i 5 o TRANSPIRATION AND ASCENT OF SAP ch. 



As it appeared quite possible that different examples 

 of the same species might have different osmotic pressures 

 in their leaves, these branches were all taken from the 

 same individual, and from a height of about six feet from 

 the ground. 



In this series of experiments there are two sources of 

 error tending to make the critical pressure appear lower than 

 it is in reality : First, there is the mechanical crushing of 

 the conduits themselves owing to the external pressure. 

 When the osmotic cells experience the pressure, they may, 

 without themselves suffering any collapse, move in on the 

 conducting tissues, which, although they are specially 

 adapted to resist external pressure as well as internal 

 tension, are elastic to some extent, and consequently will 

 become somewhat contracted. This will expel a certain 

 quantity of water from them into the vessel beneath : 

 and, as the vessel was taken away immediately after the 

 pressure in the glass cylinder was lowered, the conducting 

 tissues may not have had time to reassume their former 

 volume. By this means a quantity of water would be 

 forced back into the vessel and remain there, and would 

 tend to counteract the loss due to transpiration. As the 

 greatest amount of water I have observed forced back 

 in this way from a branch, which was larger than the 

 branch used in these experiments, was about 01 gr., as 

 will be seen later, we may place the critical pressure of 

 the branch of Cytisus laburnum at 6 to 8 atmospheres. 



Effect of carbon dioxide on the osmotic pres- 

 sure. The second source of error is more difficult to 

 allow for. The presence of the carbon dioxide surrounding 

 the leaves undoubtedly acts injuriously on the cells of 

 the leaf, so that a leaf which has been surrounded with 

 carbon dioxide for several hours sometimes shows a 

 darkened appearance, and collapses at a lower pressure 

 than one which has been put in fresh into the apparatus. 

 With this plant (Cytisus laburnum), however, the injurious 



