CRITERIA AND METHODS 113 



xylem vessels. This would seem to indicate translocation 

 in the xylem when the solute is not artificially introduced 

 through cuts. Eosin, however, is toxic and it seems proba- 

 ble that when it comes into contact with the living cells 

 of the root it kills them or makes them become abnormally 

 permeable, and then, of course, with no effective semi- 

 permeable membranes the solute would tend to be carried 

 directly with the water. The evidence that normal salts 

 are not absorbed with the water, as discussed in Sec. 7, 

 would indicate that this eosin movement is abnormal. The 

 absorption of lithium through uncut tissues might be 

 explained in a similar way. Birch-Hirschfeld (1920), for 

 example, found that, if a strong solution of lithium nitrate 

 (0.75 M) was placed on the surface of a leaf, or if crystals 

 of the salt were placed under the bark and in contact with 

 thin-walled cells, the lithium was evidently carried in the 

 xylem. Such strong solutions would tend to kill or at 

 least increase the permeability of the living cells so that the 

 tissues, though seemingly intact, are not uninjured. 



That lithium or comparable solutes can be used as tests 

 for normal solute movement may be open to serious 

 criticism. Certainly the results obtained through its use 

 seem to contradict those obtained by other experiments. 

 For example, when a lithium salt is added to the soil, it 

 seems readily to pass a ring (Curtis, 1923). This might be 

 explained in various ways. Possibly the lithium is toxic 

 and increases the permeability of cells and therefore is 

 carried with the water. But the concentration used in this 

 experiment gave no clear indication of toxicity. That 

 lithium is fairly quickly absorbed by intact tissues from 

 seemingly nontoxic solutions and that it is then carried 

 rapidly through the water-conducting tissues appears to be 

 fairly well established. Possibly certain ions are carried in 

 greater quantity in the xylem while others are carried 

 chiefly in the phloem. It is conceivable that different 

 tissues might thus absorb ions differentially. Attempts are 

 being made to determine if certain of the normally occurring 

 elements are more readily carried in the xylem than others. 



