136 PLANT PHYSIOLOGY 



into the tracheids and tracheae to make room for the water 

 which continues to be absorbed. Although the sap-pressure 

 is generally greatest at the base of a stem or the butt of a 

 branch, the pressure is clearly due to a condition in some 

 group of cells, whether these are in the root or elsewhere. 

 We should, therefore, abandon the misleading name of root- 

 pressure and use only the equally self-descriptive but correct 

 term sap-pressure. 



The amount of sap-flow is exceedingly different, not only 

 for different species and different individuals of % the same 

 species, but also for the same individuals in different sea- 

 sons. The reasons for this diversity we have already con- 

 sidered. The following figures will serve to indicate the 

 volumes sometimes obtained 



Birch in 7 days yielded 36 liters sap (Wieler) 



(12 years old) daily average 5+ 



Agave 7.5 " " (Humboldt) 



(decapitated flower in 4.5 mont hs 995.0 " 



Sugar maple in 23 days 93095.0 grs. " (Morse) 



( cuSference?; r " daily average 4047.0 



=3.6 litres " 



(Sp. gr. 1.32) 



TRANSPIRATION. 



From all their surfaces exposed to the air, plants give off 

 water-vapor. This is a physical necessity, for water-vapor 

 will be given off from any mass, lifeless or living, which con- 

 tains water, whenever the surrounding air is not saturated 

 with moisture, or when the mass has a temperature higher 

 than that of the air, or when the mass, in relatively dry 

 air, is not enclosed in a waterproof covering. Other things 

 being equal, the amount of water-vapor given off will be 

 greater the greater the exposed surface in proportion to the 

 mass. With like conditions of humidity, temperature, sur- 

 face-composition, and surface-area, equal masses of different 

 composition will dry, 7. e. lose water by evaporation, at 

 different rates, a gelatinous or slimy mass more slowly than 

 a woody one, for example. The living plant differs from a 

 dead one of exactly the same dimensions in being able to 

 control four of these five factors, and to that degree it is 



