THE METHOD OF MOVEMENT 181 



to the protoplasm and appeared later, perhaps when the 

 dying protoplasm could no longer retain it. The exudate, 

 therefore, may not be a true sample of the material moving 

 in uninjured cells. Its high content of sugar, however, 

 indicates that if some other part, such as protoplasm, is 

 the carrier its content of sugar is also likely to be high. 

 Crafts found the exudate to become weaker as successive 

 samples were taken, as did also Frey-Wyssling (1932) for 

 the exudate from the latex system of Hevea. 



The very fact that rate calculations based on dimensions 

 obtained from cut tissues have invariably led to seemingly 

 impossible figures, adds weight to the suggestion that 

 dimension measurements may be wrong. For example, 

 the calculations of Birch-Hirschfeld (1920), Dixon (1922), 

 Mason and Lewin (1926), Tincker (1928), and Crafts 

 (1931, 1932, 1933) all point to the seeming impossibility 

 of movement through the phloem. Ringing and xylem- 

 cutting experiments, on the other hand, have conclusively 

 proved that the materials do actually move at adequate 

 rates through these tissues. 



The seeming inadequacy of streaming, as indicated by 

 calculations of necessary rates, may in part also be due to 

 the low concentrations of sugar assumed to be moving. 

 This does not apply to Crafts' calculations, for he assumed 

 movement of pure sugar. Miinch has found phloem 

 exudate to contain as high as 23.8 per cent dry matter, of 

 which about 88 per cent was sugar. An average of 22 

 determinations from 8 kinds of trees gave a dry-matter 

 content of 17 per cent. Miinch (1930, p. 132) expresses 

 this as a percentage of water in which case the average 

 is 20.5 per cent. 



It IS possible that rates of natural streaming in sieve 

 tubes have been underestimated. It is true that observed 

 rates of streaming in ordinary parenchyma cells and hair 

 cells are rather low. DeVries (1885) reported streaming 

 rates of 0.2 to 0.4 mm. per minute in companion cells, and 

 I have observed rates of 0.25 to 0.36 mm. per minute at 

 18°C. in the companion cells and phloem parenchyma of a 



