120 Scientific Proceedings, Royal Dublin Society. 



It may be supposed that the leaf-cells function just in the 

 same manner as this osmotic cell continues to draw up water after 

 it is surrounded with a saturated atmosphere. When the leaves 

 of a plant are placed in a saturated space, the cells of the 

 mesophyll tissues, if not completely distended, will he capable of 

 taking in more water. Some of this water may be derived from 

 the surrounding water-vapour; but the greater part will un- 

 doubtedly be drawn from the wood of the vascular bundles, and 

 so will cause a rise of water in the conducting tracts of the plant. 

 If the osmotic membranes of the mesophyll cells are not strictly 

 semipermeable, this rise will continue, and dilution of the solutions 

 in the cells will proceed until the fluid in the adjoining conducting- 

 tubes is isotonic with the solution in them. The fact that the 

 fluid exuded into a moist atmosphere contains an appreciable 

 quantity of dissolved substances, indicates that the cells of the 

 leaves which are active in raising water into a saturated 

 atmosphere do not possess strictly semipermeable membranes. 

 Hence we may, with great probability, assume that the processes 

 occurring in leaf-cells in a saturated space, so far as the elevation 

 of water is concerned, are extremely similar to those taking place 

 in our model. Of course in leaves containing starch, or in those 

 which are in a condition to carry on photosynthesis, the equaliza- 

 tion of the concentration of the solutions in the cells and in the 

 conduits may be long or indefinitely postponed. This is quite in 

 agreement with the fact that submerged leaves are able to draw 

 up water from below when exposed to light, and hardly at all, 

 when kept in darkness. 1 



If the membranes of the model were strictly semipermeable, 

 and the space above them completely saturated, its action would 

 be different from what has just been described. Water would 

 then rise in the supply-tube only as long as the cell was not 

 distended to its maximum. As soon as it had attained its maxi- 

 mum distension, and if its membranes were capable of resisting the 

 osmotic pressures, a state of equilibrium would ensue, and the 

 number of water molecules leaving the cell through the upper 

 membrane would be balanced by the number of those enter- 

 ing through it from the saturated space. And, in the same way, 



1 Loc. ci£.,p. 633. 



