200 PHYSIOLOGY OF NUTRITION. 



the night ; or it may be entirely absent daring the day, only taking 

 place at night. This is due to the fact that during the day the 

 sap driven into the stem by the roots goes to cover the loss by 

 transpiration, while at night-time at least a. portion of the sap 

 can escape, since now the evaporation of water, as the result of 

 external conditions, is usually much reduced. As the season 

 advances and the leaves of the birch unfold, the outflow of sap 

 completely ceases. The transpiration of the tree has now become 

 very considerable. 



The observations on the flow of the sap from the birch are to be 

 associated with meteorological observations, in order that we may 

 be enabled to follow closely its dependence on external conditions. 

 We note the character of the clouds, the height of the barometer, 

 and observe the rainfall, the hygrometric condition of the air, and 

 the temperature of the air and of the soil. The necessary appara- 

 tus rain gauge, maximum and minimum thermometers, hygro- 

 meter (e.g. the well-known wet and dry bulb hygrometer of 

 August) can all be obtained of Muericke in Berlin. To determine 

 the hygrometric condition of the air from the readings of the 

 August hygrometer, we employ the tables of Jelinek, Vienna, 

 1876. 



If we evaporate birch sap we obtain a residue composed of 

 organic and mineral substances. We can easily ascertain the 

 presence of the latter by evaporating the sap to dryness in a plati- 

 num dish, and igniting the residue to decompose the organic sub- 

 stances. The mineral substances remain behind. Fresh birch sap 

 is slightly acid in reaction. On boiling it, a coagulum of proteid 

 material separates out. If we treat a small quantity of the sap 

 with a few drops of Sulphuric acid, and then boil, replacing the 

 water lost by evaporation, we obtain a fluid which, when added to 

 hot Fehling's solution, gives a precipitate of Cuprous oxide. The 

 crude sap contains, viz., cane-sugar, which, by boiling in presence 

 of Sulphuric acid, is converted into grape-sugar, and this exerts a 

 reducing action. 



77. The Influence of External Conditions on the Flow of Sap 

 from Decapitated Plants. 



We decapitate vigorous pot plants of Cucurbita, Helianthus, 

 Ricinus, or Begonia, and fit the stump of the stem with a tube as 

 in Fig. 73. The soil in the flower pots must not be too moist, 



