the Ascent of Water in Trees . 651 
The nature of each phenomenon concerned must be clearly kept in 
mind with regard to whether it is of a static or of a kinetic nature ; 
whether merely a sustaining action or one actually giving rise to 
motion. It will tend to clearness to cite from Dixon’s paper of 1896 
(on the ‘Role of Osmosis in Transpiration/ pp. 773-74) the model or 
system which he imagines as diagrammatically representing the role 
of osmosis in transpiration. 
A porous pot rendered semi-permeable is attached, water-tight, to 
the upper extremity of a glass tube. Pot and tube are filled with 
water, and the lower end of the tube opens into a vessel of water. 
Above, a bladder or other semi-permeable membrane is supposed to 
inclose the porous pot loosely, the mouth being closed by a water- 
tight binding around the tube below the pot. We suppose an aqueous 
solution of potassium nitrate in the space between the bladder and 
pot. We call the region inside the pot A ; the space occupied by 
the osmotic solution B ; the external atmosphere C. The following 
actions I will now suppose to occur, and may be described as the 
teaching of the model : — 
(1) If B is at first not quite filled with liquid, the osmotic attraction 
upon the water in A, or, according to recent views upon the nature of 
osmotic action, the osmotic pressure, will gradually suffice to quite fill 
it. Water will consequently ascend the tube. In this operation the 
membrane is at first loose and, of course, any force located in it is 
clearly unable to exert any independent lifting pull upon the water 
in A ; nevertheless water ascends. This is the case of the revival of 
a drooping leaf in which upon an adequate supply of water turgescence 
is re-established. 
(2) The bladder becomes tense under increasing pressure in the 
space B. 
I will assume now that a capillary or imbibition attraction of the 
membrane alone acts, and suffices to keep the external surface of 
the membrane moist. Evaporation now progresses upon the surface, 
and the current of water upwards in the tube is maintained. What 
of evaporation from a moist surface’ (Phil. Trans., Vol. 186 b, p. 567). The 
paragraph which follows refers to the ‘ sorting demon ’ nature of evaporation and 
the inflow of thermal energy at the evaporating surface in support of the view that 
to evaporation phenomena the elevation of the sap is to be ascribed. The same 
views are urged throughout the Abstract, and we admit that ‘ other physiological 
phenomena’ may act (p. 5). 
X X 2 
