32 THE WORK OF THE LEAF [cHAP. 



enable the green fronds of the fern to decompose 

 sufficient carbon dioxide to maintain its slow rate of 

 growth, and also to give rise to the oxygen required 

 by the plant for respiration throughout the whole year. 

 Similarly the plant is always re-creating carbon dioxide 

 by its respiration, and this carbon dioxide must be 

 undergoing a perpetual cycle of change — it is split up 

 by the plant in the light summer days, its oxygen being 

 returned to the air and the carbon held by the plant, 

 then in the darkness and the winter this change is more 

 slowly undone again and the carbon dioxide recon- 

 stituted. Of course, in such a fashion the fern could 

 never become any heavier, could not in fact have 

 reached its present size ; there has also been another 

 source of carbon dioxide within the bottle due to the slow 

 decay of the organic matter originally present in the 

 soil. The development of this enclosed fern with its 

 quiet annual ebb and flow might be looked upon as a 

 sort of perpetual motion machine, but we must not fail 

 to recognise the fact that one external factor — the 

 incidence of the light — is absolutely necessary to the 

 process. It is this light, small as its amount may seem 

 to be, which supplies the energy required to drive the 

 machine, and this fact may lead us now to consider 

 the assimilation process from the point of view of the 

 exchanges of energy which go on during the life of 

 the plant. It is clear that a plant is a storehouse 

 of energy, of potential work ; given enough of the 

 plant — a tree, for example — we can burn it under a 

 boiler and drive an engine which will do work for us. 

 To come a little closer, starch will burn when ignited 

 and supplied with air or oxygen ; in burning it will give 

 out heat, which can be made to do work. Clearly, then, 

 starch and oxygen contain more energy than the carbon 

 dioxide and water into which they are turned by the 



