Chapter VII — 79 — Osmotic Quantities of Cells 



thus acting as anchors ; the polar water soluble groups remain in the aqueous 

 phase. The forces holding the complex (protein-fat-water) together must 

 be of an exceedingly intricate nature. Other substances may be present at 

 the surface in addition to the three indicated. The complexity is increased 

 when one considers that the composition may differ from plant to plant, 

 varying with internal and external conditions. 



A similar view is the lipoid-sieve theory of Collander and Barlund 

 (1933). A lipoid-protein mosaic is postulated as comprising the surface 

 of the protoplasm, a view which tends to reconcile the essential features 

 of the "sieve" and "lipoid" theories. On such a basis many of the results 

 of permeability experiments may be explained, especially when the molecu- 

 lar volume of the permeating substances is considered. Details may be 

 found in books by Davson and Danielli, and by Hober (1945). 



Artificial membranes of many sorts have been tested in attempts to 

 simulate such properties of the protoplasm. And membranes have rendered 

 valuable service in the physical measurement of osmotic pressure (Morse, 

 1914; Tinker, 1917). Among artificial membranes that have been used, 

 some certainly partake of the nature of sieves for the pore size can be 

 regulated in their preparation (Elford, 1937; Sollner, Abrams, and 

 Carr, 1941) ; others act as selective solvents, for instance water between 

 layers of chloroform and ether (Meyer and Anderson, 1939), or oil be- 

 tween aqueous phases (Overton, 1902) ; air or other gases in a closed 

 container will act as selective diffusion media for passage of solvents be- 

 tween solutions of different concentrations. 



From the complex nature of protoplasm, as described in Chapter VI, 

 it seems possible that all mechanisms that are compatible with a continuous 

 liquid system may be involved in the protoplasm. Furthermore, from 

 studies on the nature of chemical bonds (Pauling, 1939; Remick, 1943) 

 it seems that electrostatic and other bonding forces, by repelling some com- 

 pounds, holding others in close association with the constituents of a mem- 

 brane, and allowing free passage of others, may play a role in differential 

 permeability. Ultimately the properties of all membranes must be explain- 

 able in terms of the chemical and physical properties of their constituent 

 molecules. 



Permeability to Solutes : — The limiting surfaces of protoplasts were long con- 

 sidered to be readily permeable to solutes, and absorption was pictured as taking place 

 by diffusion. Since the memorable work of Hoagland and Davis (1923), Brooks 

 (1929), Steward (1932), Collander (1939), Lundegardh (1940), and many others, 

 it is now recognized that most solutes enter the plant as a result of an active absorption 

 process utilizing metabolic energy and often acting against strong gradients in con- 

 centration. Demonstration of active solute absorption can be accomplished using vital 

 stains that accumulate in cells to concentrations far above that of the bathing solution. 



Thus, whereas the cell walls are pictured as readily permeable, the cytoplasmic 

 layer has distinctive properties which make it different from any artificial membrane 

 that has been prepared. And careful researches have established a long list of en- 

 vironmental and internal factors that determine the permeability and absorptive capacity 

 of plant cells for solutes (Hoagland, 1944). Principal among these are oxygen 

 supply, temperature, organic nutrients, the status of the cells with respect to previously 

 absorbed solutes, and the nature and concentration of solutes in the external medium. 



In the absorption of nutrient ions from soils, not only the solutes present in the soil 

 solution but the composition of clay colloids in intimate contact with root surfaces 

 is involved. Studies by Jenny and Ox'erstreet (1939) show that contact exchange 

 phenomena may enable the roots to take up ions from the solid phase, their release 

 into solution being accomplished only after they have reached the vacuoles of the root 

 cells. Though the protoplasm itself may not extend through the walls of root hairs 

 to come into actual contact with the soil, the presence in the wall of polyuronides 



