ii8 PERMEABILITY 



differentially permeable. Different substances diffuse through such mem- 

 branes at different rates. They may be, and often are, impermeable or vir- 

 tually so to some substances, while others pass through them readily. Such 

 membranes are sometimes termed selectively permeable or semi-permeable, but 

 these two terms are not as truly descriptive of their properties as the designa- 

 tion "differentially permeable." 



The Membranes of Plant Cells. — With very few exceptions the proto- 

 plasm of plant cells is enclosed by a definite, more or less rigid cell wall. 

 This cell wall fulfills all the criteria for being considered a membrane. 

 Through it must pass all substances moving either into or out of a cell. The 

 cell wall membrane is sometimes called the "non-living membrane" of a cell, 

 but the assumption that all cell walls are entirely non-living is scarcely a valid 

 one. The plasmodesms, at least, must be considered in any problem involving 

 the permeability of the cell walls of living plants. 



Lining the cell wall of all mature plant cells is the layer of cytoplasm. 

 The cytoplasm, or parts thereof, comprises the second membrane through 

 which substances entering or leaving the vacuole of a cell must penetrate. 

 This layer is sometimes called the "living membrane" of the cell. Both the- 

 oretical considerations and experimentally observed facts support the view 

 that the two limiting layers of the cytoplasm possess different physicochemical 

 properties from the interior of the cytoplasm and actually may be regarded 

 as distinct membranes. The cytoplasmic membrane adjacent to the cell wall 

 is called the plasmalemma; that enclosing the vacuole the totioplast or vacuolar 

 membrane. Since the boundaries between the cytoplasm and the cell wall and 

 the cytoplasm and the vacuole constitute interfaces, protoplasmic ingredients 

 which lower interfacial tension probably become more concentrated in these 

 layers than in the interior of the cytoplasm. For this reason alone the postula- 

 tion that the cytoplasmic membranes possess different properties from the 

 interior cytoplasm seems justified. 



Results from several types of experimental investigations support this 

 hypothesis. The immiscibility of protoplasm with water, already discussed, 

 appears to be at least partly due to the fact that protoplasm is coated with a 

 layer of material which is insoluble in water. By means of micropipettes 

 certain non-toxic dyes can be injected into the body of the cytoplasm through 

 which they will spread rapidly. They do not, however, pass through either 

 the plasmalemma or the tonoplast. Neither will they pass into the interior 

 of the cytoplasm from a solution bathing the cell, nor from the vacuole if 

 the latter is injected with the dye (Plowe, 1931). This is direct evidence 

 that the constitution of the plasmalemma and the tonoplast is different from 

 that of the interior cytoplasm. The presence of cytoplasmic membranes 



