PHYSICOCHEMICAL ORGANIZATION OF THE PLANT 11 



the protoplasm its viscosity and a certain degree of elasticity. 

 It prevents its complete solution in water and limits its swelling 

 capacity. 



Depending on the course of the processes occurring in the 

 protoplasm, its internal structure may change. With the 

 partial solution, or with the condensation of the fibrils, there 

 must be a corresponding alteration in the physical properties of 

 the protoplasm, either a decrease or an increase of its viscosity, 

 elasticity, etc. In the course of these processes, ionic reactions 

 are of great importance, being accompanied either by a partial 

 coagulation or a reverse solution of the colloids. Death of 

 the protoplast is accompanied by a coagulation of its colloids and 

 loss of its structure. In consequence, it loses the properties of a 

 continuous, viscous liquid, and a watery phase separates from 

 the coagulated hydrosol. At 



the same time, it ceases to J^^^^^^^^^^ Cell-Wall 



retain the large amount of sub- 

 stances that were dissolved in J^'^ •^^^- Cytoplasm 

 the dispersion m.edium. 



4. The Cell as a Colloidal 

 System. Osmotic Properties 



of Cell Membranes. General % ± ^^-Nudeus 



Laws of Osmosis. — When very 

 young, the whole cell is made ^^^ 3 _^ ^-^^^^^ ;; ^^ ^^^^^ ^^^^^ 



up of a solid mass of proto- ceil. Arrows show the direction of 

 1 T 4-* 1 osmotic pressure {after Sinnott). 



plasm. In time, vacuoles con- -^ 



taining a watery cell sap arise in this mass. These vacuoles 

 finally merge, forming one central vacuole. The typical vegeta- 

 tive cell, therefore, may be considered ^s a double- walled sac filled 

 with cell sap. The cellulose membrane of the cell is the outer 

 wall of this sac, and the layer of semiliquid protoplasm forms a 

 lining within it (Fig. 3). 



These two membranes, separating the cell contents from the 

 surrounding medium, are endowed with very different properties. 

 The wall is a stiff porous gel. Owing to its solidity, it easily 

 preserves its shape and has but a limited capacity for distention. 

 Being a liquid, though viscous, the protoplasmic sac has an almost 

 unlimited distention capacity. Not being able to maintain a 

 definite shape, it readily fills all hollows and projections of the 

 outer wall. 



