78 



FOEEST INFLUENCES. 



From this it appears that it is the direct sun's rays tliat most pro- 

 mote trauspiratiou. When ill dift'iised light the phiiit transpires only 

 a third as much, and in darkness only a quarter. The application of 

 this to forests is evident, because there at any time of day a considera- 

 ble part of the foliage is in shade. 



The estimates of transpiration are very numerous and it is not easy 

 to get from them an estimate of the amount for plants in terms of 

 evaporation or precipitation. In the following table those have been 

 selected which were most easily expressed in terms of quantities ob- 

 served meteorologically. The duration of the active season was taken 

 into account in each case. 



Observer. 



Hales . 



Schleiden . 



Vosel . 



Hartis 



Piafl'.': 



Hriliiiel 



Scliiibler 



Haberlandt . 



Plant. 



Transpiration expressed as 

 rainfall equivalent in inches. 



Daily. 



Annual. 



Inches. 



Sunflower O.VS 



1 Cabbage ! 0.12 



j [Grapevine ! 0.03 



[iHop 0.05 



.jClover itoats j 0.11 



( 4 vears beeches ] 0. OO:! 



I 'lycars tirs 0.002 



■) Wheat field ' 0. 62 



I iBarley field 0.56 



! 24 year.s mixed forest .... 0. 021 



Oak 



115 years beeches 



Low spear grass 



Oats ^ 



Barley 



0.21 

 0.05 

 0.08 

 0.13 

 0.07 



Mm. 

 3.30 

 3. 05 



.76 

 1.27 

 2.79 



.076 



.051 

 15.75 

 14. 22 



.533 

 5.33 

 1.27 

 2.03 

 3.30 

 1.78 



Inchefi 



1.5.2 



14.4 



4 



5.6 

 13.6 

 0.43 

 0.28 

 4.4 

 3.9 

 3.8 

 193.2 (?) 

 9.2 

 14 

 9.1 

 4.9 



Mm 

 381 

 361) 

 101 

 142 

 345 



11 



7 



112 



99 



Bti.5 



233 

 355 

 231 

 124 



Authority. 



Eberniayer. 



Do. 



Do. 



Do. 



Do. 



Do. 



Do. 



Do. 



Do. 



Do. 

 Austrian Met. Journal. 

 Sachs. 

 Duchartre. 



Do. 



Do. 



The best that can be done with these very variable measures is pro- 

 bably to take a mean of the values of Hartig and llohnel. This gives 

 the transpiration of the Ibrest as G. 5 inches (105 m.) for central Europe, 

 or about one quarter of the precipitation. That this is not much too 

 large is indicated by Pfafl's results, which appear to be excessive, 

 and that it is not much too small is indicated by those of Vogel. 



The most elaborate investigations on transpiration of forest trees were 

 made by F. B. Hohnel, and since a discussion of these appears in the 

 report of the chief of the forestry division (Mr. B. E, FernoAv) for 1889, 

 I quote his language on this subject: 



The quantity of water so used is as varial)le as the amoimt of precipitation and in 

 fact within certain limits dejyeuds largelj- upon it. That is to say, a plant will tran- 

 spire in proportion to the amount of water which is at its disposal. Transpiration is 

 also dependent on the stage of development of the plant, on the nature of its leaves 

 and amount of its foliage, on temperature, humidity, and circulation of the air, on 

 intensity of the sunlight, and on temperature and structure of the soil and on other 

 meteorological conditions. Kain and dew reduce transpiration, wind increases it. 

 The amount of transpiration depends considerably upon tiie thickness of the leaves, 

 therefore the surface of the foliage is not a reliable measure, but it should be com- 

 pared with the weight. ' 



With so many factors to vary them the values which may be given for the amount 

 of transpiration of various kinds of trees can only be appro.xinuitious of its range 



