Transpiration and the x\scent of Sap. 61 



sap. If greater, the surplus pressure will be taken up by the tension 

 or stress in the cell-walls (vide page 7). 



Few determinations have been made of the osmotic pressures of 

 leaf-cells of plants and none, so far as I am aware, of those of 

 high trees. 



W. T. Su th erst, quoted by B. E. Livingston^), determined 

 (1901) the freezing- points of the juices of différent leaves, and from 

 these the osmotic pressures can be derived: 



Leaf 



Freezing point Osmotic pressure 



Vegetetable marrow i — 01b° 



Ceiery — l-4o 



Carrot — 1-20 



Cabbag-e — I'P 



9 atm. 

 15 „ 

 14-5 „ 

 13-3 „ 



These figures agree fairly well with results obtained by the author 

 in 1896 using a completely different method. According- to the writer's 

 method a small leafy branch is enclosed hermetically in a very strong 

 glass cylinder. The end of the branch projects below and is supplied 

 with water. Air can be forced into the cylinder through a narrow 

 pressure-tube. A suitably arranged manometer indicates the air pressure 

 in the cylinder. Calcium chloride exposed in the cylinder prevents the 

 air becoming saturated with water vapour coming from the leaves. 



As one raises the pressure in the cylinder, at first the leaves 

 remain quite normal, and it may be shown by weighing the vessel 

 of supply that the branch continues to draw up water against very 

 considerable pressures and the cells remain turgid. The subjects of 

 these experiments were HeUantkus multiflorus. Acer macrophjUum, 

 Crataegus oxyacantha, Cytisus Laburnum, Tilia americana. None of these 

 lost turgidity when surrounded by air pressures up to 8 atmospheres. 

 First against a pressure of 15 atm. did some of the leaves fail to lift 

 water, and in some cases a small amount was forced back from the 

 branch into the weighed vessel below. The leaves of most however 

 did not give in at 15 atmospheres. It required an external pressure 

 of 20 atmospheres to force the leaves of fresh branches of Cytisus 

 Lahurnum and Tiha americana to disgorge water. At this pressure the 

 leaves of Helianthus assume a dull appearance, roll up at the edges 

 and slowly crumple. The petiole bends downwards and soon the whole 

 leaf presents the appearance of one which has faded for want of 

 water. These appearances are presented by the leaves of Cytisus and 



') B. E. Livingston, The Rôle of Diffusion and Osmotic Pressure in Plants. 

 Chicago 1903, p. 85. 



