6l2 MOLECULAR FORCES IN THE PLANT. 



wood as is transpired from the leaves. As long as this equilibrium lasts, the plant is 

 turgid and tense in all its parts ; and conversely it may be concluded from the unaltered 

 turgidity and tenseness of the leaves and internodes, that the conduction of water is 

 compensated by the evaporation from the leaves. Hence, under these conditions, the 

 quantity of water evaporated may be taken as the measure of the suction of the root (or 

 of a cut surface), and conversely the suction observed as the measure of the evapor- 

 ation from the leaves. Since however the tissues can be more or less turgid without 

 its being immediately perceptible, evaporation and suction are not usually exactly 

 equal. But for most observations the small occasional difference may be neglected 

 so long as no actually perceptible amount of flaccidity, i. e. of withering, caused by 

 the collapse of the cells, takes place when the evaporation is stronger and the suction 

 weaker; or so long as, in the opposite case, no exudation of drops of water results 

 on the leaves of rooted plants. It is only when longer observations are made on 

 growing plants that the comparatively small quantities of water have to be taken into 

 account which are needed for the increase in size of growing organs. 



Without going more minutely into the various cases which present themselves^, 

 it need only be pointed out in addition that withering is the consequence of the 

 quantity of water evaporated being greater than that absorbed through the roots or 

 through a cut surface of the stem. This only occurs in general when the amount of 

 transpiration is very considerable, or when the ground is very dry, or when in cut shoots 

 the power of the stem to conduct water has ceased. The exudation of drops of water 

 already mentioned is, on the other hand, the consequence of a smaller quantity of 

 water evaporating from the leaves than is absorbed by the roots and forced up into 

 the upper organs. If a branch of a potato-plant, a leaf of an Aroid, a cut stem of maize, 

 or the like, is fixed in the cork k in Fig. 439, and if, when the evaporation is weak a 

 pressure of mercury of 10 or 12 cm. is allowed to act for some time, drops of water 

 appear at the same spots on the apices or margins of the leaves, where they would 

 appear in plants with roots in the evening or night or in damp weather. In the 

 same manner the exudation of drops from plants with roots can be produced or 

 increased by warming the ground and covering the leaves with a bell-glass in order 

 to hinder evaporation^. 



The pressure due to the root which is so conspicuous in stems when cut across and 

 when the amount of evaporation is very small, can scarcely be of any considerable use 

 in promoting the current of water in the wood caused by strong transpiration. The 

 fact already mentioned that strongly transpiring plants suck up water at the cut surface 

 of their stems immediately after the upper part has been cut off, shows that the pro- 

 pelling force of the root does not act sufficiently quickly to protect even the vessels of 

 the root-stock of strongly transpiring plants from complete exhaustion ; that is, although 

 the force which drives the water into the root-stock is great, as we have seen, it acts 

 too slowly to be taken into account when the evaporation is rapid. 



The same conclusion is reached if the quantity of water which exudes in the same 

 time from the cut stem of a plant above the root is compared with that which is 

 absorbed at the lower cut surface by the upper part of the same plant. The absorption 

 of the upper part is always much more considerable in amount than the outflow from 

 the root-stock, even when the withering of the upper part indicates that the capacity 

 of its wood for conduction has diminished, and that it absorbs less than it would absorb 

 in the normal condition. Thus, for example, the water absorbed by the cut leafy top of 



* See Rauwenhoff, Phytophysiologische Bijdraden in Versagen en Mededeelingen der kon. 

 Akad. van Wetens, Afdeeling Natuurkunde, 2^0 Reeks, Deel III, 1868, where however the indis- 

 pensable thermometric observations are wanting. 



2 The exudation of drops on the margins of the leaves of plants, the roots of which are sur- 

 rounded by damp wann earth, their foliage rising into moist air, is an altogether different pheno- 

 menon, as I know from the experience of many years. 



