Grafts et al. — 78 — Water in Plants 



"plasma" membranes. One, termed the ectoplast (or plasmalemma) , com- 

 prises the outer limiting layer of the cytoplasm immediately within the cell 

 wall ; the other is the tonoplast, the inner limiting layer bathed by vacuolar 

 sap. 



While these membranes are too thin to be detected microscopically, 

 evidence for their existence has been obtained in several ways. More con- 

 vincing is the micro-injection of aqueous solutions of certain dyes, which 

 distribute themselves throughout the protoplasm, but do not pass out of the 

 cell (Chambers, 1944). Micrurgical manipulations have demonstrated 

 the tough elasticity of these layers. Seifriz (1928), using onion scale 

 tissue, was able to strip off the protoplast from the vacuole by means of a 

 micro-needle, leaving a free floating sac, limited by what appeared to be 

 the tonoplast. The appearance of such structures is evidently rather com- 

 mon (ZiRKLE, 1937). They are often produced on thin sectioning of tis- 

 sue; those from red beetroot retain pigmented vacuolar sap, and several 

 may be produced by cutting one cell. Kuster (1928) has further described 

 these structures as vacuolar envelopes (Vacuolenhiillen), and gives meth- 

 ods for isolating them from ripe berries of several species of Solanuiu. 

 Whether the limiting membranes of these structures actually represent only 

 the tonoplast is questionable. When such structures break under the 

 microscope, a very thin membranous remnant is left. If the tonoplast 

 consists of a membrane but a few molecules in thickness, as some have 

 postulated, it seems doubtful if any such remnant would be detectable. 



The phenomenon termed "Intrabilitat," leading to vacuolar contraction, 

 suggests a differential permeability between the inner and outer membranes. 

 Various substances such as dyes are absorbed freely from the bathing 

 solution by the protoplasm but do not diffuse into the vacuole. This has 

 been presented as evidence that the ectoplast is permeable to such sub- 

 stances but the tonoplast is not (c/. Osterhout, 1943). As pointed out 

 by Stiles (1937), other factors — adsorption, solubility, and partition co- 

 efficients — also could conceivably contribute to this effect. Electrical 

 measurements have demonstrated a difference in behavior of the two proto- 

 plasmic surfaces (Osterhout and Harris, 1927). 



There has been hesitancy on the part of some to accept the existence of 

 these membranes (Fischer, 1921; Lepeschkin, 1938). To some extent 

 the confusion has been a result of failure to accurately define what is meant 

 by the term "membrane." If reference is made to definite visibly distinct 

 layers, then there is little supporting evidence from plant cells. But it 

 is not necessary to postulate such structures. At the limiting surface or 

 interface of any complex solution there is a tendency for orientation and 

 accumulation of substances which act to reduce the interfacial tension. 

 Consequently, the chemical and physical properties of the interface will 

 differ from those in the body of the solution. The composition of this sur- 

 face will, therefore, depend on the substances present in the solution. In 

 this sense it is difficult to visualize the absence of membranes. 



The composition of the plasma membrane has not been established with 

 certainty. The best evidence points to an association of protein and lipoid 

 (Davson and Danielli, 1943). The lipoid molecules are believed to form 

 a film in such a way that they have a certain freedom of orientation. Ad- 

 sorbed to this film is the protein layer, composed of long polypeptide chains, 

 cross-linked so that a heterogeneous pore or sieve effect is obtained. The 

 hydrocarbon groups of the chains are oriented so as to be in the oil layer, 



