Chapter VII — 107 — Osmotic Quantities of Cells 



values for Helianthus annuus seedlings. DPD's (suction tensions) were 

 determined, by use of Ursprung's simplified method, in seven zones of the 

 hypocotyl, as indicated in Table 29. It will be noted that the values de- 

 crease consistently from the region at the base of the cotyledons to a region 

 35 mm. below. This is true for both the epidermis and cortex. Values of 

 the other two osmotic quantities are also included in the table for com- 

 parison. 



Turgor of Plant Cells : — Turgor plays several important roles in the 

 structure and function of the plant. Because no rigid skeleton is possessed 

 by herbaceous plants nor by the tender parts of woody species, the swollen 

 condition of the cells acting against the restraining influence of the cell 

 walls provides support. Growth of plants is related to turgor expansion 

 of their cells. Plant movements of various sorts may be due to changes in 

 turgor. Opening and closing of stomata are the result of turgor variation 

 of the guard cells. Seed dispersal may be facilitated in certain species by 

 "exploding" of fruits {e.g., the squirting cucumber). Quality of many 

 succulent vegetables such as celery, lettuce, cucumbers, and the like may be 

 related to their state of turgor so that some extremely important problems 

 confront the handlers of fresh vegetables, problems related to water loss 

 at varying humidity and temperature. Severe losses are sometimes caused 

 by splitting of certain fruits and vegetables, for example cherries, water- 

 melons, and tomatoes. Complete explanations for this are not known, but 

 excess water and high humidity are causative factors in that they result 

 in high turgor states. Anatomical factors are also involved (Kenney and 

 Porter, 1941). The keeping qualities of cut flowers depend on the mainte- 

 nance of turgor, and this presents serious problems to the commercial florist. 



Finally, it has been observed that plants do not thrive and their cells do 

 not function properly when their turgor is low or lacking. Although this 

 may relate more directly to the low water content of such plants and the 

 effects of water deficit upon hydration of the protoplasm, imbibition of the 

 walls, and concentrations of solutes in the protoplasm and vacuoles, study 

 of water deficient plants emphasizes the intimate and complex relations 

 existing among these several factors. Therefore, though water deficiency 

 and the attendant wilting may injure plants in more important ways than 

 through loss of turgor, the state of turgor of the plant is a valuable index 

 of its water content and supply, and hence of its physiological well-being. 



Intercellular pressure (A) may be regarded as an additional osmotic 

 quantity, related in the following manner : 



OP — DPD -f TP ± A. (22) 



It seems best, however, to consider this factor simply a component of turgor 

 pressure whenever cells are associated in a tissue. 



Measurement of Turgor: — Few direct measurements of cell turgor 

 have been made. Values of 3 to 10 atm. were obtained by a manometric 

 procedure in the massive-celled M^e/Za (Arens, 1939). Overbeck (1930) 

 measured the turgor of the fruit of the squirting cucumber by means of a 

 manometer. He found a value of 2.4 atm., which was sufficient to throw 

 the seeds 12.7 meters. Most measurements of turgor are made by differ- 

 ence, using the simplified method, to determine OP and DPD values. In 

 the normal state, OP measured cryoscopically and DPD determined by the 

 simplified method enables one to calculate turgor pressure from the rela- 



