vii OSMOTIC PRESSURES OF LEAF CELLS 147 



and a half hour's sunshine, 0*342 gr. of water from the 

 weighed vessel. 



From these experiments it follows that the osmotic 

 cells of the leaves of Acer macrophyllum were able to remain 

 turgescent and draw up water against a pressure of 8 atmo- 

 spheres. Consequently, the osmotic solution in the cells 

 must be capable of generating a tension equivalent to 

 8 atmospheres pressure, by attracting water from the 

 conduits. 



All the trees with which I have experimented do not, how- 

 ever, show that their leaves possess such high osmotic pres- 

 sures when surrounded with carbon dioxide. Thus the 

 specimens of Cytisus laburnum, investigated by means of 

 the high-pressure apparatus, showed that their cells began 

 to collapse under an external pressure of 6 atmospheres. 

 Above this pressure the leaves faded, and water 

 was forced back from them into the stem. It is, how- 

 ever, very probable that all the leaves were not put out 

 of activity in transpiration simultaneously. Thus, I have 

 observed, with Cytisus laburnum, that the old leaves 

 begin to show collapse by losing their glossy surface, and 

 rolling back from the edges at a pressure of 6 to 7 atmo- 

 spheres, while the young, small leaves, which are com- 

 posed of growing tissues, remain stiff and turgescent, even 

 at 16 atmospheres. 1 



A preliminary experiment on Cytisus laburnum showed 

 that the leaves of this plant flagged markedly after an 

 exposure of five to ten minutes to a pressure of 16 atmo- 

 spheres. The flagging in this case is indicated by the 

 folding down of a leaf from the base of its petiole, and the 

 folding back of its leaflets, so that the whole leaf has the 

 appearance of the leaf of a sensitive plant {Mimosa pudica) 



1 This phenomenon is probably correlated with the relative sizes of the 

 vacuoles in the old and the young cells ; for it will appear later that the 

 osmotic pressure of the sap of the young is less than that of the older 

 leaves. 



L 2 



