114 The Living Plant 



the swamps of the Coal Period; their walls, losing the oxygen and 

 hydrogen, have become proportionally richer in carbon and in- 

 cidentally darker in color, passing gradually through stages repre- 

 sented by peat, lignite, soft coal and anthracite, which latter is 

 almost entirely carbon. It is thus that our beds of coal have 

 been formed. Somewhat the same thing occurs, through the action 

 of heat, in the charring of wood, and a similar process produces 

 the black humus of good soils from roots and the like. When the 

 carbon of coal remains yet longer exposed to suitable conditions, 

 it becomes graphite or black lead, while if crystallized it forms 

 diamond, the end of the series. And it is interesting to note in 

 this connection that we do not yet know any natural way by which 

 pure carbon can be isolated from oxygen without photosynthesis 

 constituting a step in the process. If one were to burn the dia- 

 mond, he would form carbon dioxide again, and thus close the 

 chain of transformations through which the carbon has gone since 

 it was absorbed from the air by a living green plant long ages ago. 

 And as to this burning, it is interesting to reflect that the heat and 

 light released in the combustion of coal is energy that was rendered 

 latent by the photosynthetic dissociation of carbon dioxide when 

 the coal was first formed as a photosynthate; it has been kept 

 stored all this time in the unsatisfied affinity of its carbon for 

 oxygen; and when released in our midwinter fires, it is really the 

 heat and the light of the ancient carboniferous sun that is warm- 

 ing and cheering us. 



Gums. These are solid but very elastic sweet substances, of 

 which gum arable, used in gumdrops and on postage stamps, 

 is the most familiar example; the gum of cherry trees is another, 

 and the substance of marsh mallows another, though the spruce 

 gum, of the woods and the schoolroom, is quite different as will 

 be noted below under resins. These gums are accumulated in 

 rifts of the tissues of some trees, but it is not at all clear why the 

 plant should make them, though apparently they serve at times 

 as reserve food. Chemically they have the formula (CbIIkjOs)?!, 



