22 GENERAL INTRODUCTION 



When the stimulus is removed, anabolic processes are lessened, 

 and therefore increased katabolism now decreases also; but 

 katabolism, although decreasing, is in excess, and its reaction 

 tends to increase anabolic processes until both are in equilib- 

 rium. There is thus an internal self-adjustment of metabolism 

 in living matter. It must be borne in mind that metabolism 

 is probably not limited to the building up and breaking down 

 of the biogen, but may be brought about in other substances 

 under the influence of living matter. Such changes are desig- 

 nated contact changes. 



In order that metabolism may continue, living matter must 

 have a sufficient supply of such material as it can build into 

 its structure. These materials are called foods, and may be 

 defined as substances which, taken into the cell, aid in the 

 repair or in the formation of new biogens, adding to the sum 

 total of energy which the cell may liberate, and are finally 

 cast off by the cell in altered chemical condition. The taking 

 in of food by an organism is termed ingestion. In very few 

 cases is the ingestion of solid foods possible, so that in order 

 that they may be made use of they are digested i. e., they 

 are acted upon by complex nitrogenous bodies known as ferments 

 or enzymes, which convert them into soluble forms. Enzymes 

 are the products of animals and plants possessing the power 

 of producing chemical changes in other bodies without appar- 

 ently undergoing any change themselves. As the conversion 

 takes place within or without the protoplasm it is designated 

 as intra- or extracellular digestion. 



The steps through which dead matter passes in its synthesis 

 to living matter are very incompletely known. In green plants 

 which thrive on the inorganic compounds, carbon dioxide, 

 water, and simple nitrogenous salts, the first step is observable 

 in the cells of the leaf, where, under the influence of chlorophyll 

 and the energy of the sun's rays (yellow chiefly), the carbon 

 dioxide of the air is split into its elements and the carbon is 

 united with hydrogen and oxygen in the proportions of water 

 to form starch (C 6 H 10 O 5 ) n . The latter is visible, microscopically, 

 as minute granules, and its formation has been proved to go 

 hand-in-hand with the disappearance of carbon dioxide. This 

 forms the starting point for the formation of all other bodies 

 in the plant. Reconverted to sugars probably, it disappears 



