304 CONDUCTING SYSTEM 



like sense. This particular adaptation of structure to function is 

 especially striking in a case such as that of the Dicotyledonous woody 

 cylinder, where two systems of conducting channels cross one another 

 at right angles. There is also evidently a certain correlation between 

 the length of the individual conducting elements and the activity of 

 translocation. It would, however, be going too far to assume that 

 great elongation of the conducting elements always indicates a high 

 rate of translocation, since translocation may also be accelerated by the 

 development of more numerous pits on the septa, by an increase in the 

 calibre of the conducting elements and a consequent diminution of 

 frictional resistance, or in other ways. Nevertheless, the correlation 

 in question can be readily demonstrated in a number of cases. The 

 water-conducting channels, for example, generally consist of long vessels 

 in stems, whereas much shorter elements prevail in the bundle-ends of 

 leaves. In the same way the carbohydrate-conducting bundle-sheath- 

 cells of leaves in general adjust their length very closely to the average 

 rate at which synthetic material passes through them. 



With regard to the second principal feature, namely, the provision 

 of pits, it should first of all be stated that numerous intermediate stages 

 may be found between the normal pitted condition of the transverse wall 

 and complete elimination of the septum. Most often the septa bear simple 

 pits which are circular or oval in outline, and which not infrequently have 

 their limiting membranes perforated by protoplasmic connections. Such 

 pitted septa are particularly characteristic of the various kinds of con- 

 ducting parenchyma (including xylem-parenchyma, medullary rays and 

 parenchymatous bundle-sheaths). If a pit is large, and its limiting mem- 

 brane very thin, there is a risk of rupture, when the osmotic pressure 

 differs considerably on the two sides. In order to guard against this 

 danger, large pits are often subdivided by ridge-like thickenings of the 

 wall, which afford mechanical support to the limiting membrane. 

 Eussow, among others, has observed such areolated pits in the secondary 

 phloem of a great many woody plants. 



In the case of the great majority of pits, the limiting membrane is 

 perforated by more or less numerous, very fine pores, through which 

 the neighbouring protoplasts communicate by means of exceedingly 

 delicate protoplasmic connecting threads (cf. Chap. I. B. 6) ; many 

 unpitted membranes display the same feature. It is uncertain how far 

 these protoplasmic connections represent open channels of translocation. 

 Considering the ease with which diffusion takes place through thin 

 portions of the cell-wall and through the associated ectoplasts, and in 

 view of the extreme tenuity of the protoplasmic filaments (their total 

 cross-sectional area only amounts to a fraction of the area of unperforated 

 membrane), it seems improbable that these connecting threads can play 



