MLSUTE STEUCTURE OF LEAVES. 117 



in a condition so diluted that great quantities of water must 

 be taken in order to secure enough of the mineral and other 

 substances which, the plant demands from the soil. 



]Meado^\' hay contains about two per cent of potash, or 

 2000 parts in 100,000, while the soil-water of a good soil 

 does not contain more than one-half part in 100,000 parts. 

 It would therefore take 1000 tons of such water to furnish 

 the potash for one ton of hay.^ 



146. Accumulation of Jliiiei-al Jltrffer in the Leaf. — Just 

 as a deposit of salt is found in the bottom of a seaside pool 

 of salt water which has been dried up by the sun, so old 

 leaves are found to be loaded with mineral matter, left behind 

 as the sap drawn up from the roots is evaporated through the 

 stomata. A bonfire of leaves makes a surprisingly large heap 

 of ashes. An abundant constituent of the ashes of burnt 

 leaves is silica, a substance chemically the same as sand. 

 This the plant is forced to absorb along with the potash, 

 compound of phosphorus, and other useful substances con- 

 tained in the soil-water ; but since the silica is of hardly any 

 value to most plants, it often accumulates in the leaf as so 

 much refuse. Lime is much more useful to the plant than 

 silica, but a far larger quantity of it is absorbed than is 

 needed; hence it, too. accumulates in the leaf. 



147. Details of the, Work of the Leaf- — A leaf has four 

 important functions to perform : 



(1) Fixation of carbon. (3) Excretion of water. 



(2) Assimilation.^ (4) Eespiration. 



* SiDce the root-liairs. liy closely enwrapping particles of the soil, and by giving 

 off small quantities of acid from their surfaces, exert a powerful action in dissolving 

 from the soil whatever in it is soluble, they must take up from it a solution stronger 

 than ordinary soil-water, and therefore must actually be able to supply the mineral 

 food needed by the plant from a smaller quantity of water than is found by the 

 calculation above given. 



= See Kerner and Olivers Xatural History of Plants, vol. I, pp. 371-483. 



3 In many works on Botany, (1) and i2) are both compounded under the term 

 assimilation. 



