300 THE FOOD OF PLANTS 



of a cell-wall for more than a thousand years, and millions of years may 

 elapse before the carbon of a particle of coal is converted into carbon dioxide, 

 or before the carbonic acid of a limestone rock is set free again. Similarly, 

 atoms of oxygen, nitrogen, phosphorus, &c., may be withdrawn from 

 circulation for a longer or shorter period in some form or other which is 

 inaccessible for the time being. In this respect climatic and telluric factors 

 are of especial importance, and these are largely dependent upon cosmic 

 relationships, and especially upon the radiant energy emitted by the sun. 

 It is these agencies which determine the distribution of heat and of light 

 over the surface of the globe, thus creating over certain areas the essential 

 general conditions for the existence of living beings, of which the organisms 

 containing chlorophyll, by converting radiant into chemical energy, supply 

 the propulsive force for the entire organic world. 



Sudden eruptions of energy, producing catastrophic changes, influence 

 the surface of our planet less than does the gradual wear and tear of ages, 

 and similarly, in the organic world much more is done by slow and gradual 

 actions extended over long periods of time than by sudden and rapid 

 alterations. The development of a plant involves a gradual but incessant 

 process of construction, and it is by their unceasing activity, aided by their 

 minuteness, that bacteria are able to decompose such vast quantities of 

 organic substance in the progress of a year. Similarly, it must have 

 taken millions of years to produce the enormous deposits of organic material 

 which have decomposed to form the coal-beds. 



The changes and decompositions which occur in dead substances are 

 of great importance to the organic world, and thus the carbon of coal 

 can be assimilated by a green organism only when it has been converted 

 into carbon dioxide, either by burning or by more gradual oxidation, 

 such as may occur in nature when oxide of iron is present. Organic 

 substances are continually undergoing decomposition, and such products 

 as carbon monoxide, carburettcd hydrogen, &c., may by oxidation be 

 converted into substances which can be again assimilated. 



The \veathcring and decomposition of rocks takes place mainly by 

 mechanical and chemical means, but living organisms may also assist 

 in the process, and it is they alone which produce the admixture with 

 organic materials that constitutes a humus soil. In the weathering of silicious 

 rocks carbonic acid is usually fixed and removed for the time being, but 

 above a certain temperature silicic acid drives out carbonic. Hence, 

 when the earth was still hot, the air must have contained much more 

 carbon dioxide than it does now, and this excess was probably deposited 

 in the carbon of the coal deposits. The first organisms that appeared on the 

 surface of the earth probably had a more abundant supply of carbon dioxide 

 than now exists, and were under different conditions in other respects 

 also (Sect. 57). This antagonism between carbonic and silicic acids is of 



