52 DIFFUSION AND OSMOTIC PRESSURE 



tension of the solute particles) on the one hand, and that of 

 resilience of the cellulose membrane on the other, a rigidity 

 and firmness of the cell as a whole is brought about, just as 

 a football or bicycle tire becomes rigid and firm upon being 

 inflated with gas. This rigidity is termed turgescence, or 

 turgidity. The term "turgor" has also been applied to 

 this condition, but it is better to reserve this word to express 

 the osmotic pressure of the internal fluid. 1 In order that 

 the protoplast may retain its osmotic properties, the cellu- 

 lose wall must be permeated with water. This is absolutely 

 essential for the development of turgidity, since osmotic 

 pressure is not active beyond the limits of the solvent. 

 It is, indeed, true that the cellulose envelope of every active 

 cell is saturated with water. 



Thus far only those cells have been considered which are 

 completely filled with protoplasm. This is the condition in 

 young cells, but mature cells are not usually so filled; as 

 growth progresses, vacuoles of a watery fluid appear in the 

 protoplasm. These increase in size and fuse together, until at 

 length there is a single large vacuole within the protoplasmic 

 mass. The typical cell of plant tissues consists of a cellulose 

 wall lined internally by a layer of protoplasm, which incloses 

 a mass of aqueous solution, the cell sap, containing sugars 

 and various other solutes. The lining layer of protoplasm 

 is bounded externally, where it comes in contact with the 

 cell wall, by the ectoplast. Internally, toward the vacuole, 

 it is bounded by a similar membrane, the tonoplast. The 

 cellulose wall is readily permeable to water and solutes, but 

 the protoplasmic lining, with its two somewhat differentiated 

 limiting layers, normally acts like a semi-permeable mem- 

 brane, allowing water to pass quite freely, but hindering, and 

 often seeming absolutely to prevent, the passage of solutes. 



1 Cf. E. B. COPELAND, " The Mechanism of Stromata," Ann. Bot., Vol. XVI 

 (1902), p. 330; IDEM, "The Rise of the Transpiration Stream," Bot. Gaz., Vol. XXXIV 

 (1902), p. 173. 



