Crafts et al. 



132 — 



Water in Plants 



Kerr and Anderson (1944) studied the osmotic quantities of cotton 

 seeds in various stages of growth. In seeds less than 24 days old OP values 

 exceeded the DPD, a relationship to be expected. But in older seeds the 

 reverse was true to a marked degree (Table 42). Osmotic pressures were 

 determined cryoscopically, using sap expressed from frozen tissue ; DPD's 

 were calculated by use of the change-in-weight method (Qiapter VII). 

 The authors considered two possible explanations: (a) some sort of active 

 water absorption, and (b) imbibition. Use of respiratory inhibitors, as 

 KCN and ethyl ether, indicated that respiratory activity was probably not 

 directly related to the phenomenon. It was pointed out that the embryo 

 remains very small during the first 24 days of its growth. The excessive 

 DPD values appeared after this period, at a time when the embryo was 

 rapidly developing and coincident with the breakdown of endosperm. The 

 high colloid content of the embryo cells suggested the possibiHty that water 

 liberated on freezing and pressing produced OP values which were too 

 low. Imbibitional forces thus were believed to account for the difference 

 between the two measurements, which is "more apparent than real." 



Table 42. — Osmotic pressures of cell sap from cotton seeds of three ages before and 

 after immersion in sucrose solutions {data of Kerr and Anderson, 1944) : — 



This work emphasizes the importance of colloidal hydration in the water 

 relations of some plant tissues, a force which may be generally under- 

 estimated. 



Evidence from Studies of Frost and Drought Resistance: — The 



freezing point of living tissue has usually been found to be lower than that 

 of dead tissue or its expressed sap. Muller-Thurgau (1886) found 

 — 0.98 and — 0.55° C. for living and dead potato tuber tissue respectively, 

 and — 0.8 compared to — 0.4 for Phaseolus leaves. Maximov (1914) re- 

 ported —2.15° and —1.25° respectively for red beetroot, and —1.21° for 

 the extracted sap. Similar data are reported by Lewis and Tuttle ( 1920) , 

 Carrick (1930), Curtis and Scofield (1933), Jaccard and Frey- 

 Wyssling (1934), Luyet and Gehenio (1937). 



An adequate explanation of this behavior has not been ofifered, but there 

 are several hypotheses which variously attribute the difference to 1 ) capil- 

 lary attraction of water in the intercellular spaces, 2) the resistance offered 

 by protoplasmic membranes to the movement of water out of the cells, 3) 

 non-solvent (bound) water in living cells liberated upon death, and 4) 

 errors in method. The reader is referred, for an adequate discussion of 

 this subject, to reviews by Walter and Weismann (1935), Luyet and 

 Gehenio (1939), and Levitt (1941). 



The display of a double freezing point by living tissue, confirmed by 

 several workers, is a complicating feature in the interpretation of results. 



