THE MOLECULAR FORCES IN PLANTS. 151 



twenty- four hours) we can readily determine that the distance 

 between the marks is much less than at the beginning of the 

 experiment. Salt solutions, like glycerine or solutions of sugar, 

 remove water from the cells. The resulting loss of turgidity in 

 the cells results in contraction of the tissues. 3 



When plant structures lose water by withering, they shorten in 

 proportion to the loss of turgidity in their ceils. Pea seedlings 

 (developed in sawdust) whose roots have attained a length of 

 about 50 mm., are laid for half an hour in water, in order first of 

 all to make the cells of the root thoroughly turgescent. We now 

 carefully dry the roots with a linen cloth, and place on them two 

 ink-marks, one just behind the tip of the root, the other at a dis- 

 tance of about 25 mm. from the first. If we allow the roots to 

 wither for ten minutes in the air, it will be easy to prove that they 

 have shortened to a not inconsiderable extent. If we now lay the 

 seedlings in water, their roots lengthen again, and the distance 

 between the marks becomes the same as at the commencement of 

 the experiment. 4 



1 For a more detailed discussion of Turgor see myLehrbuch d. Pftiwenphysto- 

 logie, 1883, p. 213. 



2 See Traube in du Bois-Reymond and 'Reichert's Archiv. f. Anat. und 

 Pliysiol., 1867, p. 87. 



3 See H. de Tries, Untersuchungen iiber die mechanische Ursache der Zell* 

 streckung, Halle, 1877. 



4 See Sachs, Arbeiten des botan. Imtituts in Wiirzburg, Bd. 1, p. 396. 



60. Isotonic Coefficients. 



The osmotic energy of a cell is dependent on the quality and 

 quantity of the water-attracting substances present in the cell- 

 sap. In order to make ourselves acquainted with the components 

 of the cell-sap, we first of all select succulent plant structures 

 (e.g. leaf -stalks of Heraoleum Spondylium, young stems of 

 Rheum, leaves of Crassulacese, etc.), and, best after they have 

 been killed by heating in closed vessels on the water-bath, squeeze 

 them in a hand-press. The juice obtained is heated in closed 

 vessels on the water-bath at a temperature of 100 C., so as to 

 coagulate all the proteid matter, and then filtered.* Having 

 evaporated 10 c.c. of the clear juice, and carefully incinerated the 

 residue, we can easily detect the presence of chlorides in an 



* The heating of the plant structures and juice is to be carried out in 

 pressure bottles (to be obtained from Desaga, Heidelberg). 



