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 6i 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. 



