Chap, ii Photosynthesis 303 



hydrate may perhaps not be formed directly from the 

 simple materials absorbed but may appear as a secretion 

 product from the chloroplast. He suggested that a sub- 

 stance possibly allied to formaldehyde may be first formed 

 according to Baeyer's theory and that this may be used in 

 the construction of protein by combining with the nitrogen 

 and sulphur absorbed in the form of salts from the soil, or 

 with nitrogenous substances derived from previous de- 

 compositions of protein. This protein may be then assimi- 

 lated by the protoplasm of the chloroplast, and from the 

 latter the starch may be secreted. 



Six years later, in 1892, Crato put forward a hypothesis 

 of a different nature from either. He suggested that the 

 carbon dioxide after absorption becomes ortho-carbonic 

 acid, which exists in solution in the cell sap. This acid 

 has the structure of a closed benzene ring in which six 

 molecules are linked together. This becomes decomposed, 

 liberating six molecules of water and six molecules of 

 oxygen, and forming a hexavalent phenol C 6 H,,(OH) ( .. 

 This then undergoes a molecular rearrangement and 

 becomes glucose C 6 H 12 6 . 



The last hypothesis of the century which calls for notice 

 is that of Bach, put forward in 1893. He pointed out that 

 when sulphurous acid, H 2 S0 3 , is exposed to light, it becomes 

 converted into sulphuric acid, sulphur and water being 

 split off, and he argued that a process analogous with this 

 reaction takes place in a leaf. The carbon dioxide unites 

 with water and forms carbonic acid, which is then split 

 up in the same way as the sulphurous acid, 3H 2 C0 3 = 

 2 H 2 C0 4 + H 2 + C. The carbon and water are not liberated 

 separately, but in combination as formaldehyde. The new 

 acid H 2 C0 4 splits up into carbon dioxide and hydrogen 

 peroxide, and the latter is decomposed into water and free 

 oxygen. 



A work of very great importance appeared in 1880. This 



