262 THE MOVEMENTS OF WATER 



for it is impossible to tell how far the results recorded have been influenced 

 by the exudation-pressure, by the characters of the boring, by transpira- 

 tion, &c. It has, however, been definitely established that in spring the 

 power of exuding water travels slowly up the stem, so that sap escapes 

 from a hole made at the ba c e sooner than from one made at a higher level a . 

 When transpiration becomes active, the escape of water usually ceases 

 first in the upper borings. 



Quality of the escaping sap. In many plants the sap almost resembles spring- 

 water, but in other cases trifling or even large quantities of organic substances are 

 dissolved in it. The sap of the maple and the birch contains, in addition to small 

 quantities of organic acids and proteids, so much sugar as to repay extraction, while 

 the sap may be fermented to form an alcoholic beverage. In the sap of Acer 

 platanoides Schroder found 1-15 to 3-4 per cent, of cane sugar, which is not far short 

 of the amount present in the sap of Acer saccharinum, estimated by Clark at 

 3-57 per cent. 2 On the other hand, tobacco and potato plants, sunflowers, vines, 

 and indeed most plants, yield a very dilute and watery sap. Since these plants 

 in some cases generate a higher bleeding-pressure than the maple does, there 

 appears to be no direct relation between the osmotic concentration of the sap 

 and the exudation-pressure 8 . Any accumulation of organic substances in the water- 

 channels will render possible a rapid transference of reserve food-material to the 

 transpiring organs (Sect. 106). 



The composition of the sap is not constant, and may change as bleeding 

 continues, owing to the large amounts of dissolved material which may be lost by 

 the plant, even although a regulatory formation of fresh sugar may be excited ; still, 

 when bleeding is active, the sap gradually becomes poorer and poorer in the latter 

 constituent *, although the percentage of sugar often undergoes irregular variations of 

 indeterminate origin. Even in the watery sap of the sunflower, Ulbricht was able 

 to detect a still further dilution after prolonged bleeding c , and in other experiments 

 also a fall in the specific gravity of the sap has been noted. A change may also 

 occur after a time in the acid or alkaline reaction of the escaping sap 6 . 



Observations upon the sap are recorded by Knight, Beitrage z. Physiol. v. 

 Treviranus, 1811, p. 162; Biot, Compt rend., 1841, T. xn, p. 357; Unger, 

 Sitzungsb. d. Wien. Akad., 1857, Bd. xxn, p. 445; Peckolt, Jahresb. d. Chemie, 

 1862, p. 89; Berger, Jahresb. d. Agriculturch., 1867, p. 109; Neubauer, Jahresb. d. 

 Botanik, 1874, p. 854; Rotondi u. Ghizzoni, Jahresb. d. Botanik, 1879, n, p. 366 ; 



1 Such observations have been made by Knight (Treviranus, Beitrage z. Pflanzenphysiol., 1811, 

 p. 257) ; Briicke (1. c., p. 83) ; Schroder (Versuchsst., 1871, Bd. XIV, p. 1 20) ; Horvath (Beitrage, &c., 

 18 77, P- 55). 



a Schroder, Versuchsst., 1871, Bd. xiv, p. 118 (also for Betuld); also Jahrb. f. wiss. Bot., 

 1869-70, Bd. vn, p. 261 ; Clark, Flora, 1875, p. 509. 



:i See Wieler, Cohn's Beitrage, 1893, Bd. vi, p. 158. 



4 See also A. Fischer, Jahrb. f. wiss. Bot., 1890, Bd. XXII, p. 156. 



8 Ulbricht, Versuchsst., 1865, Bd. VI, p. 468 ; 1866, Bd. vn, p. 185. 



* C. Kraus, Ber. d. Bot. Ges., 1884, p. 115; Forsch. a. d. Geb. d. Agriculturphysik, 1888, 

 Bd. x, p. 6, &c. 



