8 TRANSLOCATION IN PLANTS 



that the phloem tissues are concerned in the backward 

 transfer of foods from the leaves. That cutting the phloem 

 prevents this backward movement of foods has been 

 repeatedly demonstrated not only by the lack of growth in 

 stems, roots, or organs when separated from the leaves by 

 rings, and by the failure of such organs to receive food, 

 as indicated by measurements of dry weight as well as by 

 chemical analysis, but also by the accumulation of these 

 foods above the ring. This has been made evident by the 

 increased diameter growth of the stem, increased growth of 

 fruits or other storage organs, and marked accumulation 

 of organic materials as demonstrated by dry weight and 

 chemical analyses. 



It seems still more probable that the phloem is the tissue 

 concerned in this backward transfer, from the observation 

 that, when only the cortex tissues external to the phloem 

 are removed, food transfer is not prevented, and this 

 conclusion is still further supported by the observations 

 (Hanstein, 1860; Weevers, 1928; and others) that those 

 plants which have an internal phloem do not fail to trans- 

 port foods backward even when all tissues external to the 

 xylem are removed. 



The fact that phloem tissues usually contain elongated 

 cells which are connected in a continuous longitudinal 

 series by pores or connections through sieve plates or fields 

 has been cited as evidence that these tissues seem well 

 adapted for food transport. The presence of a high content 

 of sugar and proteins within the tissues (Kraus, 1886; 

 Zaccharias, 1884; Hartig, 1860) is added evidence that they 

 may play a role in transport of these foods. Since there 

 are no other tissues that seem adaptable to backward 

 movement, these observations, together with the fact that 

 it is difficult to see how foods could move down through the 

 xylem where water is rising, have led to the conclusion that 

 backward movement of materials occurs through the 

 phloem tissues. 



Until recently, therefore, there was almost perfect agree- 

 ment among botanists — indeed one of the few points in 



