Grafts et al. — 100 — Water in Plants 



In nonwoody stems of Helleborus and Urtica, minimum values appeared 

 in epidermis, cortex, and pith, with highest values in xylem and phloem 

 parenchyma, companion cells, and cambium. Woody twigs (Fagus) 

 showed highest concentrations in xylem parenchyma cells, lowest in phloem 

 parenchyma. 



Averages of values exhibited by leaf, stem, and root tissues of four 

 plants are shown in Table 27. Figures represent "osmotic values," i.e., 

 number of mols of KNO3 (per liter of solution) isotonic with the cell sap 

 at incipient plasmolysis. (Note, however, remarks concerning KNO3 

 as a plasmolyzing agent, page 82). 



Table 27. — Osmotic pressure of various tissjies determined plasmolytically, expressed 

 as mols of KNOs per liter of solution. Data of Ursprung and Blum (1916a). For 

 conversion to atmospheres, osmotic pressure values of KNO3 solutions (Ursprung, 



1938, p. 1290) are included: — 



Helleborus Urtica Fagus Sedumacre 



Epidermis -484 



Spongy parenchyma -575 



Palisade parenchyma 871 



Outer cortex 522 



Inner cortex 532 



Phloem parenchyma 



Companion cells -577 



Cambium 558 



Xylem parenchyma 567 



Pith 521 



Rays, cortex 



Rays, wood 



KNO3 solutions at 20 



Concentration OP 



volume molar atm. 



0.1 4.3 



0.2 8.3 



0.3 12.3 



0.4 16.1 



0.5 19.8 



0.6 23.4 



Sap contained in the conducting elements of the xylem is very dilute. 

 Maximov (1929a, p. 54) reports osmotic pressures in root sap (exudate) 

 of Xanthium strumarium, Impatiens halsamina, and Zea mays of 0.67, 0.36, 

 and 1.46 atm. respectively. Van Overbeek (1942) recorded 0.42 atm. 

 for the initial exudate of decapitated "low salt" tomato plants, 1.32 atm. for 

 "high salt" plants. The dry matter content of xylem exudate of squash was 

 found to vary between 0.102 and 0.23 per cent (Crafts, 1936). Stock- 

 ing (1945) determined a value of 1.9 atm. for this plant growing in culture 

 solution. Crafts (1932) found an average of 7.0 atm. in the phloem exu- 

 date of pumpkin and cucumber. 



Variation of osmotic pressure among dififerent organs of plants has 

 been studied. Values exhibited by leaves generally exceed those of roots 

 (Hannig, 1912; Dixon and Atkins, 1916) ; older leaves exceed young 

 leaves (Dixon and Atkins, 1912; Korstian, 1924; Gail and Cone, 

 1929). Illuminated leaves show a higher OP than those growing in the 

 shade (Dixon and Atkins, 1912; Lutman, 1919; Halm a and Haas, 



