CONDUCTION OF WATER AND MINERAL SALTS 307 



permanently in Monocotyledons, where secondary wood is ordinarily 

 altogether absent. If the cut ends of such herbaceous stems are dipped 

 into a coloured solution (say, of aniline blue or eosin), mere inspection 

 will show, that the rapid rise of the coloured liquid which results if 

 transpiration is sufficiently active, takes place only in the vascular 

 bundles; microscopic examination, carried out with suitable precautions, 

 proves conclusively that the solution travels exclusively in the xylem 

 strands. The translucent stems of species of Impatiens, and flowers 

 with white petals, furnish very suitable material for experiments of 

 this nature. The same method was employed by the author, in order 

 to demonstrate the water-conducting capacity of the central strand in 

 the stems and setae of various Mosses. 



The identification of the tissues within which conduction of water 

 takes place is a comparatively easy matter; it is, however, much more 

 difficult to point out the actual tissue- elements through which the 

 current travels. The difficulty arises very largely from the fact that 

 the secondary woody cylinder of Gynmosperms and Dicotyledons 

 which has formed the principal subject of. decisive experiments is, 

 physiologically speaking, a heterogeneous structure, the anatomical 

 characters of which represent a compromise between two conflicting 

 intiuences. The woody cylinder is in fact responsible both for the 

 mechanical strength of the stem and for the transport of water through 

 this organ ; consequently it is not an easy matter to predicate with 

 certainty regarding each organic element of the woody cylinder whether 

 it subserves both of these functions or only one (cf. Ch. XII.). 



The mineral salts absorbed from the soil and the water rising from 

 the roots undoubtedly travel along identical paths. This conclusion 

 may reasonably be drawn from the fact that, when transpiration is 

 active, the ascending current of water (transpiration current) can be 

 shown to carry with it a considerable quantity of the absorbed mineral 

 salts ; it seems improbable that the remaining portion of the mineral 

 matter, which travels upwards in immediate response to the varying 

 demands of nutritive metabolism, should follow a different path. 



1. Histology of the Water-Conducting Tubes. 



The elements that are utilised for the conduction of water are 

 the wood-vessels or tracheae and the tracheides ; collectively these 

 form a water-conducting system which extends into every part of the 

 plant-body. We must first of all deal with the histological features of 

 these elements. 157 



Vessels and tracheides are on the whole very similar in structure. 

 The chief difference between the two consists in the fact, that a tracheide 

 is surrounded by a perfectly continuous cell-membrane or, in other words, 



