aides the roots aes a system of Teale comtinaat into : 
larger streams, and in the main stem, 1n ne case of Dicoty- 
~ 
ledons and Conifers, into a single current, in the case of 
Monocotyledons and Ferns into several : : and these then 
again ramify like the stem, and distribute themselves to the 
most delicate parts of the leaves, where, ina manner compar- 
able to the perspiration of animals, the water again disappears 
in the form of vapour, by transpiration. It has been ascer- 
FiG. 345.—Apparatus for measuring the root- 
pressure. A manometer, a, 0, c, is fixed 
into the upper part of the plant Pp growing 
in a pot, the top having been cut off. The 
difference in the height of the mercury in 
the two arms 6 and c indicates the intensity 
of the pressure by which the water sucked 
up by the roots is forced out at the cut 
section. 
tained that in actively grow- 
ing plants the quantity of 
water thus absorbed often 
amounts in a few days to 
many times the weight and 
volume of the whole plant. 
The woody parts of. the 
vascular bundles serve as 
conduits for this powerful 
stream, as may easily be 
proved in plants from 
which the bark has been 
peeled off, and whose pith 
has decayed, by causing the 
roots to suck up a coloured 
infusion. The chief causes 
of these great movements of 
water are: capillarity , and 
diffusion [osmose|; but an 
action of suction may also 
be distinguished from one 
of propulsion. In _ every 
part of a plant where growth 
or evaporation is taking 
place, the part which re- 
quires water will obtain it . 
from the surrounding parts, and as this swctvon gradually ex- 
tends to larger and larger circles, 
the most distant parts, z.e. 
