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. 



First Day 



Second Day I Third Day (Fourth Day' Fifth Day 



I 



Combustible material j 450 310 



Ash I 1 1 60 980 



Total dry weight J 1610 1290 



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 22. 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 



Date of Flow Sugar ! Protein Malic Acid Ash 



April 5 12,500 ... .... 



April 11 13.500 33? 500 



April 17 10,900 21 ... 640 



May 2 I 10,100 6 ... 1080 



May 19 9,400 6 437 



May 22 6.900 . . ... .... 



»Ulbricht, R., Ein Beitrag zur Kenntniss der Blutungssäfte einjähriger Pflanzen. Landw. Versuchest. 

 6: 468-474. 1864. [Idem, same title. Ibid. 7: 183-192. 1865.] 



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

 Landw. Versuchest. 14: 1 18-146. 187 1. 



