THE WATER-RELATION 



87 



evidence of the fact may be gained by plugging the cavities of the vessels 

 of a cut shoot with gelatin : the stream is then stopped, and the subject 

 of the experiment withers, though an ample supply of water may be 

 accessible at the cut surface. It may thus be concluded that it is 

 through the cavities of the vessels or tracheides that the quick move- 

 ment of the Transpiration-Stream passes. The mechanism of its move- 

 ment is still obscure in some points, but it may be analysed so that for 

 plants of moderate size it 

 is intelligible. Two factors 

 are recognised as effective, 

 though these probably do 

 not provide a complete 

 explanation of the move- 

 ment. There is a motor 

 influence from above, and 

 another from below. Either 

 or both may be intermit- 

 tent in their action. That 

 from above is suctorial, that 

 from below is propellent. 

 That water is drawn into 

 a cut stem may be demon- 

 strated by Farmer's bottle 

 (Fig. 65). It is closely 

 fitted with a rubber bung 

 traversed by a funnel and 

 stop-cock for supply (F), 

 and by a length of ther- 

 mometer-tubing, bent as 

 shown, with its end open 

 (r). The apparatus thus 

 forms a closed system, open 

 to pressure of the air only at the end of the tube. The cut stem of a 

 suitable shoot is passed through the bung ; it must be closely fitted, 

 and the whole apparatus completely filled with water up to the end 

 of the tube. The end surface of the water column will then, under 

 normal day conditions, be seen to move inwards along the tube. 

 Since the apparatus is closed and full of fluid, the movement of the 

 column backwards in the open tube shows that water has been 

 absorbed at the cut end of the shoot. The length of the tube 

 emptied in a given time will give a rough measure of the amount of 



Fig. 65. 



A simple Potometer, for indicating and measuring the 

 amount of water absorbed by suction at the cut base of a 

 shoot. (After Farmer.) 



