214 THE MOVEMENTS OF WATER 



certain monocotyledons which do not undergo any secondary increase in 

 thickness, the same vascular bundles must in part preserve their conductivity 

 for a very long time. No detailed investigations have as yet been made to 

 determine whether, and to what extent, a compulsory demand may prolong 

 the conductivity of the alburnum and hinder its conversion into duramen. 



The individual elements of the xylem are of unequal functional value, 

 and hence on this account alone it might be anticipated that the spring and 

 autumn xylem would neither be concerned to the same extent, nor apparently 

 in the same manner, with the transference of water l . 



From what has already been said, it appears that the tracheae and 

 tracheides are the most important agents in the conveyance of water, but it 

 has yet to be determined whether and how far the wood-fibres 2 and other 

 tissue-elements aid in the process. Physiological transitional forms may 

 possibly exist between wood-fibres, tracheides, and tracheae, for which 

 conclusion there is distinct morphological evidence. It is possible that 

 some of these elements serve more for the conduction, others for the 

 storage of water. The activity of conduction may apparently be tem- 

 porarily inhibited or permanently suppressed at an early period in 

 individual conducting elements. 



Similar results are obtained by other means, and especially by the 

 use of coloured solutions, for the colouration which the parts of the wood 

 undergo serves to indicate the path of the transpiration current. The 

 dyes are actually carried along by the latter, as is shown by the fact 

 that when transpiration is suppressed, indigo-carmine, eosin, or aniline- 

 blue are absorbed only with extreme slowness, whereas in a transpiring 

 plant they ascend with relative rapidity in those parts of the wood which 

 have already been recognized as forming the paths by which the water 

 travels. The dye gradually diffuses from the conducting channels to 

 neighbouring non-conducting tissues and tissue-elements. Nevertheless, 

 the experimental results obtained, if critically interpreted, leave no 

 doubt that the dyes are at first carried with the water along the tracheides 

 and tracheae. This is also the case with those dyes which neither injure 

 nor penetrate living cells (indigo-carmine, aniline-blue, &c.), and in such 

 cases the water current must pass through dead elements, as far as the 

 dissolved crye can be traced. 



It must be remembered that all tissues can absorb water to a greater 

 or less extent, and can surrender it again to neighbouring parts. It is 

 the feeble conducting power of the parenchyma, \\hich necessitates 

 the formation of special conducting channels (Sect. 33), and the readiness 



1 Cf. Strasburger, ]. c., p. 592 ; Schwendener, Sitzungsb. d. Berl. Akad., 1892, Bd. XLIV, p. 927. 

 and the literature there given. 



3 Cf. Strasburger, Uber d. Saftsteigen, 1893, p. 25 ; Schwendener, 1. c., 1892, p. 930. 



