106 DIFFUSION AND OSMOTIC PRESSURE 



piece of apparatus by which a current is maintained through 

 a cell, different parts of whose wall are unequally permeable. 



That exudation pressure depends upon vital activity seems 

 evident from the fact that it ceases with death. Another 

 line of evidence which points toward the necessity of active 

 protoplasts for exudation is that brought forward by Wieler, 1 

 when he records that, if a bleeding plant is deprived of oxy- 

 gen, exudation stops. He observed the same result when 

 the plant was anaesthetized with chloroform. The exuded 

 liquid varies in its concentration in different plants, usually 

 becoming weaker as bleeding continues, but there seems to 

 be no discovered relation between the exudation pressure and 

 the concentration of the exudate. 2 



The whole subject of exudation and sap pressure is viewed 

 in an entirely new light by Molisch 3 in his last paper on 

 these phenomena. He presents convincing evidence that in 

 all cases where true bleeding has been observed it is a phe- 

 nomenon connected with the stimulus of wounding or with 

 the formation of new tissue, such as callus, over the wound 

 surface. Thus, the method of decapitation and of boring, for 

 the study of exudation pressure, becomes at least of very 

 doubtful use in the investigation of the normal pressure 

 within the plant. It is impossible to insert a manometer 

 into a stem without making a wound, and, according to 

 Molisch' s conclusions, this wound itself is sufficient to cause 

 a pathological condition of the neighboring tissue such that 

 exudation ensues. In the light of these considerations, then, 

 it seems extremely doubtful whether there is exhibited in 

 the normal, uninjured plant any such phenomenon as that of 

 sap pressure. Where exudation occurs in the pathological 

 wound tissues it must be due to some such change in per- 

 meability of the protoplasts as was postulated above. 



i A. WIELER, " Das Bluten der Pflanzen," Cohns Beitrage, Vol. VI (1893), p. 158. 

 2/6id.,pp. 65,69. 



3H. MOLISCH, "Ueber localen Blutungsdruck und seine Ursachen," Bot. Zeitg., 

 Vol. LX (1902), pp. 45-63. 



