TUKGIDITY 51 



layer is so transformed (perhaps in many instances by mere 

 contact with the external solution and with surrounding 

 objects) that it is almost perfectly impermeable to many 

 solutes, but remains permeable to water. The protoplast of 

 every normal vegetable cell is thus surrounded by a more or 

 less perfectly semi-permeable layer, the ectoplast. If the 

 ectoplast is ruptured in any way, it is soon re-formed, unless 

 disorganization of the protoplast ensues. 1 In such cases 

 (e. g., in Myxomycete plasmodia, etc.), where unmodified 

 protoplasm is brought into contact with surrounding medium, 

 it is perhaps partially on account of its colloidal nature that 

 the contained crystalloids are not immediately lost by dif- 

 fusion, instead of being retained, as they are, until a new 

 surface layer can be formed. 



The internal osmotic pressure, which results when the 

 inclosed solution is more concentrated than the external one, 

 tends to stretch the surface layer and enlarge the protoplast. 

 Against this pressure is brought to bear whatever cohesive 

 and resilient force the ectoplast may possess; but this, from 

 the semi-fluid nature of protoplasm itself, must be of a 

 low order. In naked cells this fact prevents the internal 

 pressure from ever becoming very great; in such cases 

 rupture and destruction of the protoplasm would inevitably 

 result. But if the protoplasm is surrounded by a cellulose 

 membrane, as in the case of the majority of plant cells, this 

 condition is entirely altered; the swelling of the protoplas- 

 mic mass is checked at the limit of extensibility of the 

 inclosing cellulose layer. Pressure upon the ectoplast is 

 transmitted immediately to the cell wall, and the latter is 

 stretched according to its extensibility and to the pressure 

 applied. In the condition of strain resulting from the 

 interaction of the force of osmotic pressure (the diffusion 



1 W. PFEFFER, "Zur Kenntniss der Plasmahaut u. d. Vacuolen," etc., Abhandl. 

 d. k. sachs. Ges. d. Wiss. zu Leipzig, math.-physik. Klasse, Vol. XVI (1890), pp. 187-344. 



