GENERAL CHARACTERISTICS 303 



crude or nutritive sap and a descending tlow of elaborated or formative 

 sap ; the old theory of the circulation of sap modelled on these lines 

 was not only a far too extensive generalisation, but was also based upon 

 views regarding individual translocatory processes which were in them- 

 selves inadequate, and which have since been greatly modified, mainly 

 owing to the important discoveries of Sachs. 



The conducting tissues form the most complicated system in the 

 plant-body, and include the greatest variety of constituent elements, 

 comprising as they do both syncytes and single cells, and dead as well 

 as living elements. This circumstance is quite in harmony with the 

 diversity of the materials that have to be transported ; for, in accordance 

 with the principle of division* of labour, a regular and uninterrupted 

 flow of material is most likely to be maintained, where separate paths of 

 translocation are provided for the most important classes of substances. 

 There is, besides, a mechanical reason for the prevailing segregation of 

 conducting channels, inasmuch as different substances very often have 

 to be transported simultaneously in opposite directions. 



On account of their diversity, the several conducting tissues possess 

 but few characteristics in common with one another ; such as they are, 

 these common characters are all referable to the influence of a funda- 

 mental principle, which has as its aim the modification of any structural 

 features that would tend to interfere with the flow of material. 



The translocation of a given substance may depend altogether upon 

 molecular motion ; it may, on the other hand, be a movement en masse, or, 

 finally, the two modes of transport may be combined. The question as 

 to the energy employed in translocation is one which concerns the pure 

 physiologist, and which, therefore, need not be considered here. It 

 may, however, be remarked that the presence of numerous transverse 

 septa in the conducting channels must obviously retard translocation, 

 whether this process takes place by means of diosmosis or filtration, or 

 by any other mode of movement, at any rate where the material moves 

 in the cell cavities and not in the walls. Hence it is an essential con- 

 dition of the efficiency of a conducting tissue, that the number of 

 transverse septa should be reduced and their permeability towards 

 migrating materials increased as far as possible. One accordingly finds, 

 in the first place, that all conducting elements are more or less markedly 

 elongated ; further, that their transverse walls are thin, and often 

 provided with numerous pits or even with actual perforations ; and, 

 lastly, that the surface available for diffusion is often augmented by 

 some special arrangement. 



Little need be said here concerning the first of these general charac- 

 teristics of conducting tissues. Wherever translocation takes place 

 chiefly in one direction, the conducting elements become elongated in a 



