86 MctcDougal— The Water-balance of Desert Plants. 
Morphogenic reactions of a xerophytic character are well illustrated by the 
structural features of Dioscorea described in this paper. 
The specializations exhibited by sclerophyllous forms are therefore of 
a direct physiological character and entail the least morphological change. 
This group is represented in American deserts by a large number of legumi- 
nous trees and shrubs, such as Prosopis, Acacia , Calliandra , Parkinsonia , 
Cercidium , Olneya , &c., and by Covillea ( Larrea ), Fouquieria, Lycium , 
Koehberlinia , Condatia , Zizyphys , Manzanita , Quercns , Aster, Encelia, 
Franseria , Jatropha, Sapindtis, Vauquelinia , &c. 
Succulents display most of the external features of the spinose xero- 
phytes, which may be carried to their extremest limits. This is well 
exemplified by the Cacti, in which the entire shoot may be reduced to 
a short cylindrical or globose form. In addition to these reductions 
secondary morphological changes have ensued, which have resulted in 
exaggeration of the medullary or cortical tissues in roots, stems, or leaves, 
which may contain large balances of water. The sap of succulents 
generally shows a comparatively low osmotic pressure. Three to five 
atmospheres are usually found in Echinocactns Wislizeni when turgid. 
Carnegie a shows 6-8 atmospheres, and Opuntia 10-12; Agave slightly 
more ; and the pressure in all of these may increase greatly with desicca- 
tion. The greater number of succulents in South-Western America are to 
be found in the regions in which the rainfall occurs regularly within well- 
defined seasons. This is true of the plants used in the experimental 
work described in the preceding pages. The root-systems of such plants 
are generally horizontally disposed within a few centimetres of the surface, and 
are in a position where the moisture is most available during the rainy 
seasons. The cessation of the rains is soon followed by a low water-content 
of the surface layers of the soil, and the passage of water from the soil 
to the plant is reduced to a minimum. The finer rootlets perish, and the 
plant stands self-contained until the next rainy season, when new absorb- 
ing branches are formed. 
The actual physiological value of large water-balances varies widely, 
as may be seen by an examination of the experimental data given in this 
paper. The great Tree Cactus, or Sahuaro (Carnegiea gigantea), may survive 
a year or perhaps two, under certain circumstances, without receiving 
additional water from the soil. The formation of flowers in the arid fore- 
summer, however, will not take place unless the plant has received its supply 
during the previous winter rainy season. Neither would apical growth 
ensue in midsummer unless the summer rains were available. The death 
of a plant from the base upwards may sometimes result in the development 
of flowers on isolated branches which have received no water for a year, 
but this must be taken as a special case in accord with many experiences 
that approaching death stimulates reproduction. 
