356 THE FOOD OF PLANTS 



glycerine might with equal justice be regarded as an intermediate assimi- 

 latory product, for the chemist can construct this substance synthetically 

 from carbon dioxide 1 ; similarly formic 2 and oxalic acids may be syn- 

 thetized with comparative ease from carbon monoxide, while carbohydrates 

 may be produced from these by a series of chemical metamorphoses. 

 Erlenmeyer supposed that formic acid and hydrogen peroxide are formed 

 during the assimilation of carbon dioxide, a theory which the absence of 

 hydrogen peroxide from assimilating plants conclusively negatives 3 . 



Various reasons militate against the acceptance of Liebig's theory 

 that organic acids are the primary assimilatory products, and that carbo- 

 hydrates are produced from them. Urea is a substance which may be 

 comparatively readily constructed by synthesis, and the hypothesis has 

 already been put forward that urea is the primary assimilatory product 

 in nitrate bacteria 4 . 



SECTION 62. Individual and Specific Peculiarities. 



As division of labour becomes more marked, green plants undergo 

 successively increasing adaptive modification in order that the assimilating 

 chloroplastids may be subjected to appropriate illumination. The enormous 

 leaf-surface is developed for this purpose, although other factors, such as 

 the necessity of restricting transpiration, &c., may come into play and 

 either cause a reduction in the leaf-surface, or induce a marked develop- 

 ment of cuticle (Sects. 38, 52). Very commonly a compromise is made, 

 one function being more or less restricted in order that another may not 

 fall below the minimum compatible with continued existence. 



It is only necessary to mention a few of the salient features concerning 

 the position, distribution, &c. of the chloroplastids in the assimilating 

 organs. Even in a typical leaf it may be more economical to allow the 

 greater number of the chloroplastids to work in modified and weakened 

 light and hence with less energy, than to waste an enormous amount of 

 material in so increasing the surface area of the leaves that all the chloro- 

 plastids are fully exposed, for in the latter case there is a danger of over- 

 exposure, and other functions may be restricted or jeopardized. In our 



1 Meyer und Jacobson, Lehrb. d. organ. Chemie, 1893, Bd. i, pp. 579, 902. 



2 Cf. Lieben, Beibl. z. d. Ann. d. Phys. u. Chem., 1895, Bd. xix, p. 463. 



* Erlenmeyer, Ber. d. Chem. Ges., 1877, p. 634. Cf. Pfeffer, Oxydationsvorgange, 1889, p. 430. 



* Liebig, Die Chemie in Anwend. auf Agric., 1840, ist ed., p. 63; 1876, 9th ed., p. 30. Cf. 

 Sect. 56. On the production of carbohydrates from organic acids, see Ballo, Ber. d. Chem. Ges., 

 1889, p. 750. Maquenne (Chem. Centralbl., 1882, p. 329) regards methane as an intermediate 

 product, Crato holds that a benzene ring is formed (Ber. d. Bot. Ges., 1892, p. 250), and Putz 

 supposes (Chem." Centralbl., 1886, p. 774) that light acts by producing electrolytic currents. 



