178 TRANSLOCATION IN PLANTS 



is greatly reduced (see paragraph 3, Sec. 30). (h) The 

 sieve structures peculiar to phloem tissues seem of sig- 

 nificance in that they should allow streaming protoplasm 

 to move from segment to segment, but when a sieve tube 

 is accidentally ruptured, the release of pressure, allowing 

 an abnormal unidirectional mass flow, results in an accumu- 

 lation of protoplasm or other semisolid material such as 

 starch grains, which would effectively prevent continued 

 bleeding. If unidirectional mass flow of sieve-tube con- 

 tents due to internal pressure is normal, as claimed for 

 the Miinch and Crafts hypotheses, no such simple explana- 

 tion can account for stoppage when the sieve tubes are 

 cut or broken, (i) If solutes are transported by living 

 protoplasm, the indication that living cells are necessary 

 for transport is easily explained. 



Any hypothesis postulating unidirectional mass flow, 

 whether or not it involved the mechanism proposed by 

 Miinch or Crafts, would be likely to have most of the 

 weaknesses mentioned in connection with the latter 

 hypotheses, especially under paragraphs 1 to 7 Sec. 30, and 

 would be likely to lack the advantages just mentioned 

 under headings (a) to (i) of the protoplasmic streaming and 

 diffusion hypothesis. 



34. Weaknesses in the Hypothesis Accounting for 

 Transport by Protoplasmic Streaming and Diffusion. — a. 

 A serious weakness in this hypothesis is that the mechanism, 

 though immensely more efficient than diffusion alone, still 

 seems inadequate to account for the observed rates of 

 movement. Data are not available from which one can 

 make very accurate calculations, but the estimate of 

 Crafts (1933) for the increase of dry matter in potato 

 tubers attached to plants by stolons of known dimensions 

 may serve as a satisfactory basis. He estimated that the 

 tubers had increased in dry matter at a rate of 0.89 g. 

 per day. Careful measurements were made of the cross- 

 sectional area of various tissues in the freshly cut stolons. 

 These are given in Table 20. 



From these figures. Crafts estimated that, to carry 0.89 g. 

 of dry matter through the stolon in 24 hr., it would require 



