THE FOOD OF THE PLANT 23 



always be demonstrated. It can be observed only in 

 certain plants at certain seasons of the year. 



No trace of root-pressure can ever be detected on 

 cutting the stem of a plant that has plenty of leaves 

 and is growing in a dry, sunny atmosphere. 

 ■ Of course this does not necessarily show that the force 

 has ceased ; it is merely negative evidence. At the same 

 time it prevents our regarding root-pressure as an all- 

 sufficient cause of the ascent of water through the stem. 



Thus after making full allowance for the work done 

 by the pressure from the roots and the pull from the 

 leaves we are still left face to face with one of the big 

 mysteries of plant-life. 



Transpiration. — If the amount of water absorbed by 

 a healthy growing plant be observed, and compared 

 with the growth and increase in weight of the plant, 

 it at once becomes evident that all the water absorbed 

 cannot be retained. The method by which a plant 



fets rid of its surplus water must now be 

 etermined and some reason found for the 

 absorption of so much more than seems to 

 be necessary. 



Experiment 11 



Aim. — To determine how a plant gets rid 

 of its surplus water. 



Method. — A gas-jar is filled with water and 

 fitted with a cork through which has been 

 passed the stem of a leafy branch. The cork 

 itself and all the cormections are made air- 

 tight so that there may be no possibility of 

 evaporation from the water in the jar. A -«— ^ 

 second cylinder is then made perfectly dry. fis. 9 

 The groimd glass edges of both cylinders are 

 well vaselined and the dry one is inverted over the 

 other (Fig 9). 



Observation. — Very soon drops of water appear on 

 the inner surface of the upper cylinder. 



