TRANSLOCATION OF CARBOHYDRATES 285 



were present." In the above-mentioned paper, and in one on 

 the sieve-tubes of Pinus, Hill supported the view that the 

 cumulative effects of the threads are probably of great im- 

 portance in translocation. But in a later paper, on the sieve- 

 tubes of Angiosperms (1908), he considered it " . . . highly un- 

 likely that they serve for anything more than the conveyance of 

 impulses from cell to cell " but that "... owing to subsequent 

 modifications in the special cases of the sieve-tubes, they become 

 enlarged and are able to serve secondarily for purposes of 

 translocation." 



Taking into consideration the anatomical characters ot the 

 various tissues and the distribution of the connecting threads, 

 where this is known, it would seem that the differences in the 

 nature of the threads are correlated with the different motions 

 of the contents of the cells in whose walls the threads occur. 

 Normal threads appear to be adequate in those tissues in which 

 the assimilates only travel over short distances, as in the 

 assimilatory system, the bundle sheath and parenchyma in 

 general. But where a comparatively rapid translocation occurs 

 over considerable distances and in restricted channels, the fine 

 threads seem to be inadequate and the sieve-tubes prove to have 

 actual pores by which the translocation is facilitated. The actual 

 mechanism of transport in the sieve-tubes is still obscure but it 

 seems probable that the power of proteids to adsorb crystalloids 

 in solution and the pressure to which the sieve-tube contents 

 appear to be subjected are very important factors in the process. 



From the foregoing account it will be seen that the anatomical 

 and histological features of the tissues lend considerable support 

 to the theory that the sugars pass from the chlorophyll tissues 

 of the leaf into the bundle sheaths of the finer veins and are 

 thence removed by the sieve-tubes. In these elements they 

 travel from the leaf and are distributed throughout the plant 

 body. Since nearly the whole of the assimilates enters the 

 sieve-tubes near the bundle endings it is clear that there is very 

 little need for any extra-stelar translocatory tissue. The nerve 

 parenchyma is accordingly held to be primarily a mechanical 

 tissue. 



In the second part of this article I hope to show that this 

 view is in accordance with the results of various physiological 

 experiments. 



{To be continued) 



19 



