ASSIMILATION. 199 



lion of all these products, therefore, the same quantity of carbonic 

 acid is consumed, and all its oxygen restored to the air.* It is 

 more and more evident, therefore, that, by just so much as plants 

 grow, they take carbonic acid from the air, they retain its carbon, 

 and return its oxygen. 



351. In the production of that modification of cellulose called 

 Lignine (41), which forms a secondary deposit thickening the walls 

 of the cells, and which abounds in wood, if this be really a simple 

 product, and not a mixture, not only must a larger amount of car- 

 bonic acid be decomposed, but a small portion of water also, 

 with the liberation of its oxygen. For the composition attributed 

 to it shows that it contains less oxygen than would suffice to con- 

 vert its hydrogen into water, f This excess of hydrogen, and the 

 still larger excess of carbon, renders those woods that abound with 

 incrusting deposit, other things being equal, more valuable as fuel 

 than those of which the tissue in great part consists of proper cellu- 

 lose, as has already been stated. 



352. The whole class of fatty substances, including the Oils, 

 Wax, Chlorophyll (85-87), and most of the products of their alter- 



* Since all these neutral ternary substances are identical, or nearly so, in ele- 

 mentary composition, and since, with the same amount of carbon, derived 

 from the decomposition of carbonic acid, the plant can form them all, not- 

 withstanding the great difference in their external characters, it will no longer 

 appear so surprising that they should be so readily convertible into each other 

 in the living plant, and even in the hands of the chemist. But the chemistry 

 of organic nature exceeds the resources of science, and constantly produces 

 transformations which the chemist in his laboratory is unable to effect. The 

 latter can change starch into dextrine, and dextrine into sugar; but he cannot 

 reverse the process, and convert sugar into dextrine, or dextrine into starch. 

 In the plant, however, all these various transformations are continually taking 

 place. Thus, the starch deposited in the seed of the Sugar-cane, as in all 

 other Grasses, is changed into sugar in germination : and the sugar which fills 

 the tissue of the stem at the time of flowering is rapidly carried into the flow- 

 ers, where a portion is transformed into starch and again deposited in the 

 newly-formed seeds. And although the chemist is unable to transform 

 starch, sugar, &c. into cellulose, yet he readily effects the opposite change, 

 by reconverting woody fibre, &c. (under the influence of sulphuric acid) into 

 dextrine and sugar. The plant does the same thing in the ripening of fruits, 

 during which a portion of tissue is often transformed into sugar. Starch-grains 

 and cellulose never can be formed artificially, because they are not merely 

 organizable matter, but have an organic structure, or are the result of growth. 



t According to Payen, lignine, separated as much as possible from cellulose, 

 consists of Carbon 53.8, Hydrogen 6.0, and Oxygen 40.2 per cent., = (P 5 , II 24 , 

 O 20 . 



