MINUTE STRUCTURE 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. 
‘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." 
146. 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 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) Respiration. 
* 1Since the root-hairs, by 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 
ealculation above given. 
* 2 See Kerner and Oliver’s Natural History of Plants, vol. I, pp. 371-483. 
* 83In many works on Botany, (1) and (2) are both compounded under the term 
assimilation. 
