SPECIAL CASES 



411 



has suggested that surface-tension actions of this character may be 

 exercised by the wood-parenchyma cells along the path of the current. 

 This has still to be proved, however, and also whether the breaking 

 strain for continuous water-columns is the same in such tubes as the 

 tracheae and tracheides as in glass tubes of larger bore. 



The exact causation of bleeding is by no means clear, and in 

 fact it is quite possible that in some cases it 

 may be produced in the same way as the plasmo- 

 lytic excretion of water from nectaries, with the 

 exception that the osmotic substances which have 

 drawn water into the vessels may be reabsorbed in 

 their upward passage l . 



During the plasmolytic excretion of water 

 from nectaries the plant provides for the external 

 deposition of the sugar, which draws out water 

 from the turgid cells beneath and so produces 

 nectar. This physical action takes place whatever 

 the source of the sugar, and in this respect it is 

 immaterial whether the sugar is produced by 

 a metamorphosis of the cell-wall, or is formed in 

 the cell, and excreted externally. 



GROWTH. During plastic growth the stretching 

 of the cell-wall is due to the osmotic pressure in 

 the cell, whereas when the cell-wall grows by intus- 

 susception, growth may take place against the 

 osmotic pressure, as during the internal thickening 

 of cell-walls. When a growing cell encounters a 

 resistance, the tension of the cell-wall is gradually 

 counteracted until the full osmotic pressure is 

 acting against the resistance. In some cases the 

 mechanical retardation of growth produces a rise 

 of osmotic pressure, but the latter determines in 

 all cases the maximal pressure which a thin- walled films ' (After Ewart) 

 growing organ can exert. Thick-walled organs, however, so long as the 

 cells grow by intussusception, can exert greater pressures than those corre- 

 sponding to the osmotic pressures of the component cells 2 . 



If the resistance is not too great growth is resumed as soon as the 

 organ exercises a pressure greater than the resistance, and if the latter is 

 pushed in front of the growing organ the work done is equal to the product 

 of the force applied and the distance its point of application is moved. 

 With moderate resistances or loads the original activity of growth is soon 



FlG<69 . Diagrammatic longi- 

 ends 



1 Ewart, Phil. Trans., 1905, p. 42. 



a Cf. Pfeffer, Druck- und Arbeitsleistungen, 1893. 



