418 THE FOOD OF PLANTS 



ash constituents obtained from air and water were no longer regarded as the 

 sole source of the plant's vital activity. The results obtained by Marrgraff (1764), 

 Wiegleb (1774^ Senebier, and especially de Saussure, were sufficient to convince 

 unprejudiced observers that a plant contains only those elements which it derives 

 from without. The opposite view for a time found supporters although it gradually 

 disappeared, and was finally and decisively disproved by the exact experiments of 

 Wiegmann and Polstorff, who showed that when plants were grown in pure sand, 

 or fragments of platinum, the seedlings contained no more and no less ash con- 

 stituents than were originally present in the seed '. 



Senebier and de Saussure were probably the first to form correct ideas as to 

 the importance of the ash constituents. In holding that the latter entered into the 

 metabolism of the plant Senebier was considerably in advance of his time, for 

 long afterwards the mineral elements were frequently regarded simply as stimulating 

 agencies, or as solvents for the organic material conveyed through the plant. 

 Sprengel, however, was the first to state that not all of the ash constituents are 

 essential, and that the essential mineral elements have a specific importance and 

 cannot be replaced by other elements-. 



The general necessity of ash constituents was determined by the researches 

 already mentioned, but which were the essential elements was first systematically 

 investigated by Salm-Horstmar by means of the differential culture-method. A few 

 experiments in this direction had previously been performed by de Cassincourt, 

 John, Boussingault, &:c., but these were often inconclusive 3 . Both the latter 

 authors used insoluble substrata, such as sand, quartz, and sugar-charcoal previously 

 boiled with acid. Salm-Horstmar used similar soil saturated with a nutrient solu- 

 tion, and placed in tin pots lacquered with wax. Sulphur and pumice-stone have 

 also been utilized for this purpose, but sand has been usually employed in late years, 

 especially by Hellriegel (Sect. 69). In many cases sand-culture is preferable to 

 water-culture, although in the latter case it is more easy to ensure the entire 

 absence of any particular mineral constituent 4 . 



The method of water-culture was first employed by Woodward, who showed that 

 plants grew better in river than in rain-water, but best of all in a watery extract of 

 soil. Woodward also used culture-solutions containing inorganic salts, but did 

 not succeed in definitely proving the necessity of the mineral ash constituents. 

 Duhamel simply grew plants in water, and such experiments have but little im- 

 portance as compared with the previous ones by Woodward. The forgotten water- 



1 Wiegmann u. Polstorff, Uber d. anorg. Bestandth. d. Pflanzen, 1842. Cf. Kopp, Geschichte 

 d. Chemie, 1845, Bd. Ill, pp. 42, 259; Sachs, History of Botany (Garasey and Balfour), 1890, 

 pp. 453, 524; John, Ernahrung d. Pflanzen, 1819, p. 73. 



3 Senebier, Physiol. vege't., 1800, T. Ill, pp. 28, 45; Saussure, Rech. chiin., 1804, p. 261. Cf. 

 Meyen, Physiologic, 1838, Bd. II, p. 120; C. Sprengel, Die Lehre vom Diinger, 1839, PP- *> 35' & c -> 

 and also in Bodenkunde, 1837, P- 4'4- 



3 Salm-Horstmar, Versuche n. Resultate ii. d. Ernahrung d. Pflanzen, 1856; a compilation of 

 the works published from 1849 (Bd. XI.VI) to 1855 (Bd. LXIV) in the Journ. f. prakt. Chemie; 

 de Cassincourt, Journ. d. Pharmacie, 1818, p. 381 ; Boussingault in 1837 and 1838, see ref. in Agron., 

 Chim. agric., &c., 1860, T. I, p. 3. 



* Hellriegel, Beitrage z. d. naturw. Grundlagen d. Ackerbaues, 1883, Unters. ii. d. Stickstoff- 

 nahrung, 1888; Fittbogen, Versnchsst., 1870, Bd. xill, p. 81. 



