184 PHYSIOLOGY OF NUTRITION. 



in the stems can only be due to the atmospheric pressure forcing 

 the fluid into the vessels. The rise of the eosin solution is not 

 merely the result of capillarity, as may readily be proved. If we 

 sever shoots of the plants in air, and then quickly dip their ends 

 into the eosin solution, it rises but little in the vessels, because 

 air has penetrated into them, and their gaseous contents are no 

 longer under negative pressure. Naturally in this last experi- 

 ment the pigment solution will rise to a less height in the vessels 

 by capillarity, the wider these are. 



We now make our experiments in a somewhat different manner. 

 We dip the branch into mercury at a point about 50 cm. from the 

 summit, and sever it below the mercury. After two minutes we 

 remove the severed portion from the mercury, and, on peeling it,' 

 find that the mercury, which can be recognised from the outside 

 by the presence of numerous grey lines running parallel to each 

 other, has, under the pressure of the atmosphere, been forced up 

 into the vessels of the wood to a considerable height. In Robinia, 

 e.g., the mercury may in the proximal portion of the shoot that, 

 viz., remaining in connection with the planb be raised in single 

 vessels to a height of 50 cm. In the distal portion that, viz., 

 severed from the plant it does not rise so high. If branches cut 

 in the air have their lower ends dipped a few centimetres below 

 mercury, the mercury does not usually rise in the vessels, since 

 the upward mercurial pressure is generally less than the capillary 

 depression of the mercury. 



While it is easy to prove generally that the air in the vessels is 

 at a negative pressure, the determination of the actual value of this 

 negative pressure is a matter of difficulty! 6 Further experiments 

 on this point are very desirable. 



The following experiment is also a very instructive one, and it 

 is readily performed in lecture. We cut off in the air a long 

 branch of Ampelopsis, leave the cut surface in contact with the 

 air for a few minutes, and then place the branch with its lower 

 end in water. After twenty-four hours we bend the shoot into 

 eosin at a point about 10 cm. from the first cut surface, and sever 

 it under the fluid. The solution rises to a considerable height 

 in the vessels, from which it follows that under the conditions 

 described, negative gaseous pressure in the vessels is after a time 

 again set up in branches which have been cut off in the air. This 

 may be due to a variety of causes. 7 



