220 THE MOVEMENTS OF WATER 



was cut was allowed to aid in sucking in the solution employed, the transference of 

 the dissolved substance took place twice as rapidly '. 



As rapid a current as this may therefore occur in at least some of the wood 

 elements, and no doubt when transpiration is very active and the general con- 

 ditions are favourable, the rapidity might become still more marked. As it is, 

 it is extremely rapid in comparison with protoplasmic streaming or rotation. In 

 the different conducting elements water will certainly not travel with exactly the 

 same speed, while any narrowing or broadening of the conducting channels will 

 undoubtedly influence the maximal as well as the mean rapidity of the water-current. 

 It is impossible to determine the mean for the water-current until the precise sectional 

 area concerned is known. Since, however, the conducting area in the 'vascular 

 bundles of a sunflower is not very great, the water must travel with great mean 

 rapidity in order to convey upwards the 0-865 kilos (\\ Ib. or i pint) of water which 

 Hales found a sunflower could exhale in twelve hours in the form of water- vapour 2 . 

 In the stem of a beech-tree the water must travel, according to the calculation of 

 Schwendener, at a daily rate of 2 metres in order to counterbalance the average loss 

 by transpiration during the summer months. 



SECTION 36. The Mechanism of Water-transport. 



How and by what means the water is so rapidly transferred even 

 to the summits of the tallest trees has not as yet been satisfactorily 

 explained. It has unfortunately not even been determined whether the 

 aid of living cells is quite unnecessary, although if this were so, the 

 problem would be much restricted and would become almost entirely 

 a physical one. 



Bohm has shown indeed that sufficient water may be transferred 

 through a dead piece of stem to keep the upper living leafy portion 

 fresh and turgid for a few days. Strasburger 3 has obtained similar results 

 by killing portions of the stems of living trees, &c., often for a distance of 

 as much as 1 2 metres (nearly 40 feet), by heat or by watering with poisonous 

 substances. In his interpretation of these facts, however, he does not 

 mention the closely allied question, as to whether the assistance of living 

 cells is not necessary to maintain the power of the dead tissue to conduct 

 water for any length of time 4 . The gradual disappearance of the con- 

 ductivity for water may result from the loss of such cells, and need not 



1 Examples by Pfitzer, 1. c., p. 213 ; Strasburger, 1. c. Cf. Sachs, 1. c., p. 171. 



' Hales, Statics, 1748, pp. 3, 10. Cf. also Sachs, 1. c., p. 153. 



3 Bohm, Ber. d. Bot. Ges., 1889, Generalvers., p. 55; 1892, p. 622; 1893, p. 203. Hartig 

 gave in 1853 (Bot. Zeitnng, p. 313) similar observations made upon stems killed by the absorption 

 of solutions of iron salts. Strasburger, Bau u. Verricht. d. Leitungsbahnen, 1891, p. 645 ; Uber das 

 Saftsteigen, 1893, p. n. 



* For other views see Schwendener, Sitzungsb. d. Berl. Akad., 1892, Bd. XLIV, p. 932. 



