1917] HARRIS & LAWRENCE—TISSUE FLUIDS 299 
As early as 1905 CAvARA’ investigated a series of cacti by the 
freezing-point lowering method and gave values not very dissimilar 
from our own. Sap was extracted from untreated tissue. This 
may result in abnormally low values of the measures of osmotic 
concentration. 
MacDovueat and Cannon” have estimated the following con- 
centrations in atmospheres for sap of cacti at 25°C: Carnegiea 
gigantea, 6.78; Echinocactus Wislizenii, 5.72; Opuntia Blakeana, 
8.88 and O. versicolor, 11.98. 
It is interesting from the historical standpoint to note that cacti, 
which with certain other succulents are quite anomalous among 
desert plants, were perhaps the first to be considered in relation to 
the problem of the dependence of absorption of water by desert 
plants upon higher osmotic pressure of their sap. Thus Livinc- 
STON” concluded, from determinations by the freezing-point, 
boiling-point, and tissue curvature methods, that the saps of Cereus, 
Echinocactus, and Opuntia “exhibit osmotic pressures no higher than 
those commonly found in plants of the humid regions. For these 
cacti at least, therefore, adaptation to desert conditions is not mani- 
fest in increased concentration of the cell sap.” 
From the foregoing account we may say that the cacti of the 
Jamaica coast exhibit sap concentration of roughly the same order 
of magnitude as do those of other regions. Possibly they are 
somewhat higher than those of purely non-saline localities, but 
until series in which standard methods ofsap extraction have been’ 
employed are available from other habitats this cannot be asserted 
tobe the case. Certainly the cacti, with Bromelia and Bryophyllum, 
are conspicuous exceptions to the general rule of high osmotic con- 
centration in these coastal forms. ‘To this point we shall recur later. 
Results 
In the foregoing paragraphs we have shown that the sap of 
the plant species of the Jamaican coastal deserts has an osmotic 
** Cavara, F., Risultati di una serie di ricerche crioscopiche sui vegetati. Cont. 
Biol. Veg. R. Ist. Bot. ice 41-80. 1905. 
* MacDoueat, D. T., and Cannon, W. A., The conditions of parasitism in 
plants. Publ. Carnegie Ins Inst. Wash. 129. IgI0. 
_  ™Livineston, B. E., The relation of desert plants to soil moisture and evapora- 
tion. Publ. Carnegie Inst. Wash. 50. 1906. 
