THE DIOSMOTIC PROPERTIES OF THE CELL 99 



plasmolysis, this must be due either to a gradual absorption of the plasmolysing 

 salt, or to an internal production of osmotically active substances, and the latter 

 does actually occur when Fungi 1 are placed in concentrated solutions. If the 

 gradual return to the normal condition of turgidity takes place only when the cell 

 is plasmolysed by solutions of particular substances, it is evident that the re- 

 expansion of the protoplast is due to its being more or less permeable to the 

 substance in question. The researches of de Vries 2 , Wieler 3 , Klebs 4 , Janse 5 , 

 Overton 6 , and others, have been carried out from this standpoint, and it has been 

 found that many of both higher and lower plants absorb during an entire day no per- 

 ceptible amount of KNO 3 , Nad, cane sugar, &c., but that all grades of transition 

 may be found, leading ultimately to plants, whose protoplasts are readily penetrated 

 by these substances. This is shown especially well by Janse's researches and by ' 

 Fischer's 7 work on Bacteria. In the latter case, perhaps owing to the relatively 

 large surface exposed, the absorption, of the plasmolysing salt and the re-expansion 

 of the protoplast are extremely rapid. The bacterial protoplast appears to be 

 readily permeable not only by KNO 3 , Nad, and cane sugar, but also by other 

 substances, and this property, permitting as it does a rapid accommodation to any 

 change in the concentration of the external medium, must be of great importance to 

 these organisms. The plasmodia of Mycetozoa also appear to be readily permeable 

 by many salts, including Ca SO 4 and the amido-acid, Asparagin 8 . 



As de Vries and Klebs have shown, glycerine and urea are substances which 

 can penetrate all protoplasts, and often with marked rapidity. Here also all 

 degrees of permeability are exhibited, for the bud scales of Begonia manicata 

 allow these substances to penetrate only with difficulty. Overton has recently 

 determined, by means of the plasmolytic method, the extent of the diosmosis of 

 which many different substances are capable. 



Exosmosis. The occurrence even of slight exosmosis of the soluble substances 

 which -the cell contains, and to which the preservation of turgor is due, would 

 render it impossible for any alga or other aquatic plant to remain turgid, or to 

 retain its soluble reserve-food-material, while even from a beet-root a large part of 

 its sugar would be extracted by the surrounding damp soil. In other cases, how- 

 ever, plants or parts of plants are specially adapted for exosmosis, and may either 

 be constantly able to excrete sugar or other substances, or may only do so under 

 special conditions (cf. Sects. 93, 109). Hence, it is not surprising that in different 

 researches the same results are not always obtained. 



1 See Pfeffer, Druck u. Arbeitsleistungen, 1893, pp. 304, 428. 



8 De Vries, Sur la permeab. d. protopl. d. betteraves rouges, 1871 (Sep.-abdr. aus Archiv. 

 Neerland., 1871, T. vi) ; Jahrb. f. wiss. Hot., 1884, Bd. xiv, p. 441; Bot. Zeitung, 1888, p. 229, 

 and 1889, p. 309. 



3 Wieler, Ber. d. Bot. Ges., 1887, Bd. v, p. 375. 



4 Klebs, Unters. a. d. Bot. Inst. z. Tubingen, 1888, Bd. II, p. 54- 



5 Janse, Mededeelingen d. Kon. Akad. v. Wetenschappen, Amsterdam, 1888, p. 33 a - 



6 Overton, Osmot. Eigenschaften, 1895 (Sep.-abdr. a. Vierteljahrsschr. d. Naturf.-Ges. z. Zurich, 

 Bd. xi) ; Zeitschr. f. phjsik. Chemie, 1897, Bd. xxil, p. 189. 



7 A. Fischer, Jahrb. f. wiss. Bot, 1895, Bd. XXVII, p. 150. 

 * Pfeffer, Plasmahaut u. Vacuolen, 1890, p. 220. 



H 2 



