PHYSIOLOGY OF PLANTS. 
37 1 
admitting the accession of the air, restore them to verdure 
and fruitfulness. 
Having considered the course of the fluids in vegetables, 
we next proceed to examine the powers by which these 
fluids are moved. 
Capillary attraction lias generally been accounted the 
cause of the motion of the juices of plants ; and the perma- 
nence of (lie action of this power has been supposed to de- 
pend on the evaporation from the leaves. Of late years, 
indeed, botanists have ascribed to plants a vital power, 
which they believe assists the flow of the juices; and this 
opinion is supported by the following reasons : 
1. The descent of the juices, that is, their return from 
the branches to the roots, cannot be explained, without the 
supposition of a vital power regulating the motion. A flow 
of fluids, through capillary tubes, will only take place when 
the resistance at the one end is diminished. This might ac- 
count for the rising of the sap, when warmth is applied to 
the leaves, but cannot account for its descending in the same 
circumstances, that is, when the atmosphere is warmer than 
the earth. But this takes place constantly with respect to 
the succus proprius, and, it is probable, that part of the sap 
has the same course, both in the day and night. 
2 . The exertions of many plants on the application of 
stimuli, afford another argument in support of their mus- 
cular power, and the spontaneous motions of other plants 
confirm the same opinion. 
S. Light admitted to plants increases their perspiration, 
and causes a leaf, before inverted, to resume its natural posi- 
tion. The influence of darkness counteracts these effects, and 
produces what is called sleep in plants, although the heat 
of the atmosphere be not diminished. These facts seem to 
prove the irritability, or muscular power, of vegetables. 
4. If the fluids of plants are conveyed through vessels, 
can we suppose these tubes to be of so small a diameter, as, 
by capillary attraction alone, to raise the juices from the 
roots to the summits of the loftiest trees? 
5. On the supposition of the fluids being moved entirely 
by capillary attraction, how happens it, that the sap of 
the vine flows from an incision made in the spring, and not 
from one made in summer? In this case, as the vessels re- 
main the same, and the heat is at least not diminished, 
the efflux of sap ought to be equally copious in summer as 
in spring. 
6. Capillary tubes filled with liquor do not discharge their 
contents when broken across. But from the stem of a vine 
