ADVENTURES OF A CARBON ATOM 



yields up carbonic acid, so that it is constantly in circu- 

 lation. 



This is more easily seen by tracing the probable history 

 of an atom of carbon. We will suppose that it enters 

 a grass leaf as carbonic acid gas and becomes starch : 

 next evening it will become sugar and may pass from cell to 

 cell up the stem to where the fruit or grain is ripening. It 

 will be stored up as starch in the grain. This grass will 

 become hay and in due course be eaten by a bullock. The 

 starch is changed and may be stored up in the fat of the 

 animal's body. When this is eaten at somebody's dinner, 

 the fat will most probably be consumed or broken up ; this 

 breaking up may be compared to a fire, for heat is given off, 

 and the heat in this case will keep up the body-temperature 

 of the person. The carbon atom will again become carbonic 

 acid gas, for it will take part of the oxygen breathed in, and 

 be returned to the atmosphere as carbonic acid gas when the 

 person is breathing. 



Another atom of carbon might enter the leaves of a tree : 

 it will be sent down as sugar into the trunk and perhaps 

 stored up as vegetable fat for the winter. Next spring the 

 vegetable fat becomes starch and then sugar : as sugar it will 

 go to assist in forming woody material. It may remain as 

 wood for a very long time, possibly 150 to 200 years : then 

 the tree falls and its wood begins to decay. 



The bark begins to break and split because beetles and 

 woodlice and centipedes are burrowing between the bark and 

 the wood. Soon a very minute spore of a fungus will some- 

 how be carried inside the bark, very likely sticking to the 

 legs of a beetle. This will germinate and begin to give out 

 dissolving ferments which, with the aid of bacteria, attack 

 the wood. Our carbon atom is probably absorbed into the 



