SIGNIFICANCE AND EARLY IDEAS 5 



observed, and when a microchemical test was necessary 

 for demonstrating its presence, as when dilute solutions of 

 starch have been used. Nearly all data from this type of 

 experiment have clearly shown a close interrelation between 

 solute movement and water movement. In fact, the one 

 has often been used as a measure of the other. 



Experiments with solute injection have seemed to con- 

 firm the conclusion that solute movement and water 

 movement go hand in hand; for when the stem is not 

 entirely severed but is left on the parent plant so that it 

 can still obtain part of its water normally and is not forced 

 to obtain all its water from the solution supplied, even then, 

 if solutions of dyes or salts are introduced through incisions 

 of various types, these solutions are carried almost exclu- 

 sively through the water-conducting channels as in the 

 completely severed stems. A number of investigators 

 (Yendo, 1917; Birch-Hirschfeld, 1920; Rumbold, 1920; 

 Dixon, 1922; MacDougal, 1925; and others) have observed 

 that in such injection experiments the solutions may 

 move not only toward the apex of the stem but also basally. 

 These observations of backward movement in water- 

 conducting tissues have been drawn upon, as evidence for 

 a normal backward flow of solution through the xylem, 

 to support the recently proposed hypothesis that not only 

 upward movement but also movement of foods backward 

 from the leaves occurs through the xylem. This will 

 be discussed more fully in Chap. IV. 



Though the xylem readily carries solutes in solution when 

 they are introduced into it, this is no more than suggestive 

 evidence that solutes are normally carried there. The 

 finding of various solutes in the water exuding from cut 

 stems, however, seems to offer rather convincing proof that 

 solutes, both organic and inorganic, may be present there 

 normally and that the xylem, therefore, may normally 

 act as a channel for transport. Many plants have been 

 found to ''bleed" freely when the stem is severed. In 

 those woody plants that show such bleeding the sap flow 

 is most profuse in early spring before the buds break, and 



