Permeability 215 
Effect of Changes in Turgor 
To account for the exudation of water or solution from living 
cells in the case of water pores and nectaries, and in the phenomena 
of bleeding and root pressure, it has been suggested that a change in 
the permeability may be involved which is brought about when the 
turgor pressure has reached a certain amount. It is certainly con¬ 
ceivable that a stretching of the protoplasmic envelope might render 
it more permeable. Livingston (1903) thus thinks that as water passes 
into the cells in question the turgor pressure may rise to a critical 
point when a change in permeability takes place as a result of which 
cell sap exudes through the protoplasm. As a result of this the turgor 
pressure is reduced and the cellulose wall contracts. If the change 
in permeability is uniform throughout the whole of the protoplasm, 
exudation will only take place to any extent into the cavity of the pore 
of the water stoma or the cup of the nectary, as the turgor pressure 
of the surrounding cells will oppose the passage of water into them. 
After the cell wall and protoplasm have contracted so that turgor 
pressure is below the critical value, the protoplasm regains its semi- 
permeable property, water passes in from the neighbouring cells, 
and the turgidity increases until the critical value of the turgor 
pressure is again reached. During this period reabsorption of the 
water previously extruded is prevented because evaporation has con¬ 
centrated the extruded solution so that it has now a higher osmotic 
concentration, and would therefore tend to extract water from the 
cell. It must be supposed on this (and perhaps not only on this) view 
of the action of water pores and nectaries, that the solutes lost from 
the vacuole in exudation are replaced by secretion in the protoplasm. 
A similar mechanism can be supposed to act in the case of the 
exudation of water from cut stems and leaves, and in the phenomenon 
of root pressure. 
It must be emphasized that this theory was only put forward 
by Livingston as a tentative one. It is possible that no change in 
permeability is involved in the phenomena of exudation, bleeding 
and root pressure, and references to theories in which this latter point 
of view is taken have already been noticed in Chapter ix. 
Effect of Wounding 
Trondle (1921) investigated the effect of wounding on the perme¬ 
ability of cells of the young roots of Pisum , Vida , Lupinus and 
Allium by the method used by him to investigate the effect of acids 
and narcotics on permeability, and already mentioned in this chapter, 
