SIGNIFICANCE AND EARLY IDEAS 9 



which there was such agreement — that upward movement 

 of solutes occurs chiefly through the xylem, the rate and 

 direction of movement being largely determined by trans- 

 spiration, and that downward movement takes place almost 

 entirely through the phloem. There was not agreement, 

 however, as to the mechanism of this downward movement 

 nor as to the exact cells concerned, whether sieve tubes, 

 companion cells, or phloem parenchyma. Mangham 

 (1910) discusses the evidence put forward by various 

 investigators favoring or opposing specialization of trans- 

 port in special cells of the phloem. There was also partial 

 disagreement as to upward movement. Some have 

 assumed there is no upward movement through the phloem 

 (Reed and Halma, 1919a), while others (Hanstein, 1860; 

 and Leclerc du Sablon, 1906) have thought some upward 

 movement might occur through the phloem, especially 

 movement into fruits and storage organs from which 

 transpiration is low. Though Hanstein concluded that 

 certain types of materials (freshly assimilated sap as well 

 as the reserves stored in the bark) moved up almost 

 exclusively through the phloem, he assumed that salts 

 absorbed from the soil solution moved through the xylem. 

 He seemed to think that reserve foods that had been stored 

 in the xylem, especially sugars, are carried in the xylem 

 but felt the evidence to be inconclusive. Leclerc du Sablon 

 also thought some materials moved up through the phloem 

 but assumed that nutrient salts were carried chiefly in the 

 transpiration stream. 



Atkins (1916, p. 187) is rather critical of botanical texts 

 because they have failed to emphasize the importance of 

 the xylem in carrying carbohydrates. He states (p. 201) 

 that ''the transference of carbohydrates can no longer be 

 regarded as an occasional and accessory function of the 

 vessels but is certainly a continual and principal function," 

 and again (p. 214) he says, "The conveyance upwards of 

 sugars, of which sucrose appears to be the most 

 important, is a continual and primary function of the 

 tracheae." 



