Dixon — On the Osmotic Pressure in the Cells of Leaves. 67 



turgescent, as in that case it was distended by internal pressure, and 

 consequently the leaf formed of such collapsed cells is flaccid. 



After I had obtained this result, I set about to determine the 

 critical pressure for this plant, i.e. the pressure at which Cytisus 

 lalurnum would cease to draw up water in transpiration, and above 

 which the cells of the leaf would be forced to collapse, and water 

 would be driven back from them into the stem. 



(1). In the first experiment, a small branch of this tree carrying 

 9 leaves was fixed in the apparatus. The pressure was maintained at 

 16 atmospheres. During one hour of diffused light, while the condi- 

 tions within the apparatus were kept favourable to transpiration, i.e. 

 the space was dried by calcium chloride, 0-950 gr. were forced from 

 the leaves through the stem into the flask below. During the first 

 ten minutes of this experiment the leaves began to flag, and soon 

 showed all the appearances described above. 



(2). A branch of the same tree, carrying 12 leaves, some old and 

 some young, was submitted to a pressure of 8 atmospheres. After one 

 hour of bright sunshine the vessel into which the branch dipped was 

 found to have gained 0-400 grs. During this time the old leaves had 

 become flaccid, while the young leaves remained turgid. Even the 

 old leaves did not become markedly flaccid during the first forty 

 minutes of the experiment. 



(3). A branch with 8 leaves was exposed to a pressure of 6 atmo- 

 spheres during one hour of mostly bright sunshine. During this time 

 the leaves showed no signs of becoming flaccid, but the surface lost 

 some of its gloss. On weighing, it was found that the vessel below 

 had lost 0-007 gr. of water. This amount, however, comes within 

 the limits of error of the experiment, and consequently we may assume 

 that neither upward nor downward motion of water occurs in these 

 branches when the leaves are exposed to a pressure of 6 atmospheres. 

 In this experiment, when the pressure was removed, the leaves re- 

 covered their gloss. 



(4). Against 4 atmospheres, the same branch, in intermittent sun- 

 shine, transpired 0-622 gr. in one hour and twenty minutes, while all 

 the leaves remained quite turgid. 



At the conclusion of this series on this branch I|measured tlie 

 amount it transpired at normal pressures still surrounded with CO3 

 gas, and found it to be 1-244 gr. in one hour and jlO minutes. In 

 air at normal pressure the same branch transpired in one hour 

 0-96G gr. During these last two experiments, the leaves were 

 slightly faded. These experiments are summarized in the following 

 Table. 



