TRANSLOCATION OF CARBOHYDRATES 261 



and referred to an experiment made by Schimper upon Plantago 

 in terms which imply that it may be regarded as a critical one. 

 However in another part of the book he admitted that as the 

 sieve-tubes often contain sugars, they may under some circum- 

 stances exercise the subsidiary function of conducting sugar, 

 but he held that the main mass of the sugar is undoubtedly 

 translocated through the " conducting parenchyma." Hence 

 the question as to the exact paths followed by the sugars, after 

 passing from their places of formation, cannot be regarded as 

 generally agreed upon. The very formulation of the problem 

 was confused prior to Czapek's work, for apparently little 

 account was taken of the fact that movement may occur in two 

 ways. The first of these is brought about by differences in the 

 concentration of the contents of neighbouring cells, differences 

 depending upon rates of production and consumption or trans- 

 formation of the material in question. By such |means diffusion 

 is promoted and this may go on in the less specialised cells 

 through any of their walls, as far as can be gathered from their 

 structure. Haberlandt, however, thinks it highly probable that 

 in the case of elongated cells the permeability of the end walls 

 is greater than that of the lateral walls and that elongation 

 indicates the direction of conduction of cell contents. 



[In addition to simple diffusion, it is probable that a good 

 deal of adsorption of sugars goes on at the surface of the 

 protoplasm lining the cell and this doubtless assists in trans- 

 ferring these substances.] 



It is clear that for such diffusion no highly specialised cells 

 are required and also that its operation may be adequate to 

 transfer materials over short distances, as through the un- 

 differentiated cells near growing points, through endosperm 

 and parenchymatous tissues. But whenever there is need 

 for a comparatively rapid transport of assimilates over con- 

 siderable distances, to meet the demands made by vigorously 

 growing parts or by organs of storage, this slow diffusion must 

 be supplemented. Just as the need for a rapid supply of water 

 has called into existence a highly specialised water-conducting 

 tissue, so, it may be held, has the need for rapid translocation 

 of assimilates been instrumental in evolving a specialised food- 

 conducting tissue — the phloem — by means of which both 

 proteids and sugars are distributed. 



A priori there is no reason to suppose that the sieve-tubes 



