UPWARD TRANSLOCATION OF ORGANIC SOLUTES 489 



that it is the phloem and not the xylem which is the principal tissue through 

 which such translocation occurs. In one of the most convincing of his ex- 

 periments the contrasting effects of intercepting the xylem and intercepting 

 the phloem of woody stems upon upward transport of organic solutes was 

 studied (Curtis, 1925). In these experiments a number of growing shoots 



FEaiO MAR. APR. MAY JUNE JULY AUG. SEPT. OCT. NOV. DEC. JAN. 



t t 



FULL BLOOM LEAVES SHEO 



Fig. 107. Seasonal variations in the carbohydrate content of the xylem sap of pear 



trees. Data of Anderssen (1929). 



were first defoliated. Some received no further treatment, thus serving as 

 checks. In others (Fig. 108) a ring of the tissues external to the xylem was 

 removed, and in still others a segment of the xylem was excised, leaving the 

 phloem and cortical tissues intact. Every stem which was cut into was en- 

 closed in a glass cylinder as shown in the figure. This cylinder was filled with 

 water in order to keep the exposed tissue surfaces moist, and in order to supply 

 water to the top of the stems in which the xylem was cut. The outcome of 



