306 CONDUCTING SYSTEM 



way, when the transverse walls assume an oblique position, or where, 

 in other words, the conducting elements become prosenchymatous. 

 Examples of this latter condition are furnished by the sieve-tubes of 

 secondary leptome-strands, by phloem-parenchyma (cambiform cells), 

 and by ordinary conducting tracheides. Where it is the pit-membranes 

 that are enlarged, the pit-cavities are dilated in a trumpet-shaped manner; 

 this condition is analogous to the first of the above-described modes of 

 expansion of the entire septum. Dilated pits occur in many Mono- 

 cotyledonous endosperms and in the bast of certain Ltltaceae. They 

 are mentioned here principally because, physiologically speaking, they 

 represent a stage intermediate between the "simple" and the "bordered" 

 types of pit. It must be noted, however, that the peculiarities of the 

 bordered pit do not merely serve to increase the area of the diffusion- 

 membrane, but are, on the contrary, determined by special considerations 

 connected with the conduction of water. 



In the preceding paragraphs we have only dealt with the histological 

 features of the conducting system which facilitate and accelerate the 

 progress of translocation. There are, of course, also purely physiological 

 factors, that tend to produce the same result. Under this latter head 

 must be included, in the first place, those rotating and circulating move- 

 ments of protoplasm which expedite osmosis by effecting the mechanical 

 mixture of the diffusible substances ; further, all arrangements that 

 serve to maintain a concentration-gradient in a series of cells by alter- 

 ing the chemical character of the diffusing substances, or to accentuate 

 this gradient, when it has once been established. Here we become 

 involved in the still mysterious question as to the part played by the 

 living protoplasts in the process of translocation, a subject which does 

 not properly lie within the scope of the present work. 156 



77. THE STRUCTURAL ELEMENTS OF CONDUCTING 



TISSUES. 



A. CONDUCTION OF WATER AND MINERAL SALTS. 



Almost every one of the numerous researches that have been carried 

 out upon the conduction of water in plants deals mainly with the 

 woody cylinder of Dicotyledons and Gymnosperms. The familiar 

 experiment in which it is shown that the leaves of a transpiring shoot 

 remain fresh and turgid, after a ring of bark has been removed at its 

 base, whereas the foliage soon withers if the wood is cut through, was 

 known to Stephen Hales. Since similar results are obtained with the 

 stems of herbaceous Dicotyledons, it may be inferred that conduction of 

 water takes place solely in the xylem portions of the vascular bundles, 

 previous to the formation of a secondary cylinder in Dicotyledons, and 



