PHOTO-SYNTHESIS 31 



the chlorophyll in a plant, and caused the synthesis of for- 

 maldehyde to take place in ordinary sunlight. The formation 

 of formaldehyde appears to be dependent upon the finely 

 dispersed iron within the chloroplast, while the function of 

 the chlorophyll itself is probably to convert the formaldehyde 

 into sugars. If the formation of this chemical formaldehyde 

 and the amino-acids be the first step by which the living 

 plant builds up its starches and sugars, these experiments 

 at least hint at a possibility of preparing food material in 

 the laboratory, food which has hitherto been only and ex- 

 clusively produced by plants. The process of combining the 

 simpler substances into more complex compounds under the 

 influence of light is called Photo-synthesis. 



Photo-synthesis does not require direct sunlight, and it 

 has been shown that it is carried on most successfully in 

 land-plants when the intensity of light is about one-fourth of 

 the intensity of full sunlight. And the same seems to be true 

 of marine diatoms. The very brilliant light may actually 

 damage the chlorophyll and other living tissues of the plant, 

 just as the actinic rays of sunlight have a powerful effect on 

 the human body. Some plants are able to protect themselves 

 against this great intensity by manufacturing pigment in the 

 epidermis, just as the human skin may protect itself by turning 

 brown. A slight shade seems to be the best light for photo- 

 synthesis. It is curious to reflect that the sun never sees the 

 shade, and yet it is a very obvious fact. From the point of 

 view of the sun the shade is like Mr Chevy Slyme in Martin 

 Chuzzlewit, "always waiting round the corner": 



Never in its life has the sun seen shade, 



Never in its Hfe seen a shadow where it falls: 



There, always there, in the sun-swept glade, 



It lurks below the leaf; behind bodies, under walls, 



Creeps, clings, hides. Be it millions, be it one — 



The sun sees no shadow, and no shadow sees the sun. 



Laurence Housman, 



The great difference in the shape of the bodies of plants 

 and of animals is largely due to the difference in their food. 

 We have seen that plants can absorb their food in simple 

 chemicals from the air and the earth. Hence they are as a 



