6 



Henry H. Dixon. 



sists of a glass thistle-funnel. The tube of the funnel is continued 

 at right angles by a capillaiy glass tube which may be laid across 

 the stage of a microscope, and dips into a vessel of supply. The 

 opening of the funnel is closed with two membranes of vegetable 

 parchment, between which is introduced some sugar. The membranes 

 are fixed in position by binding them down on glue. When set the 

 glue is rendered insoluble hj an application of a solution of tannin. 

 The tube is filled with water and set upright as illustrated in Fig. 2. 

 The osmotic pressure of the sugar as it goes into solution above 

 the lower membrane, by creating a region of low pressure for aqueous 

 vapour, quickly draws water into the space between the membranes. 



Fig-. 2. Model to illustrate transpiration from an osmotic cell. 



As the water enters, the dissolved sugar is free to exert its diffusion 

 pressure against the membranes, and they are forced apart and 

 distended. The membranes and the space enclosed by them represent 

 a cell of the leaf in a state of turgor. The bowl of the thistle funnel 

 corresponds to an adjoining trachea filled with water. 



Supposing then that the vapour-pressure of the water in the 

 imbibed upper membrane is greater than the pressure of aqueous 

 vapour in the surrounding space, water will pass off. This loss will be 

 made good to the membrane, if the vapour pressure of the solvent in 

 the 'cell' is greater than that in the membrane. In the same way to 

 maintain the water content of the cell there must be a greater 



