22 EXPERIMENTAL PLANT PHYSIOLOGY 



Inferences. — The presence of leaves on a twig is not 

 altogether responsible for the passage of food solutions 

 up a stem, but their presence greatly accelerates the 

 rate at which the solution rises. 



Experiment 10 



Aim. — To find out if the root exerts any pressure 

 which forces the water up the stem. 



Method. — A small potted fuchsia plant is used for 

 this experiment. The whole of the shoot is cut off at a 

 distance of from one to two inches above 

 the level of the soil. A long piece of 

 :^ narrow glass-tubing is then attached to 

 the cut end and fixed uprightly by means 

 of a clamp. Rubber-tubing is used to 

 make the attachment, and is firmly fixed 

 at both ends with wire or thin twine 

 (Fig. 8). The plant is watered in the 

 usual way. 



This experiment should be set up 

 several times at different seasons of the 

 year. 



Observations. — Water rises in the ver- 

 -j tical glass-tube. The rate at which it rises 

 / is increased by warming the roots and de- 

 creased by cooling them. In spring and 

 early summer the rise is rapid. It is 

 autumn, and often ceases entirely in the 



Fig. 8 



slow in 

 winter. 



Inferences. — The root does exert a pressure which 

 forces water upwards. 



The push of the root, or "root-pressure," is regulated 

 by the amount of water absorbed by the root, since the 

 conditions that affect the rate of absorption (Experi- 

 ment 3) similarly influence the rate of rise in the tube. 

 Thus : Root-pressure exists sometimes but cannot 

 always be deTnonstrated. 



As seen in Experiment 10, root-pressure cannot 



