302 BOTANICAL GAZETTE [OCTOBER 
of the leaf sap must be due primarily to electrolytes absorbed 
directly from the substratum. The difference between them, in 
so far as facts are available, seems to be an inherent physiological 
one. Sesuvium seems to be a form less tolerant of a highly con- 
centrated soil solution than Batis. The local distribution of the two, 
therefore, is not at all comparable, and the distinctly higher con- 
centration in the leaves of Batis is probably attributable to this fact. 
The only suggestion which can be made concerning the anoma- 
lous position of Prosopis among the small trees is that it has a 
more deeply penetrating root system which taps underflow water, 
poor in solutes, derived by seepage from the neighboring lime- 
stone hills.* From the extensive observations in the deserts of 
southern Arizona it is known that the related species Prosopis 
velutina is characterized by deep root penetration. Thus SPALD- 
Inc” and Cannon” both note the wide horizontal and the deep 
vertical distribution of the root system, which may reach a depth 
of 8m. or over. CANNON” concludes that with uniform and pene- 
trable substratum the species becomes a tree where the perennial 
ground water does not lie at a depth greater than 50 ft. 
Such differences as exist between the concentration in the 
leaves of Prosopis and those of Batis maritima may be accounted 
for on the grounds of a much higher concentration of salts in the 
superficial soil layers. 
It is interesting to note in this connection that Prosopis julifior a 
from the coastal deserts gives values of osmotic concentration 1n 
general agreement with P. velutina of the Arizona deserts. Thus two 
determinations made on young leaves in the spring of 1914” gave: 
Santa Catalina Mountains, A=2.08, P=25.0 
Tucson Mountains, A=2.33, P=27.9 
This suggestion. was originally made by SHREVE (loc. cit.) to account for the 
presence of Prosopis in association — Batis — and other halophytes. 
2 SPALDING, V. M., Distributi ts of desert plants. Publ. Carnegie 
Inst. Wash. 113. 1 
Cannon, W. A., The root habits of desert plants. Publ. Carnegie Inst. Wash. 
131. IgII. 
= Cannon, W. A., Some relations between root characters, ground water, and 
species distribution. Science, N.S. 37:420-423. 1913. 
3 Physiol. Researches 2:32. 1916. 
