THE ELEMENTARY CONSTITUENTS OF PROTOPLASM 41 



The synthesis of carbon compounds from the inert carbon dioxide 

 of the atmosphere can be effected only by chlorophyll corpuscles. 

 All animals take in carbon, hydrogen, nitrogen, oxygen, and sulphur 

 in the form of the carbohydrates, fats, and proteins which have been 

 built up in the living plants. In the animal organism these food-stuffs 

 serve as sources of energy, undergoing a gradual oxidation, and finally 

 leave the body in the form of carbon dioxide, water, ammonia or 

 some related compound, and sulphates. A sharp line of demarcation 

 has therefore often been drawn between the metabolism of plants and 

 animals, plants being regarded as essentially assimilatory in character 

 while animals are dissimilatory, utilising the stores of energy which 

 have been accumulated by the plant. There is, however, no sharp 

 line of demarcation. Although, generally speaking, the green plant 

 breaks up carbon dioxide, giving off oxygen and storing up carbon 

 compounds, and the animal taking in carbon compounds oxidises 

 them with the help of the oxygen of the atmosphere to carbon dioxide, 

 which is redischarged into the surrounding medium and is available 

 for further assimilation by plants, yet this process of respiration is 

 common to all living organisms, whether plants or animals. In the 

 green plant it may be masked by the assimilatory process occurring 

 under the influence of the sun's rays, but in the dark all parts of the 

 plant, and in the light all parts which are free from chlorophyll, 

 display a process of respiration, i.e. they are constantly taking 

 up oxygen from the atmosphere and using it for the oxidation of 

 carbon compounds in their tissues, with the production of carbon 

 dioxide. 



The sum total of the processes of life tend, therefore, to maintain 

 a constant proportion of carbon dioxide and oxygen in the atmosphere, 

 the decomposition of carbon dioxide by the green plants being balanced 

 by the oxidation of the carbon compounds and the continual discharge 

 of carbon dioxide by animals. It is not certain, however, that 

 this balance will be maintained throughout all time. As Bunge has 

 pointed out, there are cosmic factors at work which are apparently 

 tending to cause a constant diminution in the quantity of carbon dioxide 

 in the atmosphere, which alone is of value to the plant. One of these 

 factors is the variable affinity of the silica and carbon dioxide respec- 

 tively for the chief bases of the earth's crust. At a high temperature 

 silica can displace carbon dioxide from its compounds. Thus chalk 

 heated with silica will give rise to calcium silicate with the evolution 

 of carbon dioxide. At an early geological epoch, therefore, it is probable 

 that the greater part of the silica was present in combination with 

 bases and that the proportion of carbon dioxide in the atmosphere 

 was very much higher than it is now. At temperatures at present 

 ruling on the earth's surface carbon dioxide is a stronger acid than 



