FUNCTIONS OF LEAVES 165 



Meadow 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 4000 tons of such water to furnish 

 the potash for one ton of 'hay. The water which the 

 root-hairs take up must, however, contain far more potash 

 than is assumed in the calculation above given, so that the 

 amount of water actually used in the growth of a ton of 

 hay cannot be much more than 260 tons.^ 



175. Accumulation of Mineral Matter 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 sur- 

 prisingly large heap of ashes. An abundant constituent 

 of the ashes of burnt leaves is silica, a substance chemic- 

 ally the same as sand. This the plant is forced to absorb 

 along with the potash, compounds of phosphorus, and other 

 useful substances contained 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 quan- 

 tity of it is absorbed than is needed; hence it, too, accu- 

 mulates in the leaf. 



176. Nutrition, Metabolism.^ — The manufacture of the 

 more complex plant-foods, starch, sugar, and so on, from 



1 See the article, " Water as a Factor in the Growth ol Plants," by B. T. 

 G-alloway and Albert F. Woods, Year-Book of U. S. Department of Agriculture, 

 1894. 



2 See Kemer and Oliver's Natural History of Plants, Vol. I, pp. 371-483. 

 Also Pfeffer'g Physiology of Plants, translated by Bwart, Chapter VIII. 



