354 PHYSIOLOGY 



Mechanical losses. • — Mechanical losses must also be taken into ac- 

 count. In all plants the drying of leaves, flower parts, rootlets, and 

 even larger parts of the body, is followed sooner or later by their falling 

 off. In annuals, the whole body perishes at the end of the growing 

 season; hence the perennials offer the best examples. In woody peren- 

 nials, particularly, the partial fall of the leaves in summer, due to heat, 

 drought, or other causes, and the complete autumnal fall, are striking 

 losses of material. Yet this is not so expensive to the plant as it might 

 seem at first sight, for a large part of the available foods have been trans- 

 ferred from the leaves before their fall, and what is left is chiefly cell-wall 

 stuff, unavailable organic matter, and ash. Nevertheless, much of that 

 represents past expenditure of energy and is a dead loss; though by 

 decay some of the materials again become available for rebuilding. 



Fall of leaves. — The once active food-making machines go to the 

 scrap heap in autumn and have no value except as junk. Their deterio- 

 ration is progressive. In the leaves of woody plants as compared with 

 other parts, there is with age, as a rule, a steady accumulation of dry 

 matter and a rising proportion of ash. 



Thus in the leaves of the European beech (Fagus sy{vatica) : 

 May June July 

 Per cent dry matter . . 23.35 40.21 43.64 

 Per cent of ash . . . 4.67 5.20 7.45 



In black locust {Robinia Pseudacacia): 



May 



Per cent dry matter 26.50 



Per cent of ash 6.25 



In 500 leaves of the plane tree ( Platanus orientalis) : 

 June July 



Grams dry matter . . . 142.53 184.70 



Grams of ash . . . 8.70 14.62 



Contrast with these figures the average ash content of the wood of such trees, 

 which is about 0.7 per cent, with a minimum of 0.2 per cent and an occasional 

 maximum of about 3 per cent. 



This high ash content of leaves is not due merely to the retention of 

 mineral matters when the water evaporated, as lime scale accumulates 

 in a tea kettle.* Rather the using of certain constituents of the salts, 

 particularly the nitrogen, sulfur, and phosphorus, left behind the bases, 

 calcium, magnesium, etc., ready to enter into new combinations and to 



' This is further e^'idenced by the fact that the ribs of leaves are usually richer in ash 

 than the mesophyll. 



