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.1 
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.2— 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 of Plants,” by B. T. 
Galloway and Albert F. Woods, Year-Book of U. S. Department of Agriculture, 
1894. 
2 See Kerner and Oliver’s Natural History of Plants, Vol. I, pp. 371-483. 
Also Pfeffer’s Physiology of Plants, translated by Ewart, Chapter VIII. 
