THE MECHANISM OF WATER-TRANSPORT 221 



necessarily be due either to any blocking of the conducting channels or to 

 changes taking place in them. If the loss of conductivity is due to the 

 absence of living cells, injection with water should temporarily restore it, 

 and indeed Strasburger" 1 has found that shoots killed in various ways and 

 dried have their conductivity restored for a time after such injection. It is 

 necessary also to determine whether the loss of conducting power in the 

 duramen is owing to the blocking of its tracheal channels 2 , or is due to 

 the gradual death of all the living cells it contains. 



On the other hand, no conclusive proof has been brought forward 

 to show that living cells aid in the transference of water through the 

 conducting channels when these are once established 3 . The apparent 

 absence of any other explanation cannot be regarded as a proof that 

 vital actions aid in the transport of water, for when dealing with 

 complicated phenomena of this kind, it is quite possible that certain 

 co-operating factors or conditions may be overlooked or insufficiently 

 considered. What now appears inexplicable or incomprehensible might 

 be easily understood were our knowledge more complete. It seems 

 probable indeed that water may perform its entire journey in dead tissue- 

 elements. The fact that dyes which do not penetrate the living cells 

 rapidly ascend with the transpiration-current points to this conclusion, 

 especially when the dye is one which, like indigo-carmine, does not injure 

 the living cells in any way. Along with the dye, the water which carries 

 it must also travel upwards through dead vessels, without entering any 

 living cells which might pump it upwards to a higher level. Even if 

 the maintenance or production of the conditions necessary for the con- 

 duction of water is due to the living cells injecting water into the vessels 

 and thus exercising a pumping action, the fact still remains that the actual 

 transference takes place almost entirely through dead tissue-elements. 



The facts already given suffice to show that water travels in the 

 tracheal elements, and largely in their lumina. This is admirably illus- 

 trated by the results of injection with a substance solidifying on cooling. 

 If a shoot is injected from the cut surface upwards for a short distance 

 with fluid gelatine, cocoa-butter, or paraffin, the leaves fade soon after the 

 injected fluid solidifies, and the withering is as rapid when the cut surface 

 is immersed in water as when it is exposed to air 4 . Water cannot be 



1 Strasburger, 1. c., 1891, p. 657. 



On the blocking of the tracheae see Wieler, Biol. Centralbl., 1893, Bd. xm, p. 586. 



8 The behaviour at low temperatures (cf. Schwendener, Sitzungsb. d. Berl. Akad., 1892, Bd. XLIV, 

 p. 945) is no proof of this, for the diminished conductive power of the tracheal channels appears to 

 be explicable solely from physical causes. Cf. Sect. 37. 



4 Elfving (Bot. Zeitung, 1882, p. 714) and Vesque (Ann. d. sci. nat., 1884, vi. ser., T. xix. 

 p. 188) used cocoa-butter; Scheit (Bot. Zeitung, 1884, p. 201) gelatine. More critical experiments 

 were carried out by Errera (Bot. Zeitung, 1886, p. 16) with gelatine. Confirmatory results have 



