CRITERIA AND METHODS 



111 



the finer capillary tubes or those with smaller openings 

 through pits or pit membranes will withdraw water from 

 the coarser ones or from those with larger pit openings, 

 thus giving the appearance that the latter has been gas-filled 

 previous to cutting. Bailey (1916) reports observations 

 on sizes of pores in pit membranes and calculations of 

 pressures necessary to rupture membranes at air-water 

 surfaces. Data on the approximate pressures at 20°C. 

 necessary to force air through water-filled, or water through 



Table 16. — Approximate Pressures Necessary to Force Air into 

 Water-filled Circular Pores, or Water into Air-filled Pores of 

 Varying Diameters at 20°C. 



2T 

 These were calculated from the formula p = — where p = pre.ssure in gram.s per square 



centimeter, T = surf.ace tension of water which is 73.5 dynes per square centimeter or 

 0.7496 g. per square centimeter at 20°C., r = radius. 



air-filled circular openings of various sizes are given in 

 Table 16. These data also indicate relative powers of 

 tubes of differing diameter to compete for water when 

 the system is cut and exposed to air. Pores through 

 pit membranes range from 0.5 to 5/x in diameter, and 

 tracheids and vessels in various woody plants range from 

 10 to 200^1 in diameter. It is obvious from these data 

 that differences in capillary forces alone up to one or more 

 atmospheres may be concerned in influencing the redistri- 

 bution of water after cutting. 



Attempts to inject solutions into the phloem may, and 

 usually do, result in complete failure, not necessarily 



