PART III. THE CROPS 



SECTION I. PHOTO-SYNTHESIS 



THE natural absorption of solar energy by plants is a process 

 called photo-synthesis, to account for which there are many 

 theories, none of which can be considered as proven. Some 

 outstanding features, however, remain without any question. 

 The sun's rays falling upon green leaves are absorbed with 

 the utilization of energy for the production of plant materials. 

 The proportion of energy used in this way is small, as is 

 shown in the following table : 



TABLE 18. PERCENTAGE OF TOTAL SOLAR ENERGY 



FALLING ON A LEAF. 



Energy used in assimilation . . . . . . 0*66 per cent. 



Energy used in evaporation of water . . . . 48-39 



Energy transmitted .. .. .. ..31*40 



Energy radiated, convected, etc. .. .. 19*55 ,, 



This table shows that the amount of energy actually utilized 

 for assimilation of carbon dioxide and its conversion into 

 organic plant matter is comparatively small, and that a 

 very great deal of the energy is used merely in evaporating 

 water (see p. no). Carbon dioxide is absorbed by the leaf 

 with very great readiness, in spite of the small proportion 

 or carbon dioxide in the atmosphere. It is often assumed 

 that one of the first products is formaldehyde. That form- 

 aldehyde can polymerize to sugars is undoubtedly well 

 proved. The mechanism by which formalderryde can be 

 produced in the plant is more difficult to discover. Oxygen 

 appears to be evolved practically simultaneously with the 

 absorption of carbon dioxide, and therefore very elaborate 

 chemical changes seem improbable. The energy that will 



