measuring the power of 
motion in the sap of roots, in witnessing the phenomena 
which regulated it, and in determining the causes that brought 
about the phenomena. 
" Atmospherical circumstances," he says, " and especially 
the absence or presence of solar light, exercise a marked in- 
fluence upon these phenomena ; but it is exceedingly difficult to 
ascertain their exact nature. Nevertheless, among them is 
one, the effects of which are so constant and undoubted, that 
they appear susceptible of being defined. This consists in the 
sudden appearance of frost immediately succeeding mild wea- 
ther, and lasting for some time. Mild weather either favours 
or brings about the ascent of the sap ; but if a sudden frost 
supervenes, it seizes upon the part of the trunk swollen with 
fluid, and forces the latter to fall back again : should the frost con- 
tinue and increase in sev^erity, the earth at the foot of the tree 
freezes ; and whether at that time the roots are mechanically 
compressed by it, or whether the duration of the cold causes 
contraction by a vital action, the roots commence causing a 
considerable discharge of fluid from the lower part of the ap- 
paratus. This goes on night and day, except when the pipes to 
carry off the sap are frozen. As soon as a thaw comes on and 
the earth is relaxed, the roots, emptied of their juices, find 
themselves below their point of saturation ; they then emit 
nothing, but on the contrary absorb the descending juices. I 
satisfied myself of this not only by my apparatus, but in 
sawing through the trunk of a large poplar tree, a 
yard from the ground. The surface of the section of the 
stump was dry, but that of the trunk itself dripped with 
water." 
The motions of the sap appears to be of two kinds: 1. ge- 
neral^ and 2. special ; these must be carefully distinguished. 
