TRANSPIRATION. 



77 



only, and is especially active when these parts are young. It is tlie 

 process by which the plant gets rid of the surplus water after having 

 drawn it from the soil in order to extract from it the nutriment which 

 is present in only a very highly attenuated solution. Botanists have 

 made many measurements of its ainount and their results are ex- 

 tremely varied, due partly to the fact that this function varies much 

 naturally, and still more i)erhaps to the fact that the experiments are 

 generally made under conditions which are not natural to the plant. 

 Sachs says that it is no rarity for a tolerably vigorous tobacco plant 

 at the time of flowering, or a sun flower of the height of a man, or a 

 gourd plant with from fifteen to twenty large leaves, to transpire from 

 one to two pints of water on a warm July day; and, so far as may be 

 judged by the use of branches with the cut end in water, it may be 

 believed that large fruit trees, oaks or i^oplars absorb, transport 

 through their stems, and transpire from the leaves, ten to twenty or 

 more gallons of water daily. He also quotes Haberlandt to show that 

 the amount transpired by a stalk of Indian corn, in its season of 173 

 days, is 3 gallons; by a stalk of hemp, in 140 days, is 6 gallons; by a 

 sun flower plant, in 140 days, is 14^ gallons. 



It is not generally practicable to comi)are the transpiration with 

 known meteorological phenomena, such as evaporation from a water 

 surface, or from the soil, or the precipitation, but some such compari- 

 sons have been made. For instance, comparing the leaf surface to an 

 equivalent Avater surface, Uuger makes transportation from the former 

 0.33 of the evaporation from the latter; Sachs for white poplar 0.36, for 

 the sun flower 0.42. Comparisons have also been made between the 

 transpiration from plants and the evaporation from the surface over 

 which the plants stand. Schleiden thought that the transpiration 

 from the forest was three times that of a water surface equal to the 

 territory covered by the forest. Schiibler thouglit it only a quarter; 

 and Pfafl', who studied a solitary oak in a garden, found that it varied 

 from 0.87 to 1.58. Comparing the transpiration of plants with the 

 evaporation from the bare soil which would be covered by them, Hartig 

 thought the transpiration of a forest less, Schiibler found it 0.0 for the 

 forest and 3.0 to 5.0 for sod. Marie-Davy found it, for firs 1.18, for 

 beeches 1.32, for sod 1.80. As to the influence of sunlight. Vines 

 quotes from Wiesner, who carried on his observations with special 

 precautious to prevent influence from other stimuli of the transpira- 

 tion. The results are given in the following small table, in which the 

 evaporation in full sunlight is taken as unity. 



