OUTLINES OF THE PHYSIOLOGY OF NUTRITION 219 



roots will be found on the smooth surface of the 

 marble, which has become corroded wherever the hairs 

 of the roots have been in contact with it. The 

 absorption by' the root-hairs is in part a merely 

 physical process (osmosis), which can be imitated 

 artificially. If we take a glass tube and close it at the 

 lower end with a piece of ordinary bladder, and if we 

 then pour into the tube a solution of salt, and dip its 

 lower end into distilled water, the liquid in the tube 

 will rise. The bladder is permeable to water, the 

 dissolved salt exerts an attraction on the water outside, 

 and a passage of water from without inwards goes' on 

 continuously through the membrane. At the same time 

 a much smaller amount of the salt solution passes out 

 through the membrane into the surrounding water. 



The primordial utricle and cellulose wall of a root- 

 hair form together a permeable membrane. The 

 cell-sap contains various substances in solution, among 

 which organic acids appear to be the most active in 

 osmosis. The cell-sap is thus denser than the water 

 outside, in spite of the salts which the latter holds 

 in solution. Consequently the external fluid passes 

 inwards, through cell-wall and protoplasm, into the 

 cavity of the cell. A similar process causes the sap 

 of the root-hair to pass on into the next inner cell, 

 and so on. 



When the root-hair absorbs water, a little acid may 

 pass out at the same time. Whether this happens 

 or not, depends upon the protoplasmic lining of the 

 cell, which controls the osmotic processes, and often 

 lets in water readily, while it quite refuses to let out 

 the substances dissolved in the cell-sap. It is this 

 control exercised by the protoplasm which distin- 

 guishes osmosis as it occurs in living cells from the 

 process as it takes place through dead membranes. 



IV. The Ascent of the Sap. The ascent of the sap 



