102 DIFFUSION AND OSMOTIC PRESSURE 



alteration in permeability which is postulated above in ref- 

 erence to water pores and nectaries less difficult of supposi- 

 tion than it might appear at first thought. Also, the varia- 

 tions in permeability which are known to occur in certain 

 cells, e.g., those which can be brought about in almost any 

 cell by chemical agents like H g C1 2 , and by change in tem- 

 perature, for example the cold plasmolysis of Spirogyra, 

 render it at least possible that excess of turgor pressure may 

 alter permeability. Indeed, a similar phenomenon to the one 

 postulated was observed by Oltmanns 1 in the case of Fucus, 

 and it is more than probable that the exudations from certain 

 "sensitive" pulvini often, if not always, contain solutes. 



(3) Exudation. If the stem of a plant be severed near 

 its base and a mercury manometer be attached to the por- 

 tion connected with the roots, an exudation of water under 

 pressure may often be demonstrated. This so-called exuda- 

 tion pressure is not uniform, but varies in an irregular man- 

 ner, sometimes sinking below the zero point of the scale 

 and at other times amounting to an atmosphere or more. A 

 similar pressure is reported in various parts of the plant, 

 even high up in the tops of trees, as was noted by Molisch 2 

 in the case of Cocos and Arengo. A series of experiments 

 upon this subject was performed by Wieler, 3 who came 

 to the conclusion that the power of exuding sap is a 

 general one among plant cells. Other experiments which 

 are not nearly so satisfactory are those made by Kraus* and 



i F. OLTMANNS, "Ueber d. Bedeutung der Koncentrationsanderung des Meer- 

 wassers fur d. Leben d. Algen," Sitzungsber. d. k.preuss. Akad. d. Wiss. zu Berlin, 

 Jahrg. 1891, p. 183. 



2H. MOLISCH, "Die Secretion des Palmenweins und ihre Ursachen," Oesterr. 

 bot. Zeitschr., Vol. XLIX (1899), p. 74. Also see W.FIGDOK, "Untersuchungen fiber d. 

 Erscheinung des Blutungsdruckes in den Tropen," Sitzungsber. d. kais. Akad. d. 

 Wiss. zu Wien, math.-nat. hist. Klasse, Vol. CVII (1898), p. 640. This is noted in 

 Oesterr. bot. Zeitschr., Vol. XLVIII (1898), pp. 359, 360. 



3 A. WIELEE, " Das Bluten der Pflanzen," Cohns Beitrage, Vol. VI (1893), pp. 

 1-211. 



*C. KBATJS, "Untersuchungen tiber den Saftedruck der Pflanzen," Flora, Vol. 

 XL (1882), pp. 2 ff. 



