Dixon — On the Osmotic Pressure in the Cells of Leaves. 69 



The decrease in the rate of transpiration with the increase of pres- 

 sure which is indicated by these results is, doubtless, more marked 

 than here appears, as it is well known that the rate of transpiration 

 of a branch falls off rapidly from the time of cutting it. In the ex- 

 periment C at 6 atmospheres which was the second to be made with 

 this branch, this decrease would have been small, but in the succeeding 

 experiments would have become more exaggerated. 



It may be noted that the amount transpired at normal pressures 

 was not diminished by the presence of the CO3 surrounding the leaves. 



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 6 feet from the ground. This last precaution is necessary, as 

 it may be that at different heights in the tree, different pressures 

 obtain. I propose investigating these points at a later date. 



In this series of experiments there are two sources of error tending 

 to make the critical pressure appear lower than it is in reality : — 

 1st. 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 provided 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. Ey 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 0*1 gr., as will be seen later, we may place the critical 

 pressure of the branch of Cystisus lalurnum at 6-8 atmospheres. The 

 second source of error is more difficult to allow for. The presence of 

 the CO2 surrounding the leaves undoubtedly acts injuriously on the 

 cells of the leaf, so that a leaf which has been surrounded with COj 

 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 lahimum), however, the injurious 

 effects of CO2 are not so marked nor so rapid in their manifestation 

 as in others. Thus the leaves do not become blackened, nor is the 

 critical pressure markedly lowered, so far as my present observations 

 have gone, within the first six hours immersion in CO2. All the 

 experiments quoted above were made within this time. 



As an illustration of how the CO2 affects the transpiration and tur- 

 gescence of the leaves, I will add the two following Tables of experi- 

 ments on Tilia americana, which I have found very sensitive to this gas. 



