THE ABSORPTION OF WATER 



31 



owing to the fact that the aerial parts are transpiring, as well as to other causes, 

 the epidermal cells of the root in ordinary land plants are always absorbing 

 water, and hence a continuous inflow of water is kept up. If the supply is 

 sufficient to replace loss by transpiration, then the amount of water in the 

 plant is thus approximately a constant quantity. As the soil becomes drier 

 the absorption of water becomes, as we have seen, increasingly difficult and 

 the plant begins to wilt. The absorption of water by the root is influenced 

 to a very considerable extent not only by the amount of water present in the 

 soil but by other external T[actors as well. Thus it has been long known that 

 low temperatures, from + 4° to + 2° C, cause certain plants, e.g. tobacco and 

 pmnpkin (Sachs, i860), to wither, and even to die, if the exposure to such 

 temperatures be prolonged. These low temperatures act injuriously on the 

 plant, very often not directly, but by retarding the absorption of water (Kihl- 

 MANN, 1890), Strictly speaking, the withering is due not merely to a diminution 

 in the absorption, but possibly also there may be an interference with con- 

 duction somewhere ; at all events the direct influence of temperature on 

 absorption itself still wants elucidation. Kosaroff (1897), to whom we owe 

 investigations on this subject, employed a simple apparatus, known as a poto- 

 meter, which we shall find of service in other investigations later on. The 

 principle of this apparatus is explained by Fig. 6. Into the end of the U-tube 

 a branch is inserted through a cork, /v, care being taken that the junctions are 

 airtight ; into the other end is inserted, also through a cork, K, a capillary 

 glass tube, Gl, bent at right angles and having a graduated scale attached to it. 

 Each absorption of water by the plant manifests itself by a backward move- 

 ment of the thread of water in front of the scale. The apparatus may be im- 

 proved by placing the root-system of a growing plant into a large glass vessel, 

 holding a water-culture solution, instead of into a U-tube (Lecture VII). By 

 means of a funnel, suitably fixed, and provided with a stop cork, it is possible 

 to replace periodically the water lost, so that the experiment may be carried on 

 for a longer time. 



When the root-system of Phaseolus muUiflorus was placed in this apparatus 

 and kept at 20-8° C, Kosaroff observed that in twenty minutes the meniscus 

 in the capillary tube had moved through 210 mm. At 0° it moved only about 

 140 mm., and other experiments gave quite similar results. The amount of 

 water absorbed at 0° C. was only three-quarters to two-thirds of that absorbed 

 at 20° C. 



How is this to be explained ? If we assume transpiration interrupted, 

 a certain time will have to intervene before the cells of the root reach osmotic 

 equilibrium, that is to say, until they have absorbed as much water as corre- 

 sponds to their osmotic activity. The amount finally absorbed by the cells 

 when equilibrium is fully re-established will be practically the same whether the 

 temperature be o°C. or 20° C, but the time which elapses before the re-establish- 

 ment of equilibrium will depend very materially on the temperature. As a matter 

 of fact, the osmotic pressure alters according to temperature in the same way 

 as gas pressure ; but as this alteration does not amount to more than 2W per 

 degree we need not consider it any further, since it is of no physiological im- 

 portance. Rysselberghe (1901), by observing plasmolysis and recovery, has 

 estimated the time taken by water in passing through the protoplasm in dif- 

 ferent cases, and has arrived at the following results : — 



Temperature .... 0° 6° 12° 16° 20° 25" 30° 



Rate of movement of water .12 4-567 75 8 



At 30° C. the movement of water was eight times as rapid as at 0° C. At 

 first sight this result appears remarkable, and does not agree with the behaviour 

 of Pfeffer's osmotic cell ; for the copper-ferrocyanide membrane exhibits no 

 such irregularities in behaviour at different temperatures. Rysselberghe, 

 however, referred the matter to purely physical causes, and he remarks 



