114 TRANSLOCATION IN PLANTS 



On the other hand, the Hthium test has been used as a 

 quahtative test only. If minute quantities of solutes are 

 carried rapidly in the xylem, whereas much larger quantities 

 are carried more slowly in the phloem, then qualitative tests 

 might indicate greater translocation through the xylem. 

 The quantitative data on ash as well as on nitrogen indicate 

 that small quantities of these solutes are unquestionably 

 carried through the xylem. Merely quahtative tests like 

 those with lithium are therefore open to criticism and may 

 lead to false conclusions. Furthermore, as pointed out in 

 Sec. 14, lithium and other elements to which membranes 

 are relatively permeable and which may not be accumulated 

 in cells, may be more readily carried in the transpiration 

 stream. 



This criticism against using qualitative tests only would 

 seem not to hold for experiments reported by Bodenberg 

 (1927), for he found that, when hthium or caesium nitrates 

 were supplied to rooted woody plants, these would not pass 

 either upward or downward through a part of stem where 

 the xylem was cut out. In these experiments he found no 

 lithium or caesium to pass through the phloem bridge even 

 in as long a period as 21 days. The suggestion that 

 larger quantities are moving more slowly in the phloem is 

 not supported by this evidence. In this same paper 

 Bodenberg reports finding that cutting the phloem delayed 

 but did not prevent the transfer of these salts, while cutting 

 the xylem completely stopped their movement in either 

 direction. Though he states that he enclosed the stems in 

 glass tubes for keeping the phloem wet he does not describe 

 the method sufficiently to give one confidence that this was 

 not the cause of the lack of transfer. In earlier experiments 

 (Curtis, 1925) I found that failure to keep the phloem 

 thoroughly wet resulted in no translocation through it. It 

 is also possible that in cutting out the xylem, the phloem 

 also was destroyed. The one instance mentioned of 

 regeneration of new xylem would, however, tend to refute 

 both of these objections. In view of the objections to the 

 use of lithium and caesium salts as discussed in this section 



