142 THE OSMOTIC QUANTITIES OF PLANT CELLS 



TABLE 20 THE EFFECT OF THE OSMOTIC PRESSURE OF THE SOIL SOLUTION UPON THE OSMOTIC 



PRESSURE OF THE ROOTS OF MAIZE (dATA OF MC COOL AND MILLAR, I9I7) 



motic pressure for any species of plant — 202.5 atmos. — was found in an alkali 

 soil species, A triplex confertifolia (Harris, 1934). Salt marsh plants also 

 generally have relatively high osmotic pressures. This is also a correlation 

 with the relatively high salt content of the substratum. 



Diurnal Variations in the Osmotic Pressures of Plant Tissues. — The 

 osmotic pressures of plant tissues fluctuate diurnally. Such variations are 



12 



NOON 



10 12 2 



MIDNIGHT 



10 12 



NOON 



Fig. 28. Daily variation in the osmotic pressure of the leaves of Andropogon scoparius. 



Data of Stoddart (1935). 



more marked in the aerial than in underground portions of plants. Their 

 magnitude and direction are very strongly influenced by the environmental 

 conditions to which the plant is subjected. Dailv variations in osmotic pres- 

 sure have been studied more extensively in the leaves than in any other organ 

 of the plant. The data for the leaves of the prairie grass, Andropogon sco- 

 parius (Fig. 28), were obtained under extreme drought conditions, but simi- 

 lar, although less marked diurnal variations in the osmotic pressure of leaves 

 are undoubtedly of regular occurrence in most species, at least on clear days. 



