112 TRANSLOCATION IN PLANTS 



because materials do not normally move there but because 

 of the nature of the cells and the probable nature of the 

 movement in them. Because the phloem cells are so 

 small, because of their high colloid content and high 

 internal pressures, and because their walls are not rigid, it 

 is practically impossible, by ordinary injection methods, to 

 introduce a solution into the cell. An attempt to cut into 

 the tissue is almost certain to rupture the system, and mass 

 injection can never be expected to be successful as a means 

 of introducing solutes into such tissues. If translocation 

 through the sieve-tube system is brought about by proto- 

 plasmic streaming, as seems to be probable, the only 

 methods that at present seem feasible would be to introduce 

 some dye that is taken up by the protoplasm or other 

 substance that can be detected in small quantities. These 

 may be absorbed through intact cells or it may be necessary 

 to introduce them by micro-injection. Even here there is 

 danger of upsetting the mechanism by the introduction of a 

 foreign substance or by the puncture of the cell, and it is 

 improbable that materials could be thus injected in suffi- 

 cient quantities to be of great aid in studying translocation. 

 Schumacher (1933), however, has found it possible to intro- 

 duce fluorescein (see Sec. 37) but the amounts carried must 

 be small as compared with the amount of sugar carried. 

 The injection, natural or artificial, of a virus, also, may be of 

 use in translocation studies because of the possibility of its 

 self-multiplication. But it would seem to be of doubtful 

 value for testing normal movements (see Sec. 38). 



Furthermore, even if a given solution could be intro- 

 duced into one or the other tissue without disrupting the 

 normal pressures or movements, and the introduced sub- 

 stance was found to be carried after injection, this would 

 merely establish the fact that the material when introduced 

 might be carried in that tissue, but it would not establish as 

 fact that it is normally so carried. 



21. The Movement of Solutes Introduced through 

 Uncut Roots. — When eosin is added to the medium around 

 a rooted plant, this is found to be actually carried in the 



