Chapter VII — 99 — Osmotic Quantities of Cells 



The Magnitude and Variation of Osmotic Pressure in Plants: — 



To secure quantitative data relative to the role of osmotic pressure in plant 

 functions, and in plant distribution, an enormous number of measurements 

 have been made. Many were obtained by plasmolytic methods, especially 

 where it was necessary to work with single cells. The cryoscopic pro- 

 cedure has been preferred by ecologists, since it is more adaptable to field 

 conditions and because average values of plant organs and whole plants are 

 more easily obtained. In such studies, osmotic pressure data are used as a 

 criterion of the plant's ability to withdraw water from the soil, and to re- 

 tain it against the evaporative capacity of the atmosphere. For reasons 

 already presented, neither the plasmolytic nor cryoscopic method is without 

 objection. The former indicates the absorptive capacity at limiting plas- 

 molysis only ; the latter must rely on total expressed sap with the obvious 

 associated errors. In both, the normal turgor pressure is unknown. It 

 would appear that the simplified method theoretically would be expected 

 to give the most valuable ecological data, but from a practical viewpoint the 

 cryoscopic procedure is to be preferred. 



Osmotic pressures of plant tissues have been found to vary from values 

 of around one atmosphere to an extreme of 202.5 atm. reported for the 

 expressed sap of Atriplex confertifolia, growing in an alkali region of 

 Utah (Harris, 1934, p. 70). The sap contained 67.33 grams of chloride 

 per liter. However, most mesophytes exhibit concentrations within the 

 range of 5 to 30 atmospheres. Desert species yield higher, hydrophytes 

 usually lower values than mesophytes. It should be stated that these gen- 

 eralizations represent averages or trends. Exceptions must be expected be- 

 cause of the multiplicity of factors exerting an effect on sap concentration. 



Contiguous cells, even in a homogeneous tissue, may not have identical 

 cell sap concentrations. Regardless of protoplasmic connections, each cell 

 maintains a structural and functional individuality, reflected in one way by 

 the osmotic pressure of its contents. Plasmolytic studies clearly demon- 

 strate this fact ; all cells do not reach incipient plasmolysis simultaneously, 

 but Og values distribute themselves in a typical S-shaped curve (Figure 

 21 ). It is usually found that Og values are greater for small than for large 

 cells within a single tissue. 



Gradients of osmotic pressure have been reported within individual 

 tissues. While such gradients may be real, they are constantly changing, 

 and may even reverse in direction due to seasonal and diurnal changes and 

 to environmental and internal factors. 



Ursprung and Blum (1916a) reported a radial gradient in the corti- 

 cal region of Helleborus foetidus root, and in xylem parenchyma of Fagiis 

 sylvatica stem, the Og values increasing toward the center. Vertical gradi- 

 ents were found in the stem of Urtica dioica, Og increasing from tip to base 

 for all tissues. A gradient decreasing downwardly was reported for root 

 tissues of Helleborus, Urtica, and Fagus. 



There may be considerable variation in osmotic pressure among the tis- 

 sues composing an organ. The distribution of Og values among leaf tissues 

 assumes the following pattern : palisade parenchyma > spongy parenchyma 

 > upper epidermis > lower epidermis (Ursprung and Blum, 1916a; 

 BUHMANN, 1935). 



Ursprung and Blum found for root tissues of Helleborus, Urtica and 

 Fagiis highest Og values in xylem and phloem parenchyma, and in com- 

 panion cells ; lowest in the outer cortical region and epidermis. 



