THE METHOD OF MOVEMENT 161 



in our own laboratory show a much retarded emptying 

 from such leaves. Emptying through cut sieve tubes with 

 their relatively large pores would very likely be different 

 from an emptying from cut parenchyma cells where the 

 death of the cut cells at the surface would probably cause 

 a very rapid formation of a surface membrane closing the 

 plasmodesma. Experiment on microdissection by Cham- 

 bers (1925) and others have demonstrated rapid reforma- 

 tion of a surface membrane at cut or punctured surfaces of 

 cells. A comparable closing of cut sieve tubes might take 

 place but would seem very improbable especially if a con- 

 tinued unilateral flow of the contents is normal, as postu- 

 lated by Miinch. 



The phloem contents, especially in cucurbits, may coagu- 

 late into a gel within a few minutes after exuding from a cut. 

 This coagulum might be expected to prevent further loss in 

 much the. same way as the coagulation of blood will stop its 

 flow. Coagulation at the surface alone, however, which is 

 effective in stopping blood flow would seem inadequate for 

 stopping flow from the phloem; because internal pressures 

 in the latter must be very high and the coagulum would 

 have no firm anchorage. The average maximum pressure 

 developed in the arteries of man does not commonly exceed 

 150 mm. of mercury, or about 0.2 atmosphere, while that 

 in the capillaries where surface coagulation is effective 

 does not commonly exceed 27 mm, or about 0.035 atmos- 

 phere. In the phloem of plants, on the other hand, the 

 osmotic concentrations are probably such as to allow mini- 

 mum turgor pressures around 2 to 5 atmospheres. Miinch 

 (1930, p. 134) estimated the phloem exudate from Quercus 

 rubra and Robinia pseudoacacia to have osmotic concentra- 

 tion values at 15°C. equivalent to 20.9 to 37.5 atmos- 

 pheres, and Dixon (1933) reports similar concentrations for 

 exudate from Fraxinus excelsior. These pressures are 

 nearly a thousand times that occurring in blood capillaries. 

 There is the possibility of coagulation of contents in the 

 tubes for some distance from the cut, but why this should 

 occur if unilateral mass flow is normal does not appear 



