162 TRANSLOCATION IN PLANTS 



obvious. If unidirectional mass flow is abnormal, this 

 cutting resulting in such a flow, accompanied by frictional 

 and other disturbances, might easily induce coagulation 

 at some distance from the cut, or the more solid particles 

 may accumulate against the sieve plates and thus clog the 

 pores. Furthermore, in many types of plants the sieve- 

 tube contents do not readily coagulate (Lecomte, 1889; 

 Munch, 1930; Crafts, 1933). 



7. Another difficulty is that, if there is a unidirectional 

 mass flow through the sieve tubes, as postulated by Miinch, 

 it is not clear why a local application of anesthetics or local 

 chiUing should stop the flow (Curtis, 1929, see also Sec. 39). 

 His hypothesis might easily explain a failure to move 

 through a dead stem, for the killing would be likely to 

 result in a complete plugging of the sieve tubes. It is, of 

 course, possible that either chilhng or anesthetics may 

 cause temporary or reversible coagulation or such a great 

 increase in viscosity as to stop the flow. Crafts (1932) has 

 made a few tests to determine the effect of chilhng on the 

 exudation from the phloem of cut stems of cucumber and 

 has found that a temperature of 2 to 5°C., which appar- 

 ently stops normal emptying of bean leaves, reduces this 

 exudation only 25 to 50 per cent or less. His data indicate 

 a gradual and continuous retardation as the temperature is 

 lowered from 25 to 2°C., whereas I found that the effect 

 on carbohydrate transport from the leaves of beans seems 

 to be rather abrupt, first appearing at a temperature near 

 6°C. This would suggest that exudation from cut phloem 

 cannot be taken as an indication of normal movement. 

 The method of flow, direction of flow, and rate of flow from 

 such cut tissues may all three be strictly abnormal and due 

 to the cutting. The danger of interpreting as normal those 

 movements in a tissue which take place after that tissue is 

 cut open may be as serious when applied to the phloem as 

 when applied to the xylem. This has already been dis- 

 cussed in Sec. 19. 



8. Still another diflfiiculty attendant upon the hypothesis 

 of Munch is that a pressure causing a unidirectional mass 



