THE MECHANISM OF SECRETION AND EXCRETION 129 



marked results 1 . Similar effects are produced by all factors which influence the 

 growth and activity of the plant. The influence of illumination upon the com- 

 position of the ash has been investigated by R. Weber 2 , &c. Analyses of the ash 

 of healthy and unhealthy plants are given by Wolff, and also comparative analyses of 

 the ash of parasites and of their host plants. 



Relation between the water and salts absorbed. When a watery solution is 

 absorbed, the water and salts are as a general rule taken up in a different ratio 

 to that existing in the external medium. The absorption of a salt is determined 

 primarily by its diosmotic properties and by the selective power of the plant under 

 observation. At the same time, an inwardly directed current of water may accelerate 

 absorption, and in such cases it is always possible to arrange conditions in which 

 either the salt or the water shall be absorbed in greater relative proportion than 

 in the solution presented to the plant. Thus, by diminishing or stopping transpira- 

 tion, it is always possible to cause relatively more salt to be absorbed than water. 



De Saussure 3 and other investigators 4 employed very dilute solutions and 

 always used transpiring plants for experimentation ; hence they concluded that 

 plants absorbed their nutriment in extremely dilute form. The so-called 'de 

 Saussure's law,' which has been accepted more especially in text-books of agri- 

 cultural chemistry, is merely a special case, and not a general law applicable to 

 absorption under all conditions. Later researches by W. Wolff with transpiring 

 plants, and by Knop and Biedermann with swelling seeds, have moreover shown 

 that when solutions of appropriate strength are used, certain salts are absorbed 

 in greater relative amount than water 5 . 



SECTION 23. The Mechanism of Secretion and Excretion. 



Excretion must always accompany vital activity, owing to the con- 

 tinued production of katabolic substances whose removal is necessary. 

 The chief excreta are water and carbon dioxide, and, in green assimilating 

 plants, oxygen; but in many cases, and especially in fermentative organisms, 

 compounds such as alcohol and various organic acids, as well as other 

 substances, may be excreted. The eliminated products may either result 

 from the plant's own secretory activity, or, in the case of Myxomycetes, 

 may be solid particles which have been previously ingested (Sect. 19). 



In addition to the uses mentioned in Sect. 20 and Chap. X, many 

 secretory products attain special importance in a variety of ways. It is 



1 See Wolff, Versuchsstation, 1865, Bd. vn, p. 193, and 1868, Bd. x, p. 3; Nobbe, ibid., 1870, 

 Bd. XIII, p. 383, &c. On the influence of chalky soils, see Malaguti et Durocher, Ann. d. sc. 

 nat, 1858, iv. ser., vol. ix, p. 230; Fleche et Grandeau, Ann. d. chim. et d. physique, 1874, v. sen, 

 vol. v, p. 354. 



- R. Weber, Versuchsstat., 1875, Bd. xvm, p. 36. 



3 Sanssure, Rech. chim., 1804, p. 247. 



4 A. Trinchinetti, Bot. Zeitung, 1845, p. in; Schlossberger, Ann. d. Chemie n. Pharmacie, 

 1852, Bd. i.xxxi, p. 172 ; Hert, ibid., 1854, Bd. LXXXIX, p. 334. 



5 W. Wolff, Versuchsstat., 1864, Bd. vi, p. 203, and 1865, Bd. VII, p. 193. See also Knop, 

 ibid., 1859, Bd. I, p. 194, and 1864, Bd. VI, p. Si ; also Biedermann, I.e., 1867, Bd. IX, p. 312. 



PFEFFER K 



