Possession of a chitinous lining on either or both of the 

 walls would be of obvious value and would explain the low 

 rates of diffusive loss for CO. 



Krogh 19 defined a diffusion constant, K, for gases 

 across animal tissues as the number of cubic centimeters 

 of gas reduced to standard conditions, dv, which traverses 

 a thickness of 1 micron, dx, and an area of 1 square centi- 

 meter, A, in one minute, dt, when the pressure difference, 

 dp, (i. e. , the partial pressure gradient of the gas in ques- 

 tion) is one atmosphere. The equation may be written 



K- dV 



dt dp /dx A 



The units are cm per minute as in the case of diffusivity 

 determinations. * 



Krogh gives a K for oxygen across chitin as 0. 013, 

 a figure lower by at least an order of magnitude than his 

 diffusion constants for oxygen across muscle, connective 

 tissue, gelatin, or water. He found carbon monoxide in 

 general to have a diffusion constant through animal tissues 

 of 25 percent less than that of oxygen. 



Diffusion constants for the floats, in two experiments 

 from which there was sufficient data, are included in table 3 

 (see Appendix D for calculations). Since the partial pressure 

 difference for carbon monoxide across the float walls was 

 not constant, K ] s have been calculated for partial pressures 

 at the start and finish of the respective experiments . The 

 ranges given, 0. 027 to 0. 013, and 0. 013 to 0. 019, agree 

 well with Krogh' s oxygen value for chitin. This suggests 

 that a chitinous lining provides the diffusion barrier in the 

 pneumatophores of N. bijuga. It still remains to be seen 

 what a partial pressure differential of several atmospheres 

 will do to the diffusion constants. At present, however, it 

 seems unlikely that they will be much elevated at higher 

 pressures. 



* Millington 20 has performed a valuable service by clarifying certain conflicting points 

 arising from confusion in the use of the standard Fick equation and the Krogh diffusion 

 constant. 



24 



