ON THE DENSITY OF THE C'ELL SAP IN SOME 



DESERT PLANTS 



W. A. CANNON 



The Deserl Laboratory, Tucson, A rizonn 



It is now well known largely because of the work of Fitting' 

 that many of the longer lived desert plants may have a cell sap 

 of great density. There also appears to be a direct relation be- 

 tween aridity of the habitat, or of the conditions sm-roimding the 

 individual, and the osmotic power developed. Thus far the re- 

 searches have apparently been limited to observations on the 

 subaerial parts, in the present note, however, the results of a few 

 tests on the roots as well as on the shoots of the same individual 

 will be presented. 



The plants studied in the present connection are one species 

 with a water balance and two species of sclerophylls. The suc^ 

 culent is Opuntia discata, native on the domain of the Desert 

 Laboratory, and one of the non-succulent species, Fouquieria 

 splendens, is also endemic here. The third species, however, 

 Peganum harmala, occurs in southern Algeria mainly along the 

 flood-plains of the oueds or other situations where there is com- 

 paratively ample depth of soil. The Opuntia and Fouquieria 

 are sufficiently well-known not to call for a special description, but 

 it may be well, in a few words, to characterize Peganuvi. This 

 species, related to Covillea tridentata of the American deserts, is 

 a half shrub, with perennial subterannean parts. The shoots are 

 50 cm., more or less, in length, and, when growing under relatively 

 favorable conditions as regards water supply, they bear dissected 

 leaves which are comparatively large as well as abundant. In 

 short, the species presents rather the appearance of a mesophyte 

 than of a successful desert perennial. 



^ Die Wasserversorgung; und die osmotischcn Druckverhilltnisse der WCis- 

 tenpflanzen. H. Fitting. Zeitsc-h: f. Bonanik. 3: 209. 1911. 



209 



