112 



PHYSIOLOGY or NUTRITION 



Citric acid thus appears to change the osmotic properties of the protoplasm. 

 No dye accumulates in the cell if the solution contains o.oi per cent, of citric 

 acid, but the dye is absorbed from the surrounding solution in the absence of 

 the acid. It is thus possible to alter at will the osmotic properties of cells. 



It is well known that plants can absorb and accumulate the essential chemical 

 elements from very dilute solutions. Some non-essential elements enter the 

 plant cell only until their effective concentration becomes the same within and 

 without, but some others, as well as the essential elements, continue to enter and 

 accumulate in the cell, even from a weak solution, since they are converted into 

 new compounds after entrance and so the internal concentration never becomes 

 equal to the external. 



An illustration of continued absorption may be found in the accumulation 

 of iron tannate in an artificial cell of collodion or 

 animal bladder filled with tanpin solution and sur- 

 rounded by one of ferric chloride. Tannin does 

 not escape through the membrane, but ferric 

 chloride diffuses into the cell and there enters into 

 combination with the tannin to form iron tannate, 

 which also remains in the cell. Ferric chloride is 

 continually consumed in the formation of the iron 

 tannate and its concentration within the cell never 

 becomes the same as that outside. If the tannin 

 solution is sufficiently concentrated, all of the 

 ferric chloride will pass from the outer solution 

 , , into the cell. In a similar way plant roots appear 



Fig. 70. — Apparatus for show- . 1 -u j.t. ■ 1 1 



ing diffusion of copper sulphate '° absorb the essential elements as well as other 

 through a membrane into a tube substances, from the surroundinsT solution 



containing zinc. _^, j 11 • . 



ine tollowmg experiment also illustrates this 

 phenomenon of continued absorption (Fig. 70). A roll of sheet zinc is placed 

 in a short glass tube of large diameter, the tube being filled with water and 

 having both ends closed with animal bladder or parchment paper. The tube 

 is placed in a dilute solution of copper sulphate, which passes through the 

 membranes into the tube. Here the copper of the salt is replaced by zinc and 

 the zinc sulphate thus formed diffuses into the outer solution. Copper sulphate 

 continues to enter until all of it, or all of the zinc, has been used up. The 

 same phenomenon occurs in the growth of bacteria and moulds on various 

 organic compounds. Of two substances having different nutritive values the 

 cells take up mostly the one with the higher value, frequently leaving the other 

 entirely untouched. For instance, Aspergillus niger absorbs only glucose from 

 a mixture of this substance and glycerine, so long as the former is present in 

 the solution.! 



Outward diffusion through the cell membranes is also subject to regulation. 

 Nathansohn's^ experiments indicate that sodium chloride easily penetrates the 

 cells of Codium tomentosum (a marine alga) but that this salt cannot be com- 



' Pfeffet, W., Ueber Election organischer Nahrstofte. Jahrb. »iss. Bot. 28: 206-268 180? 

 2 Nathansohn, AleMnder, Zur Lehre vom Stoffaustausch. Ber. Deutsch. Bot. Ges. 19 : "309-513. 1901 . 



