82 PHYSIOLOGY. 



173. Absorption of soluble substances. — Since these substances are dis- 

 solved in the A\"aler of the scjil, it is not neressar\' for us to dwell on the 

 process of absoijjtion. Tliis in ^n-neral is dwelt upon in Chapter 3. It 

 should be noted, liowever, that food suljstances in solution, during absorp- 

 tion, diffuse through the pr(.>toplasinie membrane independent!'," of each 

 other and also independently <if the rate of movement of the water from 

 the soil iiUo the root hairs and cells of the root. 



When the cells have absorbed a certain amount of a given substance, no 

 more is absorbed until the concentration of the cell-sap in that particular 

 substance is reduced. This, hoAve\'er, docs not interfere ^\■ith the absorp- 

 tion of water, or of other substances in solution by the same cells. Plants 

 have theref(jre a certain selective power in the absorption of tcjod substances. 



174. Action of root hairs on insoluble substances. Acidity of 

 root hairs. — If we take a seedling which has been grown in a 

 germinabjr, or in the folds of cloths or paper, so that the roots are 

 free from the snil, and touch the moist root hairs to blue litmus 

 paper, the i)aper becomes red in color where the root hairs have 

 come in conttict. This is the reaction for the presence of an acid 

 salt, and indicates that the root liairs excrete certain acid sub- 

 stances. Tliis acid ])ro])ert)' of ihe root hairs serves a verv im- 

 portant function in the [jrepartilion of certain of the elements of 

 plant food in the soil. Certain of the chemical compounds of 

 potash, jihosphoric acid, etc., become deposited on the soil par- 

 ticles, and arc not solulale in water. The acid of the root hairs 

 dissoh'es some of these compounds where the p)articles of soil are 

 in close contact with them, and the solutions can then be tttkeitup 

 by the roots. (\tr])onic acid ;ind other acids tire also formed in 

 the soil, and aid in bringing these substances into solution. 



175. Tins corrosive action <if the loots can be sh(.>wn b\' the ^\■ell-knowIl 

 e.xjieriment nf ".growing a jilant on a marble plate which is i.o\ered b\- sciil 

 In lieu of the marble plate, the jteas ma}' t>e planted .in t.lam or o\'ster 

 shells, whii h ai'e then Iniried in the S(>il of the pot, so 'hat the roots of the 

 seedlings will ( ome in contact with the smooth surf.ice of the .shell, .-\fter 

 a few \N'eeks, if the soil be w.-ished from the marble \\bei-e the roots ha^'e 

 been in close contact, there will be an outline of this part ol the root SA's- 

 tem. .Se\'eral dilTerent arid sultstanies arc excreteil Irom the roots of 

 plants which have been found to redden blue litmus |),i|H'r by contact 

 E.xperinicnt,-, b\' ('zai}ek show, ho\\e\e|-, tllat the c.irbonic acid L'.xereted b\' 

 the roots has the power of directly bringing about these corrosion phenom- 



