INTRODUCTION 13 



there is one which has been known for a long time and is 

 certainly also the most important; we call it photosynthesis. 



Plants grow and increase in weight and size; they are 

 composed ahnost exclusively of carbohydrates. The synthesis 

 of these substances from water and carbon dioxide in the air 

 is endothermic; this means that the plant must be suppUed 

 with energy for the chemical reaction to take place. Light is 

 essential for the purpose. Luminous energy in the form of 

 rays of visible Hght, especially orange and red, is the only 

 form of energy which green plants can assimilate and use to 

 accompUsh this extremely important process. 



Growth is promoted when conditions are favourable to 

 photosynthesis. In fact, the study of the effect of Hght brings 

 us to the core of the still mysterious phenomena which govern 

 the Hfe of the plant and its nutrition. 



It is a commonplace to say that all the energy available to 

 man comes from the sun: the waterfalls of hydro-electric 

 power stations are fed by the rain and snow proceeding from 

 evaporation under the heat of the sun; the wind which drives 

 windmills is due to the differences of temperature of the air 

 heated by the sun; even wood, coal and petroleum, vegetable 

 foodstuffs and the animals which feed on them — all the things 

 that man rightly regards as valuable resources have been 

 created, with the help of solar energy, by photosynthesis. 



If plants did not perform this function, the radiant power 

 of the sun would be converted directly into heat; we owe to 

 photosynthesis the fact that a small part is turned into chemical 

 energy — a form infinitely more valuable to us. The aim of 

 cultivation is to promote this transformation as much as 

 possible; the process is unique, because no practical means 

 has yet been found of artificially converting solar energy into 

 chemical energy, but it has certainly not reached its maximum 

 efficiency. It is not impossible that a systematic study of the 

 action of Hght would lead to great advances in agriculture, 

 comparable, for example, with those which have resulted 

 from the use of chemical fertihzers. 



Under present conditions, what is the proportion of solar 



