THE CONVEYANCE OF WATER IN TRANSPIRING PLANTS 211 



a twig immersed in water, and in the arrangement shown in Fig. 26 this 

 causes a gradual rise of the mercury in the arm a 1 . 



The mercury is seldom raised to a height of more than 10 to 30 cm., 

 especially in the case of herbaceous plants, for at that pressure air is in 

 most cases sucked in through the surface of the branch ; if, however, the 

 outer layers are more impermeable to air, the mercury may be raised still 

 higher. Thus Th. Hartig observed a negative pressure equal to 76 cm. of 

 mercury when he inserted manometers into holes bored as far as the 

 alburnum of the wood cylinder. A similar sucking-force can be traced 

 right down to the roots when transpiration is active. The amount of 

 water present increases towards the roots, and the negative pressure as 

 a rule steadily decreases, but at the same time, by 

 means of a series of manometers, an irregular rise 

 and fall of the sucking-force can be observed, 

 especially in trees, which is similar to that noticed 

 with regard to the internal gaseous tensions (Sect. 

 32). The causes which induce or influence these 

 variations are similar in both cases 2 . 



These results, the essential principles of which 

 were correctly interpreted by Hales, prove that the 

 upward current of water in a transpiring plant is 

 not due to any pumping action of the roots or 

 stem, such as produces an exudation-pressure. The 

 latter is reached only when the plant is fully 

 saturated with water, and hence at first the cut 

 surface of a stem or root sucks in water, and then 

 after a shorter or longer interval begins to bleed. 

 Moreover, this exudation-pressure is not attained 

 in many plants, and even where it is present, it is 

 usually insufficient to raise the water to the summit 

 of a tree, while, as a general rule, the amount of water which escapes from 

 a cut stem is less than would have been transpired by the upper leafy portion 3 . 

 By this backwardly transmitted suction the water in the roots is drawn 

 into the vascular conducting channels, and at the same time the conditions 

 necessary for fresh absorption from the soil outside are created by the 



1 Hales, Statics, 1748, pp. 26,48, &c. ; Meyer, Pflanzenphysiol., 1838, Bd. n, p. 70; Th. Hartig, 

 Bot. Zeitung, 1861, p. 17, and 1863, p. 280. On the corrections necessary see v. Hohnel. On ihe 

 negative pressure of the tracheal air, 1876, p. 6, see also Unger, Sitzungsb. d. Wien. Akad., 1864, 

 Bd. XLIV, p. 8 (Sep.); Bohm, Ber. d. Bot. Ges., 1889, Generalvers , p. 53, and Bot. Centralbl., 

 1890, Bd. XLII, p. 234; Strasburger, Leitungsbahnen, 1891, p. 782; Vines, Ann. of Bot., 1896, 

 Vol. x, p. 292. 



2 Cf. Schwendener, Sitzungsb. d. Berl. Akad., 1896, Bd. XXXIV, p. 583. 



3 Examples by Hofmeister, Flora, 1862, p. 107 ; Sachs, Lehrb., 3. Aufl., p. 598, and Arb. d. Bot. 

 Inst. in Wiirzburg, 1873, Bd. I, p. 28S. 



P 2 



