6 5 8 SCIENCE PROGRESS 



This type of metabolism, the katabolic type, characterises the 

 animal, but not the plant. The essential difference between 

 typical animal and typical plant is that the former possesses 

 a sensori-motor system while the latter does not. Energy of 

 the available form is made use of by the animal (and the heat- 

 engine) and transforms to the kinetic energy of moving bodies. 

 Energy of radiation is said to be made use of by the typical 

 plant organism ; but it transforms, not to kinetic energy of the 

 parts of the plant, but to potential chemical energy. The 

 typical plant energy-transformations are to be compared with 

 the exceptional endothermic transformations of inorganic systems. 



The salient character of plant metabolism is the synthesis of 

 carbohydrate and proteid from inorganic compounds. Water 

 and carbon dioxide react together in such a way as to form 

 a sugar; that is, the elements of the latter are those which have 

 been taken into the plant system as water and carbon dioxide. 

 These compounds possess far less intrinsic energy than does 

 the sugar which is formed from them, so that the plant must 

 have some source of available energy to draw upon in effecting 

 the synthesis. This source is radiation. In the absence of 

 light chlorophyll is not formed, and in the absence of chlorophyll 

 sugar is not synthesised from water and carbon dioxide. More 

 energy is represented by the radiation falling on the surface 

 of the green leaf in unit time than is represented by the energy- 

 difference between the sugar which is formed in unit time, and 

 the carbon dioxide and water which are the initial phase in the 

 transformation. Formaldehyde can be synthesised from water 

 and carbon dioxide, and it is conceivable that formaldehyde may 

 be " polymerised " to form a sugar. There is a certain function- 

 ality between the intensity of the incident light, the concentration 

 of carbon dioxide round the chloroplastids, and the quantity 

 of carbo-hydrate synthesised. It is thus probable that the 

 formation of starch in the plant organism is a " photosynthetic " 

 process, and that the requisite energy is obtained from that 

 of light radiation. 



But, it must be urged, repeated investigation has failed to 

 show clearly that the radiation is the actual source of the 

 increase of available energy due to the life processes of the plant 

 organism. Radiation falling on an inorganic surface almost 

 always transforms into low-temperature heat, unavailable for 

 such a chemical transformation as that of water and carbon 



