180 TRANSLOCATION IN PLANTS 



restriction of the sieve pores, through which he estimated 

 that sugar in the pure state must flow at a rate of 270 cm. 

 per hour to carry the 0.89 g. in 24 hr., he concludes that 

 protoplasmic streaming would be entirely too slow to act 

 as a transporting mechanism. 



On the other hand, it is probable that the estimates of 

 rates required are too high because, as pointed out in 

 Sec. 26, estimates of cross-sectional areas of sieve-tube 

 lumen and sieve pores are probably too low. Crafts has 

 found the sieve pores of Cucurbita to be smaller than 

 commonly assumed. The open net-like appearance of 

 sieve plates in Ailanthus, Liriodendron, and Populus 

 figured by MacDaniels (1918), however, indicates that 

 many kinds of sieve plates have large openings. It is 

 true, on the other hand, that the reagents used in pre- 

 paring these materials may have dissolved out callose 

 layers and this enlarged the openings. It is probable that 

 the cross section of the lumen of sieve tubes is normally 

 much greater than appears when mounted for observation 

 and measurement, because the marked reduction in turgor, 

 which undoubtedly occurs when the system is cut and large 

 amounts of exudate appear, is likely to result in considerable 

 shrinking, especially if the walls are appreciably elastic. 

 Crafts' findings (1931) that phloem cell walls contain about 

 50 per cent water indicate that they may be elastic, and 

 measurements, by both Miinch and Crafts, of the osmotic 

 concentration of phloem contents demonstrate the proba- 

 bility of high turgor pressures. Measurements of osmotic 

 concentration of phloem exudate from Quercus rubra made 

 by Miinch (1930) ranged from 20.9 to 23.7 atmospheres at 

 15°C. and from Robinia pseudoacacia between 25.8 and 

 37.5 atmospheres. It is, of course, likely that this phloem 

 exudate does not have the same composition as the natur- 

 ally moving components, but the rapidity of exudation 

 and the high concentrations found clearly indicate high 

 turgor. Schumacher (1933) has observed that the fluid 

 first exuding from sieve tubes that were transporting 

 fluorescein contained no fluorescein. This was restricted 



