QUANTITATIVE SELECTIVE POWER 127 



factors which are always at work causing mechanical admixture inside 

 the cell are in themselves sufficiently active to convey a penetrating particle 

 from one end of the cell to the other with adequate rapidity, and indeed 

 as a general rule transference within the cell involves no marked delay. 

 It is moreover important to note, that in the passage along translocation 

 tissues (composed of elongated cells), the number of walls and total thick- 

 ness of the cellulose membranes to be crossed is relatively small. The 

 passage through the cell- wall, when unassisted by water currents passing 

 in one direction, is markedly assisted by mechanical mixing movements 

 both inside and outside the cell. 



Historical. Miilder 1 was probably the first to explain the selective power as 

 being due to the co-operation of diosmosis and chemical metamorphosis. A com- 

 plete account, of which the general features still hold good, was first given by 

 Schulz-Fleeth 2 . Greater accuracy was only attained as the diosmotic properties of 

 the cell began to be understood. In connexion with nutritive processes, Pfeffer 

 developed the principles governing the selective and excretory powers, in the works 

 already quoted. That plants absorb substances in other proportions than those in 

 which they may be present in the nutrient fluid presented to the plant was, it is 

 true, already made known by de Saussure 3 and others. Considering the standpoint 

 of natural science at that time, it is not surprising that the controlling factors should 

 have been incorrectly interpreted, and thus in older writings the erroneous state- 

 ment is found, that currents of water are necessary to introduce the substances 

 which the plant accumulates 4 . 



Selection of the ash constituents. Recognition of the accumulation of ash con- 

 stituents in the plant, and of the unequal proportions present in plants growing 

 in the same soil, first called attention to the selective capacities of plants. An 

 extended series of examples is afforded by numerous ash analyses B , while instances 

 of changes of composition induced by plants in nutrient solutions will be found in 

 the literature of water-culture given later (Sect. 73 \ 



As an example, the following analyses by Goedechens, of algae obtained from 

 the west coast of Scotland, are given *. The total ash is given in percentage of dry 

 substance, the single constituents in percentage of the ash itself. 



1 Mulder, Physiol. Chemie, 1844-51, p. 678, footnote. 



3 Schulz-Fleeth, Der rationelle Ackerbau, 1836, p. 124; Rochleder, Chemie u. Physiol. d. 

 Pflanzen, 1858, p. 137, also gives a correct account. 



8 Saussure, Rech. chim., 1804, p. 248; Trinchinetti, Bot. Zeitung, 1845, p. in. 



4 Thus Woodward, Phil. Trans., 1699, Vol. xxi, pp. 221. 253; Senebier, Physiol. Veget., 

 1800, Vol. Ill, p. 28; de Candolle, Pflanzenphysiol., translated into German by Roper, 1833, 

 Vol. I, p. 62. 



8 E. Wolff, Aschenanal. v. landw. Producten, pt. i, 1871 ; pt. 2, 1880. Numerous analyses 

 are given by Liebig, Die Chemie in Anwend. auf Agric. u. Physiol., 1876, 9. Aufl., p. 522 ; 

 J. Konig, Chemie d. menschl. Nahr.- u. Genussmittel, 1889, 3 ed. 



6 Goedechens, Ann. d. Chemie u. Pharmacie. See Wolff, 1. c., 1871, p. 130. 



