TRANSPIRATION FROM PLANTS 261 



account of many openings and the exposure of much surface, are 

 the regions where water is lost most rapidly. Transpiration is 

 the chief enemy of plants and is an important factor in determin- 

 ing the form, structure, and distribution of plants. 



The loss of water is not so much under the plant's control as 

 photosynthesis and respiration are. Unless the air about the 

 plant is already saturated with moisture — and it seldom is — it 

 will take up water wherever water is available, and the moist tis- 

 sues of plants are available sources of moisture. The air circulat- 

 ing through the inter-cellular spaces of the leaf receives moisture 

 from the tissues, and consequently its moisture content becomes 

 greater than that of the air outside of the leaf. But according 

 to the law of diffusion, the water-vapor diffuses from the air 

 within through the stomata to the drier air without, and this 

 diffusion continues as long as the air within the leaf receives 

 sufficient moisture from the tissues to maintain a moisture con- 

 tent greater than that of the air without. The more the moisture 

 content of the air within and without differs, the more rapidly 

 the plant loses water. 



Transpiration can be easily demonstrated by enclosing a 

 potted plant in a bell jar, taking the precaution to cover all 

 evaporating surfaces, except the plant, with rubber cloth or wax. 

 It can be demonstrated also by enclosing a branch of a plant in a 

 flask. In a short time moisture collects on the glass, at first as a 

 mist which may later form into drops and run down the sides 

 of the jar or flask. {Fig. 237.) This indicates that the plant 

 loses water to the air, which consequently becomes so nearly 

 saturated that moisture is condensed on the glass. If a plant 

 with pot protected from evaporation is exposed to transpiration 

 and weighed at intervals, the loss in weight due to the loss of 

 water through transpiration is quite marked. 



The amount of water transpired, although varying much with 

 conditions and in different plants, is always a large proportion of 

 the amount absorbed. Despite the fact that much water is 

 used by the plant in making sugar and other compounds and in 

 maintaining the turgor of cells, much the larger proportion of 

 the water taken in by the roots passes through the plant and out 

 into the air. The amount of water transpired under various 

 conditions ranges from almost zero up to 300 grams or more per 

 square meter of leaf area per hour. For this unit of leaf area per 



