METABOLISM IN LIVING PLANTS. 455 



grouped, and imagine that the varied arrangement of the atoms building up a 

 molecule finds expression in the whole mass of the substance in question. If 

 six black, ten blue, and five red balls are placed close together in a frame, they 

 can be grouped in the most diverse ways into beautiful symmetrical figures. 

 They are always the same balls, they always take up the same space, and yet the 

 effect of the figures pi'oduced by the diflferent arrangements is wholly distinct. 

 It may be imagined, similarly, that the appearance of the whole mass of a 

 carbon compound becomes different in consequence of the arrangement of its 

 atoms, and that not only the appearance, but even the physical properties 

 undergo very striking alterations. 



A glance back at the history of the development of carbon compounds, very 

 briefly stated here, will render suflaciently clear how it becomes possible that 

 many thousand diflerent organic substances are compounded from carbon and 

 a few other elements, viz., hydrogen, oxygen, and nitrogen; and how this almost 

 infinite multiplicity of vegetable organic compounds is connected with the re- 

 markable chemical nature of carbon. The materials of which these substances 

 are formed are extremely simple, and the changes undergone by plant-substances 

 depend entirely upon the insertion and rejection, on the grouping and arrangement, 

 of the atoms of a few elements. 



METABOLISM IN LIVING PLANTS. 



In the living plant these combinations, decompositions, and rearrangements 

 are accomplished with great ease, and multitudes of substances, which cannot 

 be manufactured, either directly or indirectly, in a chemical laboratory, are pro- 

 duced in plant cells, with a hand's turn, so to speak. This applies principally 

 to those organic materials already generally described in a previous section of 

 this book, which have been formed from inorganic food, from carbonic acid 

 and water. It is exactly these, however, which have the greatest claim upon 

 our interest. They are of the utmost importance to everything which lives 

 and moves on our earth; their formation is the adjustment of one of the greatest 

 contx-cists in nature, they form the bridge which connects the inorganic with the 

 organic world, the dead with the li\äng. As a matter of course, these primary 

 organic substances, derived from carbonic acid and water, are the starting-points 

 for all the other chemical compounds of which the bodies of plants and of 

 animals are composed; or, in other words, they form the commencement of all 

 these further chemical changes in living cells which are understood by the term 

 MetabolisTn. 



The process of formation of these primary organic compounds is, on the 

 whole, easily comprehensible. It is known that carbon dioxide, i.e. carbonic 

 acid, is absorbed by plants, and that oxygen is given out; it is also known that, 

 when this process is carried on in a plant kept in a confined space, a volume 

 of oxygen is given out which is equal to the amount of carbon dioxide taken 



