142 



PHYSIOLOGY OF NUTRITION 



very interesting. In Ulbricht's 1 experiments, potato tubers planted on April 

 ii, produced stems that bloomed on July 5. On July 9 the stems were cut off 

 at from 4 to 6 cm. above the soil. The sap that exuded during the next five 

 days was collected, the exudation for each day being kept separate, so that 

 five portions of sap were available for analysis, the results of which are shown 

 in the following table. The quantities (stated in milligrams) refer to a liter of 

 sap in all cases. 



These numbers show plainly that the total solids consisted mainly of mineral 

 substances, but this statement is still further emphasized by the fact that the 

 combustible material does not represent organic matter alone, for nitric acid 

 and some other inorganic substances are vaporized during incineration, so that 

 it is certain that the sap always contained more inorganic substances than the 

 data show. This result was to be expected, since the ascending water current 

 distributes absorbed soil solution throughout the plant. The presence of organic 

 substances in sap extruded from the xylem may be explained by the fact that 

 the soil solution does not enter this tissue directly, but is transferred into the 

 wood soon after its entrance. It can hardly be supposed that parenchymatous 

 cells, which are so rich in organic substances, should excrete nothing but inor- 

 ganic materials into the vessels. 



The composition of sap excreted in early spring is very different from that 

 of sap excreted in summer. Birch sap was collected from an opening in the 

 tree trunk just above the soil surface, on each of six different days, between 

 April 5 and May 2 2. 2 The sugar, protein, malic acid, and ash contents per liter 

 of sap are given below, in milligrams, together with the dates on which the sap 

 was collected. It is apparent from these analyses that the sap contains less in- 



Date of Flow 



April 5 

 April 11 

 April 17 

 May 2 

 May 19 

 May 22 



Sugar 



Protein 



Malic Acid 



12,500 



13,500 

 10,900 



10,100 



9,400 



6,900 



21 

 6 

 6 



332 



437 



Ash 



500 



640 



1080 



Landw. Versuchsst. 



1 Ulbricht, R., Ein Beitrag zur Kenntniss der Blutungssafte einjahriger Pflanzen. 

 6:468-474. 1864. [Idem, same title. Ibid. 7: 185-192. 1865.] 



2 Schroeder, Julius, Die Fruhjahrsperiode der Birke (Betula alba L.) und des Ahorn (Acer platanoides L.) 

 Landw. Versuchsst. 14: 1 18-146. 1871. 



