Osmotic Pressure in Plants. 157 



arid conditions than on the irri,<(ated land. It barely succeeds 

 at all in extremely dry soil. \'ariations in the moisture 

 supply appear not to produce any corresponding alterations 

 iu the foliar pressure of this i)lant. 



The pressures found in plants upon the dry cultivated land 

 are much higher than those upon irrigated soil, and closely 

 approach, or actually attain, the magnitudes obtained with 

 plants from the true desert. 



That Opuntia is found at Biskra only in irrigated land 

 makes it appear that tliis form is unable to succeed under 

 the natural conditions of this locality. Like Angallis, this 

 cactus was not found with high pressures, but, as noted, Anaqallis 

 is able to grow, to some extent at least, in very dry situations. 

 The Opuntias tested by Fitting were shrivelled and apparently 

 not in good condition, from lack of nijisture. Fitting calls 

 attention to the fact that the very low concentration of the cell 

 sap (isotonic with 0.4 to 0,5 normal potassium nitrate) exhibited 

 bv his Opuntias is in excellent agreement with the results ob- 

 tained bv myself (Publication .50 of the Carnegie Institution, 

 1906) at Tucson, for another species of this same genus, as well 

 as for other succulent cacti. MacDougal (Publication 129 of 

 the Carnegie Institution, 1910) has had the osmotic pressures 

 of the expressed juices of Opuntia and other cacti determined 

 bv the f reezin ,^ point method, with the result that, though the 

 pressure was found to increase considerably with the dr-.ness 

 of the season, yet the tests yielded no very high magnitudes. 

 The xerophvtism of this sort of plants is then, not correlated 

 with high osmotic concentration (jf the cell sap. Fitting emha- 

 sises the fact that these succulent cacti are not to be considered 

 as tvpes of extreme xerphytes. 



Onlv three plants were tested for both irrigated (non- 

 saline) soil and that of wet salt spots. One of these {Mesem- 

 bryanthemiim) showed the same pressure in both situations, 

 while the others {Phoenix and Fr(i)ikctiii) showed a considerably 

 lower pressure in the irrigated, soil. The couse of the lower 

 pressure in the last two cases is considered as related to the 

 greater lesistance to water absorption produced by the high 

 concentration of the salt, swamp water, that is, to external 

 osmotic pressure. Thus high osmotic pressure of the soil solu- 



