FUNCTIONS OF LEAVES 135 
have kept pace with the evaporation from its surface. A 
little calculation will show that the amount of water thus 
daily carried off through the foliage of a large tree or the 
grass-blades on a meadow is enormous. A grass-plant has 
been found to give off its own weight of water every 
twenty-four hours, in hot, dry summer weather. This 
would make about 64 tons per acre every twenty-four hours 
for ordinary grass-fields, or rather over 2200 pounds of 
water from a field 50 x 150 feet (i.e., a city lot). 
These large amounts of water are absorbed, carried 
through the tissues of the plant, and then given off by the 
leaves simply because the plant-food contained in the soil- 
water is 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. 
160. 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 evapo- 
rated through the stomata. A bonfire of leaves makes a 
surprisingly large heap of ashes. An abundant constitu- 
ent 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, 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 quantity of it is absorbed than is needed; hence it, 
too, accumulates in the leaf. 
