Chapter VII — 75 — Osmotic Quantities of Cells 



The TP-volume relation in Figure 19 is represented by a straight Hne, 

 a relation which holds only if the cell wall is perfectly elastic. Such is not 

 usually the case and Figure 20 shows three such curves derived from ex- 

 perimental results. The first (Figure 20A) shows a curve that bends up- 

 ward with increasing volume. This represents a condition exemplified by 

 a rubber film where, as the elastic limit is approached, the high tensile 

 strength limits expansion. Another example would be a group of spherical 

 cells surrounded by a tissue of great tensile strength such as the epidermis 

 of a leaf or the bark around a stem. As such cells expand and fully occupy 

 the space within the limiting tissue the force of wall pressure that opposes 

 turgor is shifted to the confining influence of the stronger tissue and ex- 

 pansion is limited. 



The inverse curve (bending downward) would represent a situation 

 such as exists with a bubble where the film becomes thinner with stretch- 

 ing and, losing in tensile strength, finally bursts. The second curve (Fig- 

 ure 20B) starts like the first but shifts to the inverse form at the condition 

 of full turgor. The third (Figure 20C) appears to start by bending down- 

 ward and then changes as if the tensile strength were increasing. Other 

 physiological processes may be involved and the time element also may in- 

 fluence the form of this curve. If the DPD of the external environment is 

 rapidly reduced as occurs experimentally when flaccid cells are immersed 

 in water, the shape of the TP curve may be concave upward because time 

 is not allowed for plastic stretching. As full turgor is approached the slow 

 extensibility of the wall causes an increasing slope. Hofler (1920), 

 Ernest (1934c), and Broyer (1946) present such curves. 



A particularly good example of this behavior was observed in Nitella 

 cells by Stowe (from Tamiya, 1938). 



With increasing volume of the cell the DPD decreases rapidly because 

 of dilution of the cell sap and increase in turgor pressure, both of v/hich 

 increase the diffusion pressure of water in the cell. Any curvature in the 

 TP line will be reflected in the DPD since the two are related. 



Hasman (1943) used the weight method in plotting DPD curves ex- 

 hibited by storage tissues of potato tuber and roots of beet and carrot. 

 Using molarities of KNO3 and sucrose solutions as abscissae, she found 

 that positive parts of the curves may be straight, concave upwards or con- 

 cave downwards. Various causes are discussed. 



The osmotic pressure also decreases as the cell expands and its contents 

 are diluted. At full turgor cell volume and TP are at a maximum ; osmotic 

 pressure reaches a minimum and DPD = 0. In this state the diffusion 

 pressure of water in the cell equals that of pure water at the same tempera- 

 ture and at the reference pressure. 



From this discussion it is evident that there is danger of oversim.plifying 

 the relations between the osmotic quantities of the cell. The diagrams 

 shown are useful only when the time period required for the volume change 

 involved is short. Under such conditions plastic stretching and the actual 

 expansion due to growth are minimized so that they can be neglected. 



Under certain conditions the turgor pressure may pass the zero refer- 

 ence level and become subatmospheric ; in such an event the DPD is in- 

 creased over its value at incipient plasmolysis. It seems doubtful if this 

 reduction in pressure can go very low in cells such as root hairs, mesophyll, 

 and the like because of the nonrigid condition of the walls ; as the pressure 

 drops below atmospheric such cells will fold, wrinkle, or collapse and the 

 pressure remain approximately constant. In rigid, thick-walled xylem con- 



