MOVEMENT OF WATER IN PLANTS. 



607 



more rapidly and completely the younger and less lignified the part where the section 

 is made. This withering can be easily prevented by making the section under water, 

 and taking care that the cut surface does not come into contact with the air, the con- 

 duction 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 evaporation, 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 interi'uption 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 



FIG. 439.— Apparatus for showing the revival of withered shoots by forcing water into them. The U-shaped glass 

 tube is first filled with water, and the perforated stopper of caoutchouc i in which the stalk of the plant is niserted. 

 is then fixed in. When the shoot is withered, as represented by a, mercury is poured into the other arm of tlie tube, 

 so as to stand at / some 8 or 10 cm. above ?, and the shoot then revives, as represented by d, even when the level ? 

 becomes subsequently higher than g'. 



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 ligni- 

 fied, the removal of a piece 6 cm. long is usually sufficient to revive the withered shoot 

 {e.g. in Reliant bus tuberosus, 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 oi Hehanthus 

 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 



