130 THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



sufficient here to mention the ferments, acids, &c., by the solvent action of 

 which substances are rendered available for absorption, or by which, as in 

 parasitism, penetration into another organism is rendered possible. Wax, 

 ethereal oils, resin, and gelatinous envelopes l are all secretions of the proto- 

 plast, which are directly excreted as such, or attain their final composition 

 by extra-cellular metamorphosis. In this sense the cell-wall itself may 

 be regarded as a secretory product excreted by the protoplast (Sect. 84). 



Secretory products frequently undergo physiologically important 

 changes when excreted by the cell. When a plant renders a nutrient 

 fluid alkaline, substances such as earthy phosphates, &c., which are soluble 

 only in an acid solution, are precipitated, while the excretion of oxalic acid, 

 which is of frequent occurrence in fungi, causes a precipitation of calcium 

 oxalate when a soluble calcium salt is present. On the other hand, the 

 nature of the secretory products is influenced and regulated in a variety 

 of ways by internal and external agencies, as is the case with regard to 

 the production of oxalic acid (Sect. 86) and of enzymes (Sect. 91), while 

 in various carnivorous phanerogams (Sect. 65) definite stimuli induce or 

 accelerate the secretion of enzymes and free acid 2 . 



All that has been said about the causes and mechanism of the 

 exchange holds good for the special case, in which the substances excreted 

 are secretory products. As is indicated by the recovery from plasmolysis, 

 salts which are present in the fluid imbibed by the cell -wall, but which 

 cannot penetrate the protoplast, will diffuse outwards until equilibrium is 

 reached, when a plant is removed from a nutrient or plasmolysing solution 

 to pure water 3 . 



When a plant is actively transpiring, the continual introduction of new 

 traces of salts by the water current must lead to a certain accumulation 

 of these salts, since backward diffusion is but slow. Nobbe and Siegert * 

 actually found that patches of saline incrustation, mainly of potassium 

 chloride, were formed on the stem and leaves of buckwheat, and to a less 

 extent on those of barley, when the plants were grown on a watery solution 

 containing i per cent, of inorganic salts (see Sect. 38). The soluble 

 incrustations which may sometimes be seen on plants growing on saline 

 soil are probably of similar origin, although other factors, including perhaps 



1 Klebs, Unters. a. d. Bot. Inst. z. Tubingen, 1886, Bd. II, p. 404; Hauptfleisch, Zellmembrane 

 in Hiillgallerte d. Desmidiaceen, 1888. 



2 On the acid secretion of certain hairs see Stahl, Pflanzen u. Schnecken, 1888, p. 41. 



3 This has been observed by Knop, Versuchsst., 1860, p. 86, and W. Wolff (1. c., 1864, Bd. VI, 

 p. 230), in plants transferred from nutrient solutions to water. Similar researches, partly however 

 on plants in which the protoplasts had been killed by immersal in over-concentrated solutions, are 

 given by M. Macaire, Ann. d. Chemie u. Pharm., 1883, Bd. VIII, p. 789; Unger, tlber den Ein- 

 fluss des Bodens, &c., 1836, p. 147; E. Walser, Ann. d. sci. nat., 1840, ii. se'r., T. XIV, p. 106; 

 Wiegmann und Polstorff, Uber d. anorg. Bestandtheile d. Pflanzen, 1842, p. 49 ; Cauvet, Ann. d. 

 sci. nat., 1861, iv. ser., T. xv, p. 320. 



4 Nobbe und Siegert, Versuchsst., 1864, Bd. vi, p. 37 ; Nobbe, ibid., 1864, Bd. IX, p. 479. 



