98 THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



The existence of proteids in the precipitates formed in the cell-sap has not as 

 yet been demonstrated, but it is extremely probable that they are actually present. 

 When experiments are conducted under conditions physically similar to those 

 existing within the plant, as for example, in artificial tannin-gelatine cells, or in 

 capillary tubes containing tannin and closed by a tannin-gelatine precipitation- 

 membrane, it is found that irrigation with solutions of aniline dyes, caffein, 

 ammonium carbonate, &c. produces similar precipitations ' to those which these 

 re-agents induce in living cells 2 . In both cases the precipitates have the same 

 character and reactions, and show the same power of reducing silver salts, which Loew 

 and Bokorny have supposed to be a satisfactory test for active albumin (Sect..n). 



Colouration and precipitation tests for diosmotic absorption. Free acids (acetic, 

 tartaric, phosphoric, &c.), as well as carbonic acid, and the caustic alkalies, readily 

 and rapidly penetrate the living protoplast. This can be easily demonstrated by 

 allowing very dilute acid or alkaline solutions to act upon cells containing blue or red 

 cell-sap (floral leaves of Pulmonaria or Rosa, sections of beet-root, or staminal hairs 

 of Tradescantid}. The presence of a cuticle renders penetration more difficult, but 

 otherwise a change of blue to red colouration or red to blue, as the case may be, 

 soon takes place, and as a rule the original colour returns when the tissue is well 

 washed with water, while, if the reagents are sufficiently dilute, and the action not 

 too prolonged, the cells are uninjured and remain living. Solutions of Iodine and 

 Mercuric chloride also rapidly and readily penetrate the cell, but exercise a very 

 pronounced injurious and poisonous effect :1 . 



The absorption of Ammonium carbonate, Caffein, Antipyrin, &c. is indicated 

 by the formation of a precipitate within the cell, without any injurious effect being 

 necessarily produced. In many plants the rapid penetration of Hydrogen-peroxide 

 is made evident by the colourations or decolonizations produced within the cells 4 , 

 and similarly dissolved oxygen also readily diosmoses through typical cell-walls. 



Chemical methods. Absorption and passive secretion may in many cases be 

 estimated or determined by macro- or micro-chemical methods. The formation of 

 starch in chloroplastids or leucoplastids may serve as an indication of the absorp- 

 tion of various sugars and certain other substances (Glycerine, etc.), (Sect. 54), while 

 in other plants a supply of sugar leads to an accumulation of glucose '' or other 

 carbohydrates. Similarly in many plants nitrates* may accumulate to such an 

 extent that the dried substance burns like touch-paper. Other plants again store 

 up large accumulations of phosphates, sulphates, &c. 7 inside living cells. 



Plasmolytic tests. When the contracted protoplast slowly re-expands after 



1 [The formation of these precipitates is usually included among the phenomena of ' aggre- 

 gation,' but it is better to restrict the latter term to the formative changes occurring in the proto- 

 plast, since the one is a vital but the other a purely physical phenomenon.] 



3 Cf. Klercker, Klemm, and Pfeffer, 1. c. 



3 Pfeffer, Osmot. Unters., 1877, p. 140. 



4 Pfeffer, Zur Kenntniss d. Oxydationsvorgange, 1889. 



5 Schimper, Bot. Zeitung, 1885, pp. 743, 758 ; Puriewitsch, Ber. d. Bot. Ges., 1896, p. 206. 



Wolff, Landw. Versuchsst, 1864, Bd. VI, p. 220; Frank, Ber. d. Bot. Ges., 1887, p. 472; 

 Schimper, Bot. Zeitung, 1888, p. 121. Further literature sect. 70. 

 7 Schimper, Bot. Zeitung, 1888, I.e. 



