PHYSIOLOGICAL 319 



ways in which it works is probably in removing substances whose 

 accumulation would interfere with continued growth and activity. 

 The final results of the breaking down are CO2 and H^O; but it is 

 characteristic of the very economical metabolism of plants that 

 early stages in the breaking down of complex molecules may be 

 arrested and rebuilt into proteins again. 



At one of the early meetings of the Royal Society, in 1667, Robert 

 Hooke produced experimental evidence to show that in certain 

 circumstances a dog would die because of "the want of a sufficient 

 supply of fresh air". Soon afterwards (1674) John Mayow recognised 

 that there was in the atmosphere a particular constituent [spiritus 

 nitro-aereus) , which was indispensable for the continuance of vital 

 activity and for ordinary combustion, "Animals and fire draw 

 particles of the same kind from the air." "With respect, then, to 

 the use of respiration, it may be affirmed that an aerial something, 

 whatever it may be, essential to life, passes into the mass of the 

 blood. And thus air driven out of the lungs, these vital particles 

 having been drained from it, is no longer fit for breathing again." 

 The spiritus nitro-aereus, the "vital particles", the "aerial some- 

 thing", must be identified with what we call oxygen, which was 

 again discovered (after Mayow) by Priestley in 1774, who also 

 showed that air "spoilt" by animal respiration may be restored by 

 the activity of green plants in the sunlight. But Priestley's re- 

 discovery of oxygen, which he called "dephlogisticated air", was 

 wrapped up with the erroneous "phlogiston" doctrine, that burning 

 meant the liberation of a special compound, phlogiston or fire-stuff. 

 Thus it was reserved for Lavoisier, a little later, to explain the real 

 nature of oxidation, and to bring into line the burning candle, the 

 living animal, and the plant as well, the last being studied at night 

 when photosynthesis is in abeyance. In all three cases, oxygen from 

 the air is uniting, directly or indirectly, with the carbon of carbon 

 compounds, and forming an oxide of carbon (CO2) which Black had 

 discovered in 1755 and called "fixed air". Yet the student must 

 seek to avoid the false simplicity of identifying internal respiration 

 and combustion; for not only does the respiratory reaction in the 

 living cells go on at a low temperature, but it is very unlikely that 

 the oxygen rushes directly into union with the carbon of carbon 

 compounds. The "fire of life" is subtler than that of the hearth. 



Thus as regards plants there is reason to believe (i) that a carbon 

 compound, especially a carbohydrate, may be split, apart from the 

 action of oxygen, into carbon dioxide and a readily oxidised sub- 

 stance; and (2) that the latter, hy the action of oxygen, may then 

 undergo combustion. It is also held by some physiological botanists 

 that a distinction must be drawn between the breaking down of a 

 food-substance like sugar, comparable to fuel, and the more funda- 

 mental breaking down in the protoplasm itself. 



