52 THE BIOLOGY OF FLOWERING PLANTS 



fluctuations and may fall as low as 079 per cent, after heavy 

 rains. 



The osmotic pressures of the plants of the desert round 

 Biskra have been investigated by Fitting (191 1). He was 

 unable to examine the roots, and made his determinations 

 on the leaves. It has been shown by Hannig (1912) that 

 the osmotic pressure of the leaves is in general 2 to 4 

 atmospheres higher than that of the root cells ; but it is of 

 the same order, and so may be taken as indicating the order 

 of pressure probably obtaining in the root hairs. 



An abstract of his results for plants growing in different 

 situations is given in Table X. The numbers refer to the 

 number of species found to have the osmotic pressures 

 shown. 



TABLE X 

 Osmotic Pressures of Desert Plants 



Osniotic pressure in 

 atmospheres. 



Extreme desert plants 

 Salt swamp plants . . 

 Dune plants 



9 

 7 



It will be seen that the great majority of these plants 

 have very high osmotic pressures, and that a number have 

 the enormous value of more than 100 atmospheres. These 

 plants, too, have considerable powers of accommodation. 

 Thus the date palm growing in the salt swamp had a pres- 

 sure of about 50 atmospheres, while in the irrigated culti- 

 vated land round Biskra its pressure was between 28 and 42. 

 Mesembryanthemum nodiflorum had a pressure of over 100 

 atmospheres in the desert, and of 35 to 40 in the cultivated 

 land. Faber (191 3) found the osmotic pressure of the 

 leaf cells of the mangrove Rhizophora mucronata to range 

 from 24 to 27 atmospheres. For steppe plants Iljin, 

 Nazarova and Ostrovskaja (19 16) found the osmotic 

 pressure of the root cells to be about 15 atmospheres, 

 compared with 10 atmospheres for meadow and 6 atmo- 

 spheres for swamp plants. An individual growing on the 



