38 PHYSIOLOGY 



carbon dioxide, water, ammonia or some related compound, and sulphates. 

 A sharp distinction 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 definite 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 respectively 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 evolu- 

 tion 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 silica. The action of water 

 charged with carbon dioxide on a silicate is to cause its gradual decomposition 

 with the formation of carbonate and silica. Both these products, being in- 

 soluble, are deposited as part of the earth's crust, the silica in the form of 

 sandstone, the carbonate as chalk or limestone. The carbon dioxide is 

 being constantly removed by water from the atmosphere and being locked 

 up in this way in the earth's crust, the process of separation of calcium car- 

 bonate being aided to a marked extent by the agency of living organisms 

 themselves. The whole of the extensive deposits of limestone and chalk 

 have been separated from the sea-water by the action of living organisms. 

 With the cooling of the earth's crust which is supposed to be going on, the 

 discharge of carbon dioxide by volcanoes must get less and less, so that one 

 can conceive a time when the whole of the carbon dioxide will be bound up 



