MacDougal.—The Water-balance of Desert Plants. 85 
tubers, and the bases of the primary branches were expanded into tuberous 
formations in the same manner as the primary stems, but the adventitious 
roots formed did not progress beyond the form of papillate outgrowths. In 
no instance did the branches attain a length beyond a centimetre. - 
An examination of my material showed that the xerophilous stems 
had attained a degree of differentiation of the tissues much less than 
the normal, which may be attributed directly to the lack of formative 
material, including water. The pericycle might not be distinguished, 
and the cortical tissue was composed of cells of greater angularity of 
outline. Despite the lack of development in the stelar elements, the sub- 
epidermal parenchymatous elements showed marked collenchymatous 
thickenings, not seen in normal stems, while the outer walls of the epidermis 
were much heavier and were distinctly cutinized. (See PI. IX, A and B.) 
It is thus to be seen that, despite the lack of nutritive material and the 
capacity for photosynthesis, the small resources of the shoot were directed 
towards the formation of elements which would operate to husband the 
minute supply of water and diminish transpiration. The differences in 
structural reaction in D. Batatas and D. alata may be accounted for chiefly 
by the fact that the tuberous formations of the first are thickened roots, 
while those of D. alata are undoubtedly stem structures. 
General Discussion. 
The vegetation of almost all desert regions usually includes a number 
of rapidly maturing forms, indigenous or of ancient or recent introduction, 
which carry out their entire cycle of existence during regular or irregular 
periods of rainfall, and are physiologically mesophytes. Other forms that 
require much moisture are to be found along streamways. 
The specialized forms not affected by the streamways and more 
or less active during the dry seasons comprise two types, viz. the sclero- 
phyllous and the succulent. The first includes a large number of woody 
and spinose herbs and shrubs with reduced branches, restricted spread 
of leaves, and indurated surfaces. These xerophytes have a very small 
water-balance, and the cell-sap may show extreme concentration. Determi- 
nations of the osmotic pressure of the sap of a number of such types 
at Biskra , 1 in Africa, by Fitting, gave pressures of 100 atmospheres and 
over. The roots of such plants are usually in continuous absorptive contact 
with soil particles from which some moisture may be withdrawn, and 
sclerophyllous forms are notably difficult to transplant, since they wilt 
so quickly when removed from the soil. Mesophytic plants, when grown 
under arid conditions, simulate sclerophylls to some extent, although the 
extremes of osmotic pressure of the sap are probably not approximated. 
See Fitting, H. : Die Wasserversorgung und die osmotischen'Druckverhaltnisse der Wiisten- 
pflanzen, Zeitschr. f. Botan., vol. iii, 1911, p. 209. 
