THE MOLECULAR FORCES IN PLANTS. 199 



ment, and if the soil in which it is rooted contains plenty of water, 

 that, the level of the fluid in the tube soon rises. To demonstrate 

 the outflow of sap from the stem-stump, we may also use the 

 apparatus indicated in Fig. 74. On the stump s of the decapitated 

 plant is fixed, by means of a short piece of rubber tubing, a 

 T-shaped glass tube, t. In order to make the connection abso- 

 lutely air-tight, the tubing must, if necessary, be bound with wire. 

 In many cases one is obliged to employ a narrow T-tube. The 

 vertical limb of the T-tube is closed at the top with a cork, 

 through which passes a glass tube provided with a glass stop- 

 cock. The horizontal limb is connected up with the outflow tube 

 r, which dips down through a loosely fitting cork, a, into a gradu- 

 ated measuring cylinder, b. The T-tube and the outflow tube 

 must at the commencement of the experiment be filled with water. 



To determine the pressure exerted by the sap flowing from the 

 stem-stump, the outflow tube is replaced by a manometer contain- 

 ing mercury, as indicated in Fig. 75. Water is poured into the 

 T-tube, which is then closed with a cork, provided with a short 

 tube, g, drawn out to a capillary point. The end of the capillary 

 must be fused up, so that no air is left in the apparatus. 



Proceeding in the manner described, we find that in many plants 

 the sap only flows for a few days. I have, however, often observed 

 that the flow may continue for more than a week. If plants are 

 decapitated, and we observe their behaviour without further pre- 

 paration, we shall find that the sap escapes from the wood, and 

 especially from the vessels, 



76. The Flow of Sap from Injured Trees Growing in 

 the Open, 



In March or early in April we make a boring in the trunk of a 

 still leafless birch tree, reaching to about the middle of the trunk. 

 In my experiments holes 7 mm, wide were made at a height of 

 about 40 cm. above the ground, where the trees were about 40 cm. 

 in circumference. One limb of a glass tube bent at right angles 

 is cemented air-tight into the boring, while the other limb passes 

 into a flask standing on the ground. Under some conditions a con- 

 siderable quantity of fluid at once flows from the tree, but in any 

 case it is very instructive to watch the behaviour of the plants for 

 several weeks. Addition of water to the soil by rain favours the 

 outflow of the sap ; in the daytime it is usually less than during 



