174 THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



Czapek, the acid reaction of the roots of Hy -acin thus orientalis is due to a compound 

 of oxalic acid (probably the acid potassium salt). 



No other secretory products are excreted in perceptible amount by the 

 roots of terrestrial plants T , and since a soil rich in humus would neutralize 

 and nullify small quantities of poisonous secretions, there is all the less 

 reason to assume that any injurious effect can be exerted upon the growth 

 and development of neighbouring plants in this manner 2 . Although the 

 solvent action of the roots of higher green plants is not as energetic as it 

 was formerly supposed to be, it is nevertheless by no means of trifling 

 importance, although Liebig's 3 conclusion that terrestrial plants absorb all 

 their ash constituents from the soil by means of the solvent action of the 

 roots, is certainly incorrect. It is indeed certain that, in a rich soil, and 

 with a sufficiently active circulation of water, the roots could absorb all the 

 nourishment they require without the necessity of any intimate union with 

 the particles of soil, whereas in a poor and dry soil a close application 

 and approximation of the root-hairs to the soil particles is of the utmost 

 importance. 



As a general rule, the plant is able to obtain possession of substances 

 absorbed and retained by the soil, whereas insoluble and non-decomposable 

 compounds are on the other hand unavailable for use, so that a plant 

 may die for want of potassium, phosphorus or nitrogen, in a soil which 

 is found to contain large quantities of these substances on analysis. The 

 constituents extracted from a soil by dilute acid or water do not afford 

 an accurate representation of its fertility, for such analyses do not take 

 into consideration the substances which may be gradually rendered available 

 by slow decomposition and disintegration 4 . 



In order to obtain food materials, many plants exert very energetic 

 solvent and disintegratory action, and thus lichens are able to create 

 for themselves a suitable habitat on bare rocks, and to obtain a supply 

 of nutriment from them (cf. Sect. 27). The rhizoids of mosses and lichens 

 slowly push their way between the sandy particles which their own activity 

 has loosened, and the commingling of these particles of dust with the dead 



1 In contradiction to Molisch's assumption (Sitzungsb. d. Wiener Akad., 1887, Bd. xcvi, p. 84) 

 Czapek finds ( Jahrb. f. wiss. Bot., 1896, Bd. xxix, p. 374) that in the root-secretions of Phanerogams 

 enzymes are not present at all, or not in perceptible amount. On the supposed oxidizing secretions, 

 see Pfeffer, Oxydationsvorgange, 1889, p. 406, and Hoveler, Jahrb. f. wiss. Bot., 1892, Ed. xxiv, 

 p. 313. See also Sect. 101. 



2 This conclusion, which formerly found common acceptance, has long been proved to be 

 erroneous. For literature see Mohl, Veg. Zelle, 1851, p. 95. Cf. on fungi, &c., Sect. 92. 



3 Liebig, Ann. d. Chein. u. Pharm., 1858, Bd. CV, p. 138; Die Chemie in Anwend. a. Agric. 

 u. Physiol., 1876, p. 1 20. Details on these points by A. Mayer, Agr.-Chem., 1895, 4. AufL, 

 Bd. n, p. 103. 



* Details in the quoted works of agricultural chemistry. Of the later researches, see Konig u. 

 Haselhoff, Landw. Jahrb., 1894, Bd. xxm, p. 1009. 



