480 MANUAL OF BOTANY 



driven off, a certain inorganic residue is left, which is composed 

 of several metals and some other elements. An analysis of this 

 residue, w^hicli is spoken of as the ash, will not, however, tell us 

 in what condition these different constituents exist in the living 

 plant, on account of the various chemical changes which go on 

 during the combustion. 



The ash of plants is found, when analysed, to contain always 

 the four metals — potassium, magnesium, calcium, and iron. 

 These are not present in the metallic condition, but in combina- 

 tion with various acids, forming nitrates, sulphates, chlorides, 

 carbonates, phosphates, &c. 



Besides these, various plants may individually contain greater 

 or less quantities of many other elements variously combined. 

 We find sodium very generally present ; less frequently so 

 aluminium, copper, zinc, manganese, silicon, bromine, iodine, 

 and others. Indeed, the composition of the soil in which a 

 plant grows determines to a very great extent what minerals 

 enter it. If a substance is soluble in the liquid which the root- 

 hairs absorb, a certain quantity will, by ordinary physical pro- 

 cesses, be taken up by them. 



The quantity of each substance a gi^■en plant will absorb 

 will depend upon whether the plant makes use of it in any way. 

 If so, it will be quickl}^ removed from the absorbing cell and 

 more will enter. If not, the cell- sap of the absorbing root-hair 

 will soon have taken up as much of it as it can contain, and the 

 absorption of that particular substance will cease. 



Some of the materials found in the soil are readily soluble in 

 the water which the soil contains. Such can enter the plant 

 without difficulty. Others are soluble only in water containing 

 CO.^, and as considerable quantities of this gas are continually 

 being generated in the soil, the water there is charged with it, 

 and bodies, otherwise intractable, are thereby brought into solu- 

 tion and absorbed. A third factor in the process of absorption is 

 the acid sap which the root-hairs contain. Not onh' does this 

 acid cause water to enter osmotically, but a little of it exudes in 

 the same way, and this has a certain solvent action upon the 

 particles to which the root-hairs cling. Tims certain salts can 

 be absorbed, though soluble neither in pure water nor in water 

 containing CO,. 



The solutions taken in are excessively dilute. There is, 

 however, a certain relation necessary between the substance and 

 the water, for we have already noticed that strong solutions 

 cannot pass the lining layer of protoplasm. Every salt is taken 



