METABOLISM AND GROWTH 219 



place in the tissues. The zoologist had, apparently, a 

 quite straightforward problem to solve, for he had to 

 consider only one gas entering the organism and one gas 

 escaping from it. The problem presented to the botanist 

 was a much more complex one. If you look back to 

 Ingen-Housz's account of gaseous interchange between 

 the air and the plant you will, I think, realise that he 

 was far from having a clear conception of the meaning 

 of the various phenomena he observed taking place. 

 Even De Saussure uses the terms " inspiration " and 

 " expiration " in relation to the entry of carbon dioxide 

 and the exit of oxygen in the photosynthetic process. 

 Considering the condition of chemical knowledge in these 

 early days, it was not to be wondered at that physiologists 

 should be puzzled by observing in plants one kind of 

 " respiration " taking place by day and another by night. 

 It was Dutrochet, so far as I can see, who first (in 1837) 

 asserted that respiration was fundamentally identical 

 in both the plant and the animal world, and yet Liebig, 

 four years after Dutrochet, flatly denied the occurrence 

 of respiration in plants. Sachs, in his famous textbook 

 pubhshed in 1868, cleared up the whole situation, and 

 showed that respiration was, as Dutrochet had stated, 

 essentially the same in both sets of organisms, but that 

 in the green plant, when exposed to hght, the respiratory 

 phenomena were masked by the much more vigorous 

 inhalation of carbon dioxide and exhalation of oxygen. 



As soon as this fundamental thesis had been estabUshed, 

 enquiry began to be made as to what exactly took place 

 in the tissues — how was the actual oxidation effected ? 

 Working on the idea that respiration was essentially 

 an oxidation of carbon — a kind of slow combustion in 

 fact — investigators from about 1870 onwards poured but 

 paper after paper, giving data as to the amounts of 

 oxygen inhaled and carbon dioxide exhaled under different 

 conditions of temperature, food-stuffs, and so on. The 

 animal physiologists were also busy with the same 



