1917] HARRIS & LAWRENCE—TISSUE FLUIDS 301 
ments, indicates that the absorption of any considerable quantity 
of salts and their retention in solution is not a-necessary result of 
existence in a saline substratum. Some physiologists have sug- 
gested that the high osmotic concentration of the fluids of desert 
plants is due primarily and directly to greater quantities of soluble 
material in the substratum than generally occurs in regions of higher 
rainfall. The validity of the conclusion is rendered highly improb- 
able by the high concentrations demonstrated for the plants of the 
rocky hillsides. 
While in general it is better to reserve hypotheses concerning the 
peculiarities of individual species until theoretical discussions of 
their relation to environmental factors can be replaced by inductions 
from actual quantitative data secured in the particular habitat 
under investigation, it may be useful to other workers, especially 
in the case of a problem requiring so many different kinds of 
specialized observation in a habitat not easily accessible to most 
botanists, to point out certain possible interpretations of the 
observed phenomena. 
The question of greatest interest is that ing the difference 
in behavior of the several species of the same habitat, say the 
Coastal flats. For example, the leaves of Prosopis and Caesalpinia 
yield sap of a distinctly lower concentration than do those of 
Guaiacum and the two species of Capparis. Jatropha gossypifolia 
has sap of only about one-fourth of the concentration of that of 
Batis maritima, with which it is so generally associated. The cacti 
and the terrestrial bromeliad exhibit only a fraction of the freezing- 
Point lowering shown by the hard and succulent leaves of the 
arborescent and suffrutescent species among which they are 
interspersed. 
Any suggestion in interpretation of these phenomena must be 
purely tentative and be substantiated by, or discarded on the 
basis of, actual field studies. Those which are here called to the 
attention of ecologists are not at all speculative, but merely 
the result of an attempt to correlate the results of studies by a 
number of specialists in the various fields of desert botany. 
_ Sesuvium Portulacastrumand Batis maritima are both species with 
highly succulent leaves. In both, the high osmotic concentration 
