ABSOKPTION AND TRANSMISSION OF WATER 103 



Pitra. 1 The former found that various parts of stems 

 and roots exuded water when cut out from the plant and 

 placed partly submerged in wet sand. Details of the experi- 

 ments are not given, and there are many circumstances 

 in the somewhat superficial account which lead the reader 

 to doubt whether the author was dealing with true bleed- 

 ing ; for there are other factors, such as the expansion of 

 gases in the wood, which may cause exudation under cer- 

 tain conditions. Pitra's experiments are much more con- 

 vincing than those of Kraus, but it is still somewhat 

 doubtful whether very much true bleeding was observed by 

 either of these authors. Pitra makes one observation which 

 is of interest here, however. If a cut shoot be inverted with 

 its leaves under water and its stem in air, bleeding from the 

 cut surface of the stem will ensue. That is, if leaves are 

 placed in a position to absorb water, they can do so in a 

 manner entirely similar to that exhibited by roots, and a 

 leaf-pressure corresponding to the normal root-pressure 

 seems to be developed. The observations of Pitra in this 

 regard have been substantiated by Molisch, 2 who finds the 

 same to hold true if the leaves are not placed in water but 

 are surrounded by moist air. 



The evidence seems to be good that bleeding may occur 

 (1) in the case of cut stumps to which active roots are still 

 attached; (2) at the cut surface where the crown or inflor- 

 escence has been removed from certain palms (Molisch), and 

 (3) at the cut surface of certain stems whose leaves are sub- 

 merged in water (Pitra). It may occur in other parts, but 

 it seems that true bleeding has not been unquestionably 

 demonstrated elsewhere in tall plants. 



Exudation pressure has often been ascribed to osmotic 



1 A. PITRA, "Versuche fiber d. Druckkraft d. Stammorgane bei d. Erscheinun- 

 gen des Blutens u. Thranens," Jahrb.f. wiss. Bot., Vol. XI (1877), pp. 437-530. 



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

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



