120 DIFFUSION AND OSMOTIC PRESSURE 



that KNO 3 was taken out of a mixed solution two to six 

 times as fast as NaNO 3 or Ca(NO 3 ) 2 . He also observed that 

 wheat plants absorb K ions two to three times as fast as 

 those of Ca in equivalent solution. On the other hand, 

 maize absorbs somewhat more Ca than K. Pfeffer 1 per- 

 formed a series of experiments with fungi, which gave the 

 general result, that the plant takes out of a mixed solution 

 those solutes which are the best food for it. These would 

 naturally be the ones whose diffusion tension would be first 

 to diminish in the active cells. 



There may be substances, however, such as certain 

 poisons, 2 which react upon the membranes in such a way as 

 to cause them to become impermeable. The membranes 

 must, of course, be more or less permeable to such sub- 

 stances at first, else they could not react upon them. But 

 such cases are very rarely met with. The so-called select- 

 ive power is thus probably active usually, not in the absorb- 

 ing organs, but in the cells where the substances are used in 

 metabolism. 



III. TRANSMISSION OF SOLUTES 



An internal atmosphere exists in the plant, occupying the 

 intercellular spaces, which are in communication throughout 

 its body and which connect with the lenticels of the bark 

 and with the air chambers and stomata wherever these occur. 

 By means of this internal atmosphere, gaseous oxygen may 

 reach the more deeply lying parts of the body, and the 

 gaseous product of respiration in such parts may find its way 

 to the surface. Thus, it is probable that the comparatively 

 slow process of hydro-diffusion of gases is replaced by the 

 much more rapid gas diffusion wherever the internal atmos- 



1W. PFEFFER, "Ueber Election organischer Nahrstoffe," Jahrb. f. wiss. Bot., 

 Vol. XXVIII (1895), pp. 205-68. 



2 PTJLST has recently shown that, while copper ions are readily absorbed by 

 Mucor, Aspergillus, and Botrytis, they are not taken in by Penicillium. See C. 

 PULST, " Die Widerstandsfahigkeit einiger Schimmelpilze gegen Metallgifte," Jah; b. 

 f. wiss. Bot., Vol. XXXVII (1902), pp. 205-63. 



