172 THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



and oolitic limestones. When plates of gypsum are used, the course of the roots is 

 represented by raised lines, since the roots partially protect the gypsum from the 

 solvent action of the water in the soil. If, however, plaster of Paris and powdered 

 chalk are made into a paste and poured upon a glass plate, the roots form similar 

 etchings to those formed on marble upon the shining surface thus produced. 

 Czapek l used calcium phosphate, aluminium phosphate, &c., instead of chalk, in 

 order to determine the behaviour of the roots towards these substances. He found 

 that they exercise no effect upon aluminium phosphate, which is corroded by all 

 inorganic acids, and by oxalic, formic, tartaric, malic, citric, and butyric acids, but 

 is not attacked by carbonic, acetic, or propionic acids. 



According to these and other researches of Czapek's, it appears that, with the 

 exception of carbonic acid, the roots of land-plants excrete no free acids. The 

 permanent reddening '-' produced by many roots when pressed upon litmus-paper is, 

 as a general rule, due to acid phosphate of potassium (KH 2 PO 4 ), which is usually 

 given off in minute traces from young roots, or to a similar acid phosphate. The 

 reddening of a neutral watery litmus solution induced by living roots usually dis- 

 appears on boiling, and can be used, together with the decolourization of red 

 phenolphtalein solution, to demonstrate the excretion of carbonic acid by them. 



Certain proofs of the excretion of other free acids by the roots have never 

 been brought forward, for free acetic acid (Oudemans and Rauwenhoff, 1. c.) and 

 butyric acid (Boussingault) are, according to Czapek, not present in the normal 

 root secretions, and although Czapek finds formic acid to be commonly present, as 

 stated by Goebel, it is always combined with bases s . 



As a matter of fact all the solvent actions hitherto observed can be brought 

 about by the action of carbonic acid. The sharpness of the outline formed upon 

 marble is readily explicable when we remember that the most energetic action of 

 the CO., takes place at the point of contact, and that by the continuous removal 

 of the soluble products a marked result can finally be produced. Thus a well- 

 defined etching may be produced when a small glass jar covered with printed 

 parchment and filled with water impregnated with carbonic acid is inverted upon 

 a marble slab, if the water in the jar is continually renewed. The parchment 

 may be covered with writing in wax, and to produce a more rapid reaction a dilute 

 solution of hydrochloric acid may be used, while by means of the apparatus given 

 in Fig. 1 8, it may be shown how the soluble products formed by the exosmosing 

 acid may be diosmotically removed and transferred to the interior of the cell 4 . 



1 Czapek, Jahrb. f. wiss. Bot., 1896, BcL XXIX, p. 331. 



a Becquerel, Archiv. de Bot., 1833, T. I, p. 400; Oudemans u. Rauwenhoff, Linnaea, 1859-60, 

 Bd. xxx, p. 220 ; Molisch, Sitzungsb. d. Wiener Akad., 1887, Bd. xcvi, Abth. i, p. 105 ; Czapek, 1. c. 

 From certain unproved statements of Moldenhawer (Beitrage z. Anat. d. Pflanzen, 1812, p. 312) as 

 to the solvent actions of roots, Sprengel (Die Lehre vom Diinger, 1839, p. 23) assumed that they 

 were able to excrete fixed acids. See also de Candolle, Physiol., 1833, T. I, p. 186 ; Liebig, Ann. 

 d. Chem. u. Pharm., 1858, Bd. cv, p. 139. 



3 Boussingault, Die Landwirthschaft, 2. Aufl., 1851, Bd. I, p. 24; Goebel, Pflanzenbiol. Schilder- 

 ungen, 1891, Bd. n, p. 211. 



4 First employed for demonstration purposes by Zoller, upon Liebig's recommendation : Ver- 

 suchsst, 1863, Bd. V, p. 45. 



