30 LECTURES ON BIOLOGY 



the definition of metabolism holds, therefore, still good as a 

 function specific only to living substance. 



Nevertheless, this criterion does not suffice for the strict 

 definition of the animate and inanimate, for though real 

 metabolism is not to be found in dead nature, we shall see 

 later that there are nevertheless organisms in which this 

 function is, for shorter or longer periods, absent. 



We have already heard that the normal course of life 

 depends upon the presence of certain conditions, the absence of 

 which leads inevitably to death, or at least to the temporary 

 discontinuance of the life-functions. One of these conditions is 

 nutrition. Without nutrition there is no life. The substances 

 which furnish the different organisms with the required force and 

 heat and renew the substances which have been used up, are of 

 a very varied nature ; further, the substances required for the 

 nutrition of animals are different from those required by plants. 

 Whilst the former are only able to use complex organic com- 

 pounds for building up living albumen, the food-sources of plants 

 lie in the surrounding air, water, and the salts of the soil. But 

 however great the wealth of such food-sources may be, the plants 

 are unable to use them except under the influence of the sun- 

 light. It is only with the help of the sunlight that the chloro- 

 phyll is able to decompose the carbon dioxide of the air into 

 carbon and oxygen the first step that must be taken in the 

 building up of the organic compounds. And as all nutrition of 

 animals depends finally upon this activity of the plant cell, we 

 must see in the sun one of the fundamental conditions of life. 



Next to nutrition stands respiration as a function specific to 

 all organisms. All that lives breathes. In a room wholly 

 devoid of air, in water wholly devoid of gas, life is impossible. 

 But just as no particular article of nutrition can be mentioned as 

 being of absolute necessity to all organisms, so there is no gas 

 which is absolutely necessary to the life of all organisms. Often 

 that which is necessary to the life of one brings death to another. 

 Formerly it was thus believed that oxygen is needed for the 

 growth of all organisms. This is still true of all higher plants 

 and animals, for though plants give off oxygen during the day 

 they absorb it again, like animals, during darkness. We know, 

 however, thanks to Pasteur, that among the lowest known 



