ADJUSTMENT TO WATER 59 



root. Diffusion in the leaf is due to osmotic pressure, arising in 

 part from the active production of organic acids and salts in the 

 cells, but chiefly, it would appear, from the increased density 

 of the sap caused by evaporation. The latter reason doubtless 

 causes diffusion to set most strongly toward those areas in which 

 evaporation is greatest. Here, as elsewhere in the plant, diffusion 

 currents are always in the direction of greatest use. 



76. Transpiring surface. In ordinary leaves, especially those 

 found in the sunshine, the cutinized wall of the epidermal cells 

 either entirely prevents transpiration from them or reduces it 

 to an insignificant amount. The transpiring surface, therefore, 

 is not the epidermis of the leaf, but it is formed by the cells that 

 lose water rapidly and in relatively large amounts. It is composed 

 of the aggregate cell surfaces that border on air-spaces, both in 

 the sponge and the palisade tissue. At these places, the cell-sap, 

 which fills the cell walls, passes into vapor whenever the air in 

 the passages is not completely saturated. The moist air that 

 fills the spaces gives up some of its moisture through the stomata 

 to the drier air outside. It seems probable, however, that a 

 more important factor in water loss is the passing of the moist 

 air itself through the stomata, owing to the constant movement 

 of leaves in the wind. In this case, drier air at once passes in to 

 take its place. Consequently, while the number, size, and position 

 of the stomata determine the ease and rapidity with which air 

 and moisture pass out, the stomata do not form part of the trans- 

 piring surface. On the outside, the guard-cells are protected 

 against evaporation in exactly the manner of epidermal cells, 

 and the surfaces next the opening are also cutinized. The inner 

 surface of the guard-cell next the air-chamber is usually exjjosed 

 to the air of the latter, and consequently contributes very slightly 

 to the transpiring surface. 



Experiment 16. Measurement of the actual transpiring surface. Find 

 the linear extent of the air-spaces shown in tlie druwinj!; of a sunflower 

 leaf by attaching a needle-point to a thread, and allowing the thread 

 to run out as each side of an air-space is measured. If the result thus 

 obtained is squared, it will represent roughly the area of transpiring sur- 

 face for a square segment of the leaf of the width of the area studied. 

 The two surfaces of this segment woiild represent the eorresi)on(ling 

 leaf surface. Determine the ratio between the actual leaf surface and 

 the transpiring surface. 



