684 MOLECULAR FORCES IN THE PLANT. 
of water through the stem thus suffering no interruption. If care is taken that while 
the section is being made in the air the leaves and upper parts of the stem lose 
only a very small quantity of water by transpiration, withering does not begin till later 
and increases only slowly after the cut surface is placed in water and the leaves again 
transpire.' 
It results from these experiments that the cause of withering is the interruption in 
the conduction of water from below ; and this interruption produces withering not only 
from the conduction of the water ceasing for a short time, but chiefly also from the 
power of conducting water in the stem being diminished by the loss of water above the 
cut surface, which loss cannot be restored simply by placing the cut surface in contact 
with water. 
If the cut surface does not remain too long in contact with the air, the diminution 
of the capacity for conduction takes place in only a short piece of the stem above the 
cut. When placing in water ends of shoots which have begun to wither after being cut 
off, it is only necessary to remove by a new cut a sufficiently long piece above the first 
cut, but this time beneath the water, for the shoot to revive. In the case of shoots 
20 centimetres or more in length which at this distance from the apex are not hgni- 
fied, the removal of a piece 6 cm. long is usually sufficient to revive the withered shoot 
{e.g. in Helianthus tuber osus, Sambucus nigra, Xanthium echinatum, &c.). This experiment 
proves beyond question that the change, whatever its nature may be, takes place only 
in this relatively short piece above the cut. That it consists in a diminution of the 
power of conducting water is shown by the following experiment : — When a sufficient 
number of the lowest and largest leaves have been removed from a stem of Helianthus 
tuberosus cut off in the air and placed in water, and which has begun to wither, the 
leaves that are left and the terminal bud will after some time begin to revive even 
without again cutting the stem. The water which is required for the transpiration of 
a great number of leaves can therefore no longer be conducted through the stem after 
it has been cut off in air, although that which is wanted for the transpiration of a few 
leaves can be. 
The cause of this phenomenon is therefore a diminution in the power of conducting 
water in a short piece above the cut surface of the stem. This is evidently occasioned 
by the loss of water from the cells caused by the suction of the higher parts not being 
compensated by absorption from below. All circum.stances which favour this loss of 
water increase also the loss of power of conducting it, and cause the shoot which is 
placed in water to wither more rapidly and completely. It must therefore be assumed 
that the conducting power of the cells depends on the quantity of water they contain. 
The probability of this hypothesis is increased by the fact that by artificially increasing 
the amount of water in the cells of this piece its conducting capacity can also be in- 
creased, as is proved by forcing in water from below. If the modified portion is dipped 
in water of from 35° to 40° C, the withered shoots soon revive, and if then placed in 
water of 20° C, remain fresh for days (as in the case of the Elder), or at least wither 
more slowly {e.g. the Artichoke). 
{d) Water retained in the wood by Capillary Attraction. If the capillarity of the cavities 
in the wood must be considered as without any immediate action on the currents of 
water, this force must nevertheless be taken into account with respect to other processes 
connected indirectly with the movement of water in the plant. In winter and after 
long-continued rain in summer a large quantity of water is found in the cavities of the 
wood together with bubbles of air which occupy the wider spaces. It is not known how 
this water has reached the higher parts of the trees, though it is possibly by the forma- 
tion of dew as the temperature varies ; it is however to a great extent retained by 
capillarity. A part of the water flows out in many cases through holes bored in the 
stem if they are not placed too high, as in the Birch, Maple, Vine, &c. It may be sup- 
posed that the water which flows out has been forced up by the root-pressure which 
must also be taken into account ; though how far up this pressure extends is not yet 
