120 THE MECHANISM OF ABSORPTION AND TRANSLOCAT1ON 



absorbed in proportions altogether different from those obtaining in the water 

 in question. Similarly the large quantities of organic material, which green 

 plants produce, form an indication of the extent to which plants manage to 

 ultimately accumulate relatively enormous quantities of a substance presented 

 in excessive dilution, as is the case with the carbonic acid gas of the air. 



This selective and accumulative power may be very clearly demonstrated 

 by means of certain aniline dyes (see Sect. 16), because the colouration 

 enables the process of absorption to be followed, and the region where 

 accumulation takes place to be determined. As has already been described, 

 methyl-blue penetrates all plant-cells, but accumulates by passive secretion 

 only when it combines with substances present in the cell-sap to form either 

 a coloured precipitate, or a non-exosmosing, though soluble and coloured 

 compound. The latter takes place in the roots of Lemna minor, for in 

 the epidermal cells, the dye may reach a concentration of one per cent, 

 in from one to three days, although the external fluid contained only ooooi 

 per cent, of methyl-blue. Hence, in this time as much dye is carried to the 

 cell-sap and retained there, as is present in one thousand times the quantity 

 of external fluid l . 



Even when the water contains in 100,000,000 parts only one part of 

 methyl-blue, the cells of Lemna accumulate the pigment substance, if only 

 sufficient quantities of the solution are employed, or if the percentage of the 

 dye present is kept constant. It is true that each individual cell absorbs 

 the dye in trifling absolute amount, for a single cell weighs less than 

 o-ooi mgr., but a mass of cells weighing i mgr. may passively secrete as 

 much of the dye as is present in a litre of the dilute solution, so that 

 100 litres of the latter contain as much dye as 100 mgr. weight of cells can 

 absorb and accumulate. 



In a similar manner plants can obtain the potassium, phosphoric acid, 



and other substances, which they require, from extremely dilute solutions, 



provided either that fresh fluid is frequently supplied, or that the percentage 



of salts present is continually being readjusted and kept constant. These 



substances and -the aniline dyes mentioned, as well as other bodies, may be 



( completely removed 2 from a limited amount of a dilute solution. In the 



I same way living plants enclosed in a confined space may absorb every 



\trace of oxygen from the air surrounding them. 



This power of accumulating particular substances is shown not only by 

 plants, but in all cases in which the absorbed substances are fixed as rapidly 

 as they are taken in. Thus potash withdraws carbonic acid from the air 



1 Pfeffer, Unters. a. d. Hot. Inst. z. Tubingen, 1886, Bd. II, p. 198. 



a For examples see Versuchsstat, 1865, Bd. VII, p. 93, and Beitr. d. Sachs. Ges. d. Wiss., Leipzig, 

 l8 75> i> P- 76; Nobbe and Siegert, Versuchsstat., 1864, Bd. VI, p. 43. Fungi may devour all the 

 sugar present in a nutrient solution. On the protection of certain substances, when others preferable 

 to the plant are present, see Sect. 67. 



