VESSELS AND TRACHEIDES 321 



elements of the wood, thereby confessing his adherence to the views 

 previously formulated by Westerrnaier, Godlewski and Janse, who all 

 maintain that the ascent of sap is a vital and not a purely physical 

 process. 



Strasburger, on the contrary, has endeavoured to disprove the 

 vitalistic theory experimentally. He has succeeded in showing that 

 poisonous liquids, such as solutions of copper sulphate or picric acid, 

 which rapidly kill the living cells of the wood, may nevertheless rise for 

 days in succession, in trunks 20 metres (65 feet) in height, right up to 

 the transpiring foliage. Strasburger's experiments are, however, open 

 to many objections, and are certainly not conclusive. On the contrary, 

 recent experiments carried out by Ursprung appear to support the 

 opposite view, according to which the living cells of the wood do take 

 an active part in raising water. Ursprung finds that, if a portion of a 

 branch or trunk is boiled, etherised, or cooled down sufficiently, so that 

 its living cells are rendered inactive, the transference of water is 

 invariably retarded to such an extent that the experimental plants 

 soon wither and die. 



3. The special uses of vessels and of tracheides. 



Between vessels and tracheides no very sharp distinction can be 

 drawn in respect either of length. or of diameter; nevertheless, these 

 two types of water-conducting element are so clearly differentiated in 

 most Angiosperms, that one is forced to enquire whether both are 

 employed for precisely the same purposes or not. Anatomical investi- 

 gation supplies an unequivocal answer to this question : for as 

 Schwendener long ago remarked, wherever this differentiation exists, 

 the vessels serve mainly to transport water over considerable distances, 

 whereas the tracheides are chiefly employed in satisfying local require- 

 ments. A comparison between a typical leaf and a typical stem brings 

 out this difference of function very clearly. In the stem the principal 

 channels of the transpiration-current consist mainly or exclusively of 

 vessels, while the lateral offshoots of these main channels forming in 

 the leaves a dense network, which serves to effect a uniform distri- 

 bution of water throughout the mesophyll are largely or entirely com- 

 posed of tracheides. Tracheides also predominate in the lateral anasto- 

 moses that connect adjacent vascular bundles, or different parts of the 

 secondary woody cylinder (the latter case being exemplified by the 

 Oak), with one another. 



The fact that vessels represent the principal water-paths, is indi- 

 cated not only by the great length of these structures, but also by their 

 width, which is on an average greater than that of tracheides. This 



argument is in no sense vitiated by the circumstance that tracheides 



x 



