410 



THE FOOD OF PLANTS 



The fact that nitrogenous assimilation occurs mainly in the green leaves 

 may be of advantage in many ways, especially because the products of the 

 assimilation of carbon dioxide may be at once utilized, and hence an 

 injurious accumulation avoided (Sects. 54 and 55). When, however, no 

 assimilatory products are present, or when the leaf is in a pathological 

 condition owing to the continued absence of light, it is hardly surprising 

 to find that nitrogenous assimilation ceases (cf. Sect. 55, and for the 

 regeneration of amides, Sect. 80). 



Sachs' observation that proteid substances are continually removed from an 

 assimilating leaf indicates that an active power of assimilating inorganic nitrogen- 

 compounds must be localized there. Schimper has since shown that the absorbed 

 nitrates disappear from the attached as well as from the detached leaf 1 , and at 

 the same time, according to Chrapowicki, the amount of proteid present increases, 

 while Emmerling's results indicate that amides are produced in especial abun- 

 dance 2 . Schimper holds that the appearance of calcium oxalate favours the 

 synthesis of proteids in the leaf. It is not yet certain to what extent nitrates 

 may be assimilated in other organs, but an experiment by Miiller-Thurgau seems 

 to indicate that this process does actually occur in the root, and it is certain 

 that proteids may be formed from the amides conveyed to the roots and other 

 organs. Whether the synthesis of proteids takes place mainly in the sieve-tubes, 

 as A. Fischer 3 suggested, has yet to be proved. 



PART V. 



THE ASH CONSTITUENTS. 

 SECTION 73. The Essential Elements. 



In addition to carbon, hydrogen, oxygen, and nitrogen, there are 

 certain other essential elements which are found in the ash left behind 

 when a plant is burnt (Sect. 50). These ash constituents form 1-5 to 

 5 per cent, of the dry weight in plants supplied solely with essential salts, 

 but may reach as much as 10 or even 30 per cent. 4 when large quantities 

 of unnecessary salts are absorbed (Sect. 22). Hence it can only be found 



1 Sachs, Flora, 1862, p. 298, and Eot. Zeitung, 1862, p. 372; Schimper, Bot. Zeitung, 1888, 

 p. 130. From the mere distribution or accumulation of nitrates no conclusion can be drawn; cf. 

 Schimper, p. 120, and the literature quoted on p. 405. 



3 Emmerling, Versuchstt., 1880, Bd. XXIV, p. 113, and 1887, Bd. XXXIV, p. 73 ; Chrapowicki, 

 Bot. Centralbl., 1889, Bd. XXXIX, p. 352. 



3 Schimper, I.e., p. 89; Flora, 1890, p. 257; Miiller-Thurgau, Bot. Jahresb., 1880, I, p. 319; 

 A. Fischer, Ber. d. Sachs. Ges. d. Wiss. zu Leipzig, 1885, p. 280. 



* A complete series of ash analyses is given by Wolff, Aschenanalysen, 1871 and 1880, and 

 also by Ebermayer, Physiol. Chemie, 1882, p. 727; Ad. Mayer, Agr.-Chem., 1895, 4. Aufl., 

 p. 307. See also Sect. 22. 



