176 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



So the cycle is completed. Starting with certain simple mate- 

 rials, the plant protoplasm builds up complex plant substances, fixing 

 the sun's energy by means of the transformer chlorophyll ; this is 

 the constructive part of the cycle. Later on, the other part — the 

 destructive change — begins ; the complex substances break down 

 and are resolved finally into the simple substances with which the 

 change began Things are now as they were, excepting only that these 

 simple substances have for a time been caught up in the wheel and 

 have served for the manifestation of life ; they have then been put 

 down again, ready to serve as nutrients for a new generation of 

 plants. 



The soil is the medium in which this second part of the great 

 cycle of life goes on. It is so intimately bound up with this cycle 

 that one cannot separate them, or think of the soil apart from the 

 changes occurring within it. This is the great distinguishing feature 

 of the soil, marking it off sharply from a heap of mineral matter and 

 bringing it out of the province of the mineralogist into the sphere 

 of the biochemist. In the language of the laboratory, the soil has 

 to be studied as a dynamic and not as a static medium, and we have 

 to think of it as the seat of perpetual change. 



Cycle of Changes between the Soil and the Plant. 

 Complex Plant Substances. 



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Substances. Compounds. 



The processes involved in the formation of soil may be seen at 

 work on the little landslips that occur periodically on clay cliffs. 

 With each new slip a new surface of subsoil is exposed as a heap of 

 virgin mineral matter. Before long vegetation begins to spring up : 

 frequently colt's-foot and Equisetum. This dies down and starts the 

 process of soil formation ; finally a very different type of vegetation 



