466 



NATURE 



[February 14, 19 18 



phyll and the degree of openness of the stomata 

 are sometimes significant. 



When the magnitudes of the three external 

 factors are known or controlled, there arises the 

 important question of the nature of their inter- 

 action when the magnitudes of them vary inde- 

 pendently — a problem which has been elucidated 

 largely by English investigations. In any possible 

 combination of these factors, the rate of photo- 

 synthesis at any moment is not an expression of 

 their combined magnitude, but only of the magni- 

 tude of a particular one of them acting as a 

 "limiting factor" to the rate of functional 

 activity. Which of the factors happens to be the 

 limiting factor in any combination of them can be 

 determined experimentally by application of the 

 principle that increase of the magnitude of the 

 limiting factor, and of this factor only, can in- 

 crease the rate of photosynthesis. 



With high rates of photosynthesis, yet a new 

 factor has to be brought into account, as internal 

 causes produce a regular falling off of the power 

 of photosynthesis from moment to moment of 

 time. Until the internal causation of this is fully 

 explained it may pass by the non-committal name 

 of the "time-factor." 



There is yet another important aspect of our 

 attack on the problem of photosynthesis which is 

 still in its infancy, and that is the "energetics" 

 of the process, dealt with in chap. vi. of the 

 pamphlet. 



The essential human value of the chloroplast 

 activity lies, of course, over and above the indis- 

 pensable accident that its products are edible, in 

 the high energy content of these carbohydrates. 

 Therefore, the energetic aspect of the process is 

 the fundamental one, and the whole problem 

 should be investigated on this basis. This in- 

 volves measurement of the energy incident on the 

 leaf-surface, with determinations of the amount 

 transmitted, or reflected, or used in transpiration, 

 as compared with the fraction stored up in photo- 

 synthesis, which last finds expression in the in- 

 creased heat of combustion of unit-area of leaf- 

 surface enriched by carbon assimilation. In this 

 field of work progress can be made only by elab- 

 orate physical apparatus and critical determina- 

 tion of physical constants. 



Let us now turn to ihe economic aspect of 

 photosynthesis regarded as a problem of industrial 

 or applied science. In these times, when cereal 

 food supply threatens to become a limiting factor 

 to the endurance or free existence of nations, the 

 question of what science can do to multiply the. 

 number or heighten the activity of the chloroplasts 

 subserving the cause of humanity acquires a 

 poignant interest. 



It cannot be said that the physiological study 

 of chlorophyll activity has yet enabled any im- 

 provement to be made in the applied science of 

 agriculture. The conditions of present-day agri- 

 culture are too little intensive, and not yet such 

 as to make it worth while to attempt to exploit 

 the researches of plant physiologists. Cultivation 

 NO. 2520, VOL. 100] 



I of new acreage, selection of types, and increase 

 .of transport facilities are the present solutions 

 of the limitation factor of carbohydrate food 

 supply. 



The utilisation of researches on the augmenta- 

 tion of photosynthesis would be of profound im- 

 portance in the imaginary case of a self-contained 

 or strictly isolated community of limited acreage, 

 a wealthy and intelligent community with a large 

 population on a small area of soil for sunshine or 

 artificial illumination. Their problem would have 

 to be solved on the basis of investigations on the 

 factors controlling photosynthesis of the type we 

 have already mentioned. 



In such a community the relation between plant 

 physiology and agriculture would become some- 

 thing like that holding now between human physi- 

 ology and medicine. To-day every progress in 

 human physiology is eagerly correlated with medi- 

 cine, and lavish endowment and encouragement 

 are extended to pure physiological science on 

 account of its generally recognised applicability 

 to medicine. The outlook of medicine and hygiene 

 is, however, individual, and not commercial ; there 

 is a desire to save every life and continue the 

 activity of every individual, however worthless it 

 may be to the community. With agriculture and 

 plant communities there is no such outlook, and 

 with regard to any application of plant physiology 

 it is required that the intensification of the syn- 

 thetic activity of the plant aimed at shall pay 

 economically. 



We see, then, that it is probable that the main 

 cereal crops will for a long time be left to the 

 mercy of natural vagaries of light, heat, water, 

 and carbon dioxide, but minor activities of inten- 

 sive food cultivation are now utilising deliberate 

 or unconscious control of one or more of the 

 factors of photosynthesis. It becomes, therefore, 

 highly important that there should be carried out 

 a comprehensive investigation of the physiology 

 and energetics of carbon assimilation dealing with 

 the possibilities of intensive photosynthesis under 

 all artificial combinations of factorial conditions. 

 From what we have said as to the complexity of 

 this matter it is clear that no one or even two 

 investigators are likely to have all the special 

 chemical, physical, and physiological experience 

 required for rapid progress, so that this would 

 have to be an organised combined research, and 

 continued over a number of years with good equip- 

 ment and liberal endowment. 



In conclusion we may express the opinion that, 



I in the eyes of all who know the results of modern 

 work on chlorophyll, Germany has acquired last- 

 ing credit for her great achievement with this 

 difficult and elusive problem. Under the inspira- 

 tion of Prof. Willstatter many workers have 

 striven for years in the National Research In- 

 stitute, and thousands of pounds have been spent, 

 on a novel type of investigation involving tons of 

 leaf material. Their credit is not the less for this, 

 that the results have not at once proved to be of 

 economic importance : one more province of igno- 

 rance has been strenuously conquered and an- 

 nexed to the empire of knowlcd.o-c. 



