148 TRANSPIRATION AND ASCENT OF SAP ch. 



which has been stimulated. Besides these motions, the 

 surface of the leaf loses its gloss and becomes dried-looking, 

 the edges of the leaf roll up, and the expanded portion 

 becomes crumpled. The general appearance of the leaves 

 after twenty minutes' exposure to 16 atmospheres is that 

 of a leaf which has been exposed to a high temperature 

 and afterwards dried. Microscopic examination of the 

 cells of these leaves shows the protoplasm contracted from 

 the cell- wall just as it is in plasmolysed cells. This appear- 

 ance is probably brought about by the cell-wall being 

 pressed in on the protoplasm, and causing the latter to 

 force out its watery contents. When the pressure is 

 relieved, the cell-wall, by virtue of its elasticity, recovers 

 its form, while the protoplasm remains contracted within. 

 The space included by the cell-walls does not, however, 

 attain the dimensions it possessed when the cell was turge- 

 scent, as in that case it was distended by internal pressure 

 and consequently the leaf formed of such collapsed cells 

 is flaccid. 



After obtaining this result, I set about to determine 

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

 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 pres- 

 sure was maintained at 16 atmospheres. During one hour 

 of diffuse light, while the conditions within the apparatus 

 were kept favourable to transpiration, i.e., the space was 

 dried by calcium chloride, 0950 gr. was 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 



