96 TRANSLOCATION IN PLANTS 



synthate made within a leaf during the day is moved from 

 the leaf each 24 hr., a 10 per cent solution would have to 

 move at the rate of 140 cm. per hour if restricted to a tube 

 the size of the cross-sectional area of the phloem. (He made 

 an error in his calculations so that the figure should be 14 cm. 

 not 140 cm., but even this lower figure seems impossible.) 

 Still more accurate calculations were made by Mason 

 and Lewin (1926). They made accurate determinations 

 of the rate of increase in dry weight of the greater yam 

 and found that between the thirty-first and thirty-fifth 

 weeks the average weekly increase of dry weight of tuber 

 per stem was over 45 g. Since 95 per cent of the material 

 was combustible, 42.7 g. of organic material must have 

 been transported each week through the stem. The 

 average total cross-sectional area of sieve tubes and vessels 

 per stem was 0.05747 + 0.0019 sq. cm. Of this the sieve 

 tubes occupied less than 20 per cent, or 0.0115 sq. cm. 

 Then making the very liberal assumption that a 25 per cent 

 sugar solution was being carried through the sieve tubes, 

 this would have to move at an average velocity of 88 cm. 

 per hour in order to carry the solutes at the rate they were 

 carried. A movement at this rate seems of course impos- 

 sible. The writers are not justified, however, in assuming 

 that translocation is limited to sieve tubes only, for it is 

 possible that all of the elongated living cells when in con- 

 tinuous series act as conducting cells. It is possible that 

 by sieve tubes they mean entire phloem. In some plants 

 the phloem consists largely of sieve tubes with companion 

 cells, whereas in other plants the proportion of sieve tubes 

 is very low. What the condition is in the yam used by 

 Mason and Lewin I do not know, and they give no indi- 

 cation as to how much of the phloem consists of sieve tubes. 

 Yet even if the entire cross section of the Hving part of the 

 stem is assumed to carry sugar, it seems difficult to see 

 how materials are carried so rapidly (see Sec. 34 for other 

 calculations of rates). 



The seeming inadequacy of the phloem as a conducting 

 tissue and the ease and rapidity with which solutions can 



