PHYSIOLOGICAL 307 



must be admitted that the leaf remains far from ideally efficient 

 when we consider the relation of the energy absorbed to the amount 

 of work done. In fact this limited productivity inspired the great 

 chemist, Berthelot, to predict a Utopia, in which the chemical 

 factory should replace the field ! 



As to the products formed in the chloroplast laboratories, sugar 

 and starches are among the first stable substances to appear, 

 probably, as we have said, through the intermediation of formalde- 

 hyde. But there are many plants in which starch is not formed at 

 all, or only in exceptional conditions. Instead of starch there may 

 be oil, or it may be that the simpler carbohydrates are diverted 

 from the path leading to starch formation to one which leads to 

 amino-acids or other nitrogenous compounds. 



As all living matter includes protein substances, the most abund- 

 ant and probably the most important of its constituents, we must 

 continue our outline-inquiry a step further, and ask how the plant 

 makes its protein food. Since proteins are nitrogenous carbon- 

 compounds, whose molecules often include some sulphur atoms, 

 and in many cases some phosphorus as well, it is evident that they 

 cannot simply be built up from carbohydrates. The question is the 

 source of the nitrogen, and the first most obvious a priori answer 

 was -the atmosphere; for this contains 78 per cent, ol the element 

 required, and is present in the endless air-spaces of the soil, and 

 dissolved in the soil-water, and so in the sap that ascends and 

 transpires. Yet there is no evidence that any plants, save certain 

 fungi and bacteria, and the leguminous plants (with a few others) 

 with which these bacteria have formed a root-symbiosis, can utilise 

 free nitrogen. 



For ordinary plants, and not entirely excluding those with 

 symbions, the saprophytes, the parasites, and still less the insecti- 

 vorous species, which after all grow normally in the main, the 

 essential source of nitrogen is by way of ordinary root-action which 

 brings up the nitrates of the soil ; and these are mainly traceable to 

 organic matter decomposed by microbes. These nitrate salts are 

 carried in the sap to the living cells of the plant, and in the presence 

 of unstable or nascent carbohydrates, are built up into proteins. 



Though our knowledge is still very insufficient, it seems reason- 

 able to regard the upbuilding of proteins as a climax of the anabolic 

 and synthetic processes which occur in the green leaf. Starting from 

 its synthesis of carbohydrates: these may be converted into amino- 

 acids by the reduction of the sap nitrates to ammonia, and 

 these amino-acids, by linkages with others and by incorporation 

 of sulphur and phosphorus molecules from the sulphates and 

 phosphates also in sap-solution, may build up proteins. But along 

 this line of inquiry there is as yet more of scientific imagination and 

 endeavour than settled result. 



