118 WATER-ABSORPTION AND TRANSPIRATION. 



water, and water passed into the potometer-tube, when required, by 

 releasing the clip. 



* (h) For experiments with small plants (e.g. Bean seedling) or even 

 single leaves, the apparatus sketched in Fig. 41 A is convenient. THfe 

 rate of flow is roughly measured by the paper scale fastened to the 

 long horizontal tube ; care must be taken to make the cork and the 

 joinings of plant and tube with it air-tight by using plasticine or wax. 

 This apparatus lias, of course, the disadvantage that air-bubbles collect 

 below the cut end of the plant and stop the water-current, hence it is 

 not so convenient to use as the potometer. 



(i) By the aid of a simple piece of apparatus 

 (Fig. 43) and a weighing balance, the amount of 

 water lost by a leafy stem can be determined, 

 and at the same time it can be proved that the 

 amount transpired is approximately the same as 

 that absorbed by the roots. A plant is fixed by 

 means of an air-tight indiarubber cork in a bottle 

 filled with water. As water evaporates from the 

 leaves and is absorbed by the roots, the level of 

 the water in the graduated tube falls. This 

 gives the amount absorbed by the roots, and on 

 weighing the apparatus it will be found to corre- 

 spond to the total amount lost by evaporation 

 from the leaves and stem. 



Each of the larger divisions on the graduated 

 tube corresponds to a cubic centimetre of water, 

 so that when the roots have absorbed that amount 

 the entire apparatus will have lost about one 

 gramme in weight. The vertical tube attached 

 to the side of the vessel, if not graduated by 

 engraved lines, may have a scale drawn on a 

 narrow strip of paper and fixed to it. The 

 divisions should represent definite volumes, such 

 as cubic centimetres. Fill the vessel with water, 

 replace the stopper firmly, and set aside for 

 observation. The volume of water becomes less, 

 and the amount of loss may be seen by observing 

 the scale. Enter in a book the amount of loss in 

 equal intervals of time, and note that it is less 

 during darkness than in the light, and also that 

 it varies with atmospheric conditions. 



(j ) Fix a Wallflower or large Bean seedling in 

 the apparatus shown in Fig. 43, and determine 

 the amount of water lost under the optimal 



conditions (i.e. in a warm, airy, well-illuminated room), either by weigh- 

 ing at intervals, or by watching the attached register. Then remove 

 all the leaves and cut out twice their area in thick note-paper, to which 

 add a piece the same length as the stem and three times its breadth. 



Fig. 43. Apparatus for 

 estimating the Absoi-p- 

 tion of Water by the 

 Roots, and the Loss 

 by Transpiration from 

 the Leaves. 



