172 TRANSLOCATION IN PLANTS 



Although I have never accepted Crafts' suggestion 

 that the walls form an important path for transport, 

 or that the sieve tubes are completely permeable so that 

 the entire cross section is available for transport, or that 

 exudation in any way indicates a normal flow, or that the 

 proposed mechanism accounts for the flow, I think these 

 probably mistaken interpretations should not blind one to 

 the fact that his evidence indicates that the exudation 

 from cut phloem, as well as from cut cambium and young 

 meristematic tissue, can hardly have come from normal 

 sieve pores or plasmodesma if these are of the size observed 

 in fresh cut material. The exudation from Cucurhita 

 stems, in which most of the phloem had heavy callus on 

 the sieve plates, indicates that the exudation perhaps does 

 not come from sieve tubes. This is supported by his 

 observations, and those of others, on the exudation from 

 young tissues in which the sieve tubes have not become 

 differentiated. Although Crafts obviously has accepted 

 the translocation mechanism proposed by Miinch and 

 suggested, first that the walls, and later that the entire 

 phloem might be the channel, because his measurements 

 and calculations pointed to the inadequacy of the sieve 

 pores or plasmodesma to allow for the exudation observed, 

 it seems to me that his findings really tend to throw doubt 

 on the vahdity of the hypothesis of transport by a pressure 

 flow. If, on the other hand, sieve pores and plasmodesma 

 are distended under natural conditions and much larger 

 than observed in cut material, many of the calculations 

 may be misleading. 



Crafts is not the first to have suggested that walls may 

 form the path of transport. Bokorny (1890) found iron 

 to be deposited in the thick-walled elements of both xylem 

 and phloem when the plants had previously been supplied 

 with a solution of iron sulphate. He concluded that this 

 supported the earlier suggestion of Sachs that water rose 

 through the walls by imbibition. Priestley (1929) pro- 

 posed a flow of solution within walls to supply the shoot 

 meristem and Scott and Priestley (1928) proposed that 



