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 inci eases 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 bej^eath 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 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 nvood 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 



