146 THE LIFE OF THE PLANT 



The form of the leaf finds its physiological signifi- 

 cance in this function of light absorption. For this 

 the flat form is more efficient than any other. The area 

 of the light-absorbing leaf-surface is in some plants 

 eighty times as large as the area of the soil they cover. 



It is only now that we can fully appreciate the 

 significance of the processes that take place in the 

 leaf. In the first place there is the assimilation of 

 one of the most important elements that enter into 

 the composition of the plant — carbon — and at the 

 same time a transformation of inorganic into organic 

 matter. As we have already said, all organic matter 

 in plants and animals comes directly or indirectly 

 from the leaf ; while the process of its manufacture 

 in the leaf links up the whole organic world to the sun. 

 The leaf serves as intermediary between any mani- 

 festation of energ}^ in the organic world and the sun, 

 the universal source of energy. It is not the plant 

 alone, but also the whole of the animal world, man 

 included, that profits by the energy of the sun stored 

 up by the plant. We have seen that a seed gets warm 

 during germination. This heat is derived from respira- 

 tion, the combustion of some part of the organic matter 

 bequeathed to the seed by the parent plant. In the 

 formation of this organic matter, the sun's energy has 

 been employed. The seed, therefore, germinating in 

 the soil, profits by the heat of the rays of the sun 

 absorbed by the parent plant. Similarly, when we use 

 organic matter as food, we also take in the rays of the 

 sun latent in it, and use them to warm our bodies or 

 set them in motion. 



This means that the leaf, the only natural laboratory 

 where organic matter for both animal and vegetable 

 kingdoms is prepared anew, in this assimilation of car- 

 bon stores up the energy of the sun's rays, thus becom- 

 ing the source also of energy, the retailer of heat and 

 light to the whole organic world. 



