How Plants Draw in Various Materials 185 



acts as a check, either partial or total, to the further diffusion of the 

 dissolved substance while allowing the liquid itself to pass freely, as 

 a result of which the dissolved substance, whether by gas-like ex- 

 pansion or direct attraction, draws liquid through the membrane, 

 swells, and exerts an osmotic pressure proportional to its strength. 



After this lengthy but needful discussion of physical principles, 

 we turn to the actual osmotic phenomena displayed by plants, 

 and here the reader who is skipping the hard parts must resume 

 the narrative. The absorption of water by roots is the most 

 important of these phenomena, but there are others of little less 

 consequence. First among them is the maintenance of rigidity 

 in very soft parts such as leaves, young stems and flowers. These 

 parts consist mostly of water (fully 90 per cent), while the re- 

 siduum of solid matter (about 10 per cent) is too small and un- 

 substantial to supply rigid support. Even the moderately firm 

 veins, as everyone knows, are quite unable to keep a wilted 

 leaf from collapsing. But every young cell, soft and weak 

 though it is, can absorb water powerfully through its semi- 

 permeable protoplasmic membrane into its sugar-holding sap, 

 and thus swell to turgescence, stretching the walls until they are 

 tense, and the structure is stiff. Again, osmotic pressure supplies 

 the energy by which young cells can expand their walls in growth, 

 overcoming the resistance of older cells around them; by which 

 buds or flowers can swell and unfold; by which young roots can 

 force a way through hard soil and even destroy masonry and lift 

 curbstones ; and by which soft-bodied fungi can burst pavements. 

 Osmotic pressure is the mechanical power used by those parts in 

 effecting their work. 



Of minor osmotic phenomena in plants, some of them familiar 

 in the household, there are many. Thus, if one places dry sugar 

 on fresh strawberries, pretty soon it becomes a syrup, and the 

 berries look shrunken; evidently the sugar, moistened by con- 

 tact with the berry, makes a dense solution which draws water 

 from the cells. The collapse of berries from this cause is very 



