488 TRANSLOCATION OF SOLUTES 



apical portions of the plant in which rapid growth is taking place. Likewise 

 upward transport of foods invariably occurs during the earlier stages of shoot 

 growth from bulbs, tubers, rhizomes, and other types of underground organs. 



The "classical" view that upward translocation in plants takes place in 

 the xylem has generally been accepted, at least until comparatively recently, 

 as referring to mineral salts and organic solutes as well as to water. The 

 concept tiiat upward transport of carbohydrates occurs principally through 

 the xylem is based largely on phenomena which have been observed in cer- 

 tain woody species. Large quantities of soluble and insoluble carbohydrates 

 are stored in the wood parenchyma, wood rays, and (in the younger stems) 

 pith cells of many varieties of trees and shrubs. At certain seasons soluble 

 carbohydrates are also found in the xylem conduits, as illustrated by data of 

 Anderssen (1929, Fig. 107). As found by this investigator the concentrations 

 of both sucrose and free reducing substances (probably largely hexoses) in the 

 tracheal sap of pear trees were highest in the winter and early spring. Both 

 fell to a zero value during the summer months, and increased slowly during 

 the autumn. The sugars found in the conducting elements undoubtedly 

 come from the storage tissues of the pith or xylem. The relatively high 

 soluble carbohydrate content of the xylem tissues in the winter and spring is 

 probably due largely to shifts in the starch <^ sugar equilibrium towards the 

 sugar side, as a result of the relatively low temperatures prevailing during 

 these seasons (Chap. XXII). 



Since in the early spring the soluble carbohydrate content of the sap is 

 relatively high, and by the time the shoots are well developed has dropped to 

 a low or zero value, this seems to indicate that soluble carbohydrates have 

 been conducted through the xylem up to the developing buds. Although this 

 assumption appears to be superficially plausible there are good reasons for 

 doubting if this phenomenon actually is very strong evidence that upward 

 translocation of organic solutes does occur in the xylem. In the first place 

 the concentration of organic solutes in the xylem sap is always very low. 

 In the sugar maple the proportion of sugar in the sap of the vessels may at- 

 tain 8 per cent, but is usually less and in most other species seldom exceeds 

 2 per cent. Furthermore the highest concentrations of sugar occur in the 

 sap during the winter when there is little or no upward translocation of 

 water, and it is not at all certain that these solutes do not largely disappear 

 from the xylem sap in the spring before its upward flow begins to take place 

 at an appreciable rate. 



Recent investigations by Curtis, summarized in 1935, point to a con- 

 clusion regarding the upward transport of organic solutes which is directly 

 the opposite of the long accepted view. His experiments all seem to indicate 



