196 METABOLISM 



will easily pass through the outer walls of the cells. The matter is not so certain 

 in the case of water plants because the oxygen surrounding them has only 

 a low tension. An investigation of the intercellular space system in all plants, 

 whether aerial, subterranean, or submerged, teaches us that accumulation of 

 carbon-dioxide and deficiency in oxygen never reach a degree worth considering, 

 and hence that the means at the disposal of the plant are always sufficient for main- 

 taining a gaseous exchange. Carbon-dioxide to the extent of 5 per cent, and 

 oxygen as low as 8 per cent, are only seldom met with in intercellular spaces, 

 and Pfeffer and Celakowski have shown that in the interior of individual 

 cells oxygen is never wanting. Pfeffer (1889) studied Rotifera living in the 

 cell-sap of Vaucheria, which moved actively under normal conditions, but whose 

 movements ceased when oxygen is prevented from entering. Celakowski (1892) 

 studied the protoplasmic streaming in cells of Tradescantia which had been 

 absorbed by the plasmodium of a Myxomycete, and found that these movements 

 continued inside the plasmodium ; the cells must, therefore, have been abun- 

 dantly supplied with oxygen. 



Having now gained some acquaintance with the general occurrence of 

 respiration we have to inquire next as to the substances which undergo respira- 

 tion and the products resulting therefrom, products which naturally are related 

 to those arising from ordinary combustion. In many cases it may be shown that 

 starch and sugar disappear during respiration. If these were completely burnt 

 we mustexpectcarbon-dioxide and water as the final products. From the formula 



C.H^A + 60, = 6CO2 + 5H,0 

 it may be seen that for every volume of oxygen taken in one volume of carbon- 



CO 

 dioxide must be produced, and in many cases the respiratory quotient -~ 



has actually been found to be unity. The contemporaneous formation of water 

 may also be demonstrated. Saussure (1804, p. 17) long ago remarked that 

 germinating seeds lost more weight than one would expect from the amount of 

 carbon-dioxide formed, and he thought that this was due to loss of water ' which 

 previously was united with the substance of the seed '. Laskowsky (1874) in- 

 vestigated the origin of the water by exact methods and found it to be about 

 equal to the amount which was to be expected from the formula given above. 



CO 

 It would be quite wrong to expect unity as the value of the fraction -~ 



in higher plants, on the assumption that carbohydrates were exclusively used up 

 in respiration. The average value for the fraction is the result of a number 

 of processes, each of which varies from unity, being sometimes greater, some- 

 times less. 



The higher plants do not lend themselves readily to such experiments, because 

 it is difficult to say in most cases what substances are undergoing combustion. It 

 is quite otherwise with Fungi ; in their case we have it in our power to supply the 

 organism sometimes with one kind, sometimes with another kind of material. 

 On this subject we owe much to Puriewitsch's (1900) thorough researches on 

 Aspergillus^ and the following table gives a summary of his results : — 



Relation of Carbon-dioxide to Oxygen in Aspergillus. 

 Nutrient. 

 Dextrose 

 Cane sugar 

 Raffinose 

 Starch 

 Glycerine 

 Mannite 

 Tannin 

 Tartaric acid 

 Lactic acid 



