CH. V, 5] OSMOTIC PROCESSES 233 



The most striking utilization of osmotic pressure by plants 

 consists in the maintenance of the form and rigidity in leaves, 

 young stems, flowers, and other soft herbaceous parts. So 

 small is the percentage of solid matter in such tissues (not 

 over 10 per cent, with 90 per cent of water), and so thin and 

 flexible the cell walls, that they cannot alone sustain their 

 own weight, as shown by their collapse in wilting. These 

 herbaceous parts are held tensely stretched and outspread 

 in their characteristic forms by the osmotic pressure of their 

 sugar-containing sap inside the thin-walled cells, the needful 

 water being supplied from the ducts . That herbaceous tissues 

 owe their stiffness to osmotic turgescence may be proven 

 conclusively by the simple experiment of immersing them 

 in a solution having a greater osmotic strength than the sap, 

 in which case of course an osmotic movement out of the 

 cells will take place. The result is always a collapse of the 

 tissues even more striking than wilting produces. It is true 

 the experiment works badly with leaves and stems, because 

 the waterproof epidermis almost prevents osmotic move- 

 ment; but the effect is perfect in parts without epidermis, 

 such as strips cut from Potatoes or Beets. These become 

 soft and flexible after only a few minutes' immersion in 

 strong sugar or salt solution, although comparison strips are 

 rendered stiffer and harder than ever by immersion in 

 pure water. Not only do such tissues become flaccid by 

 wilting or immersion in strong solutions, but they also 

 shrink in area, thus proving that the tense cells are held 

 actually stretched by the osmotic pressure within them. 

 The stiffness which pressure of water can give is familiar 

 also in fire-hose. 



Equally important is the role of osmotic pressure in growth, 

 for it supplies the mechanical power whereby the newly formed 

 cells expand in size, often against much resistance of the 

 overlying tissues. The young cells osmotically absorb 

 water, and the resultant pressure stretches the wall, in 

 which new cellulose is continuously laid down by the proto- 



