66 Proceedings of the Royal Irish Academy. 



equivalent to 8 atmospheres pressure, "by attracting water from the 

 conduits. Sucli a tension would be capable of drawing up a column 

 of water 240 feet high, provided the column of water was submitted 

 to such conditions that it would not break. Dr. Joly and myself 

 have shown elsewhere that these conditions obtain in the conducting 

 tissues of plants.^ 



All the trees I have experimented with up to the present do not, 

 however, show that their leaves possess such high osmotic pressures 

 when surrounded with COj. Thus the specimens of Cytisus lalurnum, 

 investigated by means of the high-pressure apparatus, showed that 

 they were unable to transpire against an external pressure of more 

 than 6 atmospheres. Above this pressure the leaves begin to collapse, 

 and water is forced back from them into the stem. It is, however, very 

 probable that all the leaves are 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 6-7 atmospheres, while 

 the young, small leaves, which are composed of growing tissues, remain 

 stiffi and turgescent, even at 16 atmospheres. This is quite in accor- 

 dance with Wieler's observations on the internal pressure of the cells 

 of the cambium, which he estimated at 13-16 atmospheres. 



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 atmospheres. 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) 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 atmosjiheres 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 appearance 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 



1 Phil. Traus. Roy. Soc, vol. 186 (1895), B. 



