94 THE MECHANISM OF ABSORPTION AND TRANSLOCAT1ON 



with substances, with which normally the plant does not come into contact, 

 which are not used in metabolism, and which yet penetrate to the cell-sap 

 without any vital activity being involved. Further, it is not every pigment, 

 nor all the compounds that a pigment substance may form, which may 

 be thus absorbed and passively secreted '. This subject is dealt with in 

 greater detail later (Sect. 22), but it may be mentioned that, for example, 

 methyl-blue accumulates in some cases in dissolved form in the cell-sap 

 (roots of Lemna minor, root hairs of Trianea bogotensis), while in others, 

 a blue precipitate is formed (Spirogyra, roots of Azolla). This reaction 

 begins in a few minutes in a o-coi to 0-0005 per cent, solution of the pigment 

 substance, and rapidly leads to a marked accumulation in the cell-sap. 

 The minute traces of methyl-blue diosmosing through the plasma are 

 not sufficient in amount to cause any perceptible colouration of the latter. 

 In the plasma of the root hairs of Trianea^ however, methyl-violet 

 and cyanin accumulate in sufficient amount, using solutions of similar 

 dilution, to cause the appearance of a perceptible violet tinge 11 , and hence, 

 in such cases, the actual passage of the pigment through the plasma is 

 directly visible. If the root hairs are placed in pure water as soon as 

 the absorption of the dye has commenced, the colouration of the plasma 

 gradually disappears, as the dye passes inwards to accumulate in the 

 cell-sap. The above-mentioned dyes penetrate all living cells, but it is 

 only where a certain amount of passive secretion takes place, that the 

 pigment substance accumulates in perceptible amount. Such accumulation 

 is only possible when the dye is converted, as rapidly as it is absorbed, 

 into a modification, which is unable to diosmose through the plasma 

 membranes, or escape from the cell. The precise character of the still 

 soluble combination in which pigment accumulates in the cell- sap of 

 Triatiea, Lemna, Elodea, &c., is as yet unknown. The blue precipitate 

 which methyl-blue produces in the cell-sap of Spirogyra and Azolla, is 

 a compound of tannic acid and methyl-blue, and if this compound is 

 presented to the cell in solution it is found to be incapable of diosmosing 

 through the plasmatic membranes. That the diosmotic properties of the 

 different soluble compounds of any given substance may markedly differ, 

 can be directly demonstrated by experimentation with artificial preci- 

 pitation membranes 3 . It may also be mentioned that the protoplast is 



1 [In passive or indirect secretion, substance presented to the cell is absorbed and accumulated 

 by means of the formation of non-diosmosing compounds. In active or direct secretion the sub- 

 stance is produced by the cell itself, and may either accumulate or be excreted by it. The two 

 processes are not always perfectly distinct.] 



a For further details see Pfeffer, 1. c., p. 247. On the staining of the living nucleus, Campbell, 

 Unters. a. d. Bot. Inst. z. Tubingen, 1888, Bd. II, p. 569; Lauterborn, Uber Bau u. Kerntheilung 

 d. Diatomeen, 1893 ; Palla, Jahrb. f. wiss. Bot., 1893, Bd. XXV, p. 535. See also Pfeffer, 1. c., p. 273. 

 On the changes in the mode of pigment absorption coincident with death, see Pfeffer, 1. c., p. 276. 



" See for example Walden. Zeitschr. f. physik. Chemie, 1892, Bd. X, p. 699. 



