96 The Living Plant 



oxygen outside, those two elements having originally been separated 

 by the kinetic energy of the sunlight in photosynthesis and kept 

 separate through all the subsequent transformations and trans- 

 portations of the food through the bodies of plants and animals; the 

 original source of respiratory energy is therefore the sunlight, and 

 food is primarily a storage battery, charged 'by the sun in green 

 leaves and discharged by respiration at the places of need. 



It will doubtless ere this have occurred to some philosophic 

 reader to ask whether carbon dioxide and water are the sole 

 substances by which organisms can thus store and transport 

 energy, and whether, accordingly, life is dependent solely upon 

 them. There is, however, no chemical reason why organisms 

 might not use in the same way any other decomposable and 

 oxidizable substances, and indeed even in our common plants some 

 small quantity of energy is no doubt derived from the oxidation of 

 other elements, while certain Bacteria exist which can use the 

 energy derived from the oxidation of sulphur compounds. Plants 

 probably use carbon in photosynthesis and respiration chiefly 

 because its chemical transformations, which are very susceptible 

 to temperature, happen to be easily under control at the temper- 

 atures now prevailing on the earth's surface. Under markedly 

 higher or lower temperatures carbon would be unavailable for this 

 purpose, but it is conceivable that life might still exist by the 

 similar use of other substances whose combinations would be 

 under control at those temperatures. It is only a step farther to 

 assume that life might even exist in this way in the flames of a 

 nebula, or the awful cold of interplanetary space, and hence 

 that its origin may be contemporaneous not only with the origin 

 of the earth, but even with the origin of matter itself. It is not 

 at all likely that life is something which results incidentally from 

 the properties of carbon; it is far more probable that it is some- 

 thing which uses the properties of carbon as the most convenient 

 tools for its own ends. This is a phase of the super-vitalism of 

 which I have spoken in the first chapter. 



