316 CONDUCTING SYSTEM 



feel disposed to place most reliance upon those experiments in which 

 the rise of the coloured solution itself is observed, rather than some 

 secondary phenomenon such as the coloration of the walls. Elfving, 

 for example, has shown that when a watery solution of eosin is sucked 

 through the wood of Taxus baccata or other Conifer, the cavities of the 

 tracheides become filled with the deep-red liquid, while their thickened 

 walls remain unstained. In any case the method of coloured solutions 

 is inadequate, where it is desired to determine the individual water- 

 paths, and not merely the general course of the transpiration-current. 



Various physiologists have devised experiments for the special 

 purpose of proving that the transpiration-current moves in the cavities of 

 the vessels and tracheides, and not as has been alleged especially by 

 linger and by Sachs within the lignified walls of these elements. All 

 these experiments consist in observing the effect of artificial occlusion of 

 the tracheal cavities upon the transpiration-current. Elfving injected 

 branches of Yew, Ash, Oak, etc., and also haulms of Indian Corn, with 

 melted cacao-butter (previously coloured red) ; after allowing the fat to 

 congeal, he cut a fresh surface and attempted to force water through 

 the stem, but invariably without success. Wood is thus evidently 

 rendered impervious to water, when the cavities of the vessels and 

 tracheides are blocked with fat. In order to meet the objection that 

 the injection of fat interferes with the permeability of the cell-walls, 

 both Scheit and Errera replace the cacao-butter by gelatine coloured 

 with eosin or Indian ink. In Errera's experiments, a branch was 

 allowed to absorb the gelatine through its cut end by the force of its 

 transpiration, with the result that the leaves soon drooped. In this 

 way it has been clearly demonstrated that the transpiration current 

 travels in the cavities of the conducting tubes. 



In searching for arguments in favour of the water-conducting function 

 of vessels and tracheides, anatomical and histological evidence can only 

 be regarded as of secondary importance in comparison with experi- 

 mental data. Nevertheless, though the evidence of anatomy is not in 

 itself conclusive, it cannot be altogether ignored in a professedly 

 anatomical treatise. In this connection we may first of all lay stress 

 upon the circumstance, that the vessels and tracheides invariably form a 

 continuous system in every part of the plant-body, from the finest 

 rootlets up to the extreme tips of the uppermost leaves. If these 

 structures were merely reservoirs of water, as has been suggested in 

 certain quarters, this continuity would be unnecessary, not to say 

 incomprehensible. A further argument may be derived from the 

 elongated tubular form of the vessels and tracheides, a fact which, con- 

 sidered in conjunction with the experimental evidence, clearly indicates 

 their conducting function. Another noteworthy circumstance is the very 



