120 STRUCTURE AND FUNCTIONS OF LEAVES 



to the weight of the plant itself. During 173 days of growth a 

 corn plant has been found to give off nearly 3 1 pounds of water. 

 During 140 days of growth a sunflower plant gave off about 

 145 pounds. 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 6^ tons per acre every twenty- 

 four hours for an ordinary grass field, or rather over 2200 pounds 

 of water from a field 50 X 150 feet,- -that is, from a tract not 

 larger than a good-sized city lot. Calculations based on obser- 

 vations made by the Austrian forest experiment stations showed 

 that a birch tree with 200,000 leaves, standing in open ground, 

 transpired on hot summer days from 700 to 900 pounds, while 

 at other times the amount of transpiration was probably not 

 more than 18 to 20 pounds. 1 



145. 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, 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 accumu- 

 lates 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. 



146. The fall of the leaf. In the tropics trees retain most of 

 their leaves the year round ; a leaf occasionally falls, but no con- 

 siderable portion of them drops at any one season. 2 The same 



1 See B. E. Fernow 's discussion in Report of Division of Forestry of United 

 States Department of Agriculture, 1880 ; also the article, " Water as a Factor 

 in the Growth of Plants," by B. T. Galloway and Albert F. Woods, Year- 

 Book of United States Department of Agriculture, 1894. 



2 Except where there is a severe dry season. 



