MOVEMENT OF WATER IN PLANTS, 689 



transpiration from the leaves. Since however the tissues can be more or less turgid 

 without its being immediately perceptible, transpiration and absorption 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, /. e. of withering, 

 caused by the collapse of the cells, takes place when the transpiration is stronger and 

 the absorption 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 transpired 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. 468, and if, when the transpiration 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 transpiration 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 

 a Tobacco-plant amounted in five days to 200 cubic cm., while the root-stock exuded 

 only 157 cubic cm. In the same manner in Cucurbita Pepo (when much withered) the 

 amount absorbed was 14 cubic cm., the exudation from the root-stock only 11 '4 cubic 

 cm. The withered upper part of a Sunflower absorbed in a few days 95 cubic cm., 

 while the root-stock exuded only 52*9 cubic cm. The result is also the same when 

 the relative amounts which extend over a shorter time are compared'. 



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

 Akad. van Wetens., Afdeeling Natuurkunde, 2^^ 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 warm earth, their foliage rising into moist air, is a very common phenomenon, 

 as I know from the experience of many years. 



^ For a more complete account see Arb. d. Bot. Inst, in Wiirzburg, I. 3, 1873. 



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