TRANSLOCATION OF CARBOHYDRATES 265 



udation of the contents. In the case of the gourd stem, in which 

 the sieve-tubes are of large diameter, Fischer found that this 

 partial emptying extended over one or two internodes from the 

 cut. This could be seen by comparing the distribution of the 

 contents of the sieve-tubes in such cut stems and in stems which 

 had been dipped in boiling water for a few minutes to produce 

 coagulation previous to cutting. After the latter treatment a 

 fairly uniform distribution was found and. the tubes were well 

 filled. On the other hand in unboiled stems it was found that 

 some of the contents had been expelled through the sieve-plates, 

 which acted as filters and withheld the larger granules against 

 their opposing surfaces. 



Hence the contents of active sieve-tubes appear to be under 

 pressure. Sachs drew attention to the pressure caused by the 

 turgidity of the neighbouring cells of the parenchyma and 

 thought that the driving power of translocation came from this 

 pressure being less at growing points than at the places of 

 formation of assimilates. But Czapek remarked in his paper 

 that it is very improbable that this is the sole cause of the 

 movement of the contained assimilates. He considered that the 

 protoplasm itself in some way takes in the substances, combines 

 with them and gives them out on the other side where the 

 process is repeated in the next element. However, it appears 

 impossible to explain satisfactorily the mechanism of translo- 

 cation until much more is known about the physical chemistry of 

 plant cells, in particular the phenomena of specific permeability 

 of the protoplasm and adsorption of dissolved substances by 

 colloids. 



Previous to the observations of Fischer and Lecomte the 

 aggregation of contents more towards one end of the elements 

 than the other, as seen in ordinary spirit material (fig. 8), had 

 been held to indicate the actual direction of motion in the sieve- 

 tubes. But these two investigators showed that such effects 

 were due only to the cutting of the stem and to the withdrawal 

 of water from the sieve-tubes by the alcohol, which thus caused 

 the protoplasmic lining to contract. 



Coinpanion-cclls\ — Associated with the sieve-tubes of the 

 flowering plants are certain cells known as " companion-cells " 

 (figs. 8-10). These are in reality sister cells of the sieve-tube 

 segments, formed by the longitudinal division of a mother cell 

 to give two unequal daughter cells. 



