ioo THE VEGETATIVE ORGANS 



secure the tube with the shoot to a retort-stand or other 

 suitable support. Fix a scale to the tube and take readings 

 at intervals. When the mercury has reached its maximum 

 height, ascertain the weight of the column of mercury 

 raised by the force of transpiration. 



After a time, air will collect in the upper part of the tube. 

 Where can this air have come from ? Is it possible that 

 the tension of the liquids has resulted in air being drawn 

 through the stem, and that this has accumulated on the 

 top of the column ? Or did the water contain air which 

 may have risen to the surface ? The value of the latter 

 suggestion may be tested by using water which has been 

 previously boiled and allowed to cool. If air then accumu- 

 lates it must have come from some other source. The 

 former suggestion may then be considered. Do air-channels 

 exist in shoots ? and, if so, is it possible to draw air through 

 them ? The following experiment will help us to answer 

 this question. 



Suction of air through a shoot. Fix a suction-pump 

 firmly to the water-tap and connect it to a bottle by means 

 of thick-walled rubber tubing. Fill the bottle with water 

 and insert a rubber stopper, through the hole of which 

 is passed the stem of a Laurel or the stalk of a single leaf, 

 as shown in Fig. 62. Turn the tap gently, then steadily 

 increase the flow. Note what happens at the end of the 

 shoot. Where is the air coming from ? Is a stream of 

 air passing through the shoot ? 



This experiment may be reversed. Place the leaves 

 in the water and the cut end of the shoot in the air; 

 observe the air-bubbles coming out from the numerous 

 tiny points on the under surfaces of the leaves and more 

 vigorously at any broken or injured places. Suddenly turn 

 off the water and note the change of colour in the lower 

 surface of the leaf. Why is this ? Turn on the water again 

 and the leaf regains its colour. Repeat the experiment 



