1094 Appendix Il: Phytogeography and Geology. 
The desert is characterized by a vegetation of fairly uniform 
character in its main features’). The means whereby the existence 
of these desert plants is preserved resides rather in the peculiarities — 
of their organisation than in any specially favouring influences of 
the environment. The most prominent feature of this organisation 
is the capacity which the vegetative organs have acquired to resist 
factors so inimical to life as heat and drought, factors whose com- 
mon tendency is to annihilate all living things. Though the minute 
details of these multifarious protective arrangements are not visible 
to the naked eye, they find obvious expression in the external con- 
formation of the various organs of the plants. Thin-stemmed plants 
of delicate appearance have tubers or tuberous roots (Hrodium hirtum 
and Hrodium arborescens) snuk deep in the strong ground for the storing 
of reserves of nutriment adequate to maintain them alive through 
long months of absolute drought. The same end is gamed in other 
delicate herbs by the possession of an enlarged woody basal portion. 
Then again, the tendency to general lignification through all the 
parts of the plants affords a capacity for resistance to many members 
of the families Cruciferae and Compositae, families known to us at 
home by their herbaceous, unprotected representatives. To restrict 
evaporation due to wind and solar radiation the desert flora exhibits 
a high degree of reduction in the surface area of its members. 
This principle is illustrated in numerous instances by poverty of 
foliage and considerable spininess, whilst in apparent contradiction 
of this tendency, one often finds the surface of the plant clad in 
a hairy covering or with glands and superficial excretions of wax 
or resin or strongly aromatic substances (Hrodium arborescens, Haplo- 
phyllum tuberculatum, Tvrigonella stellata, Odontospermum graveolens, 
Pulicaria undulata, Francoeria crispa, Iphiona mucronata, Achallea 
jragrantissima, Artemisia herba alba, Artemisia judaica, Lavandula 
pubescens and Lavendulu coronopifolia). Further we tind plants with 
smooth or shiny, thick and flesly leaves. Nature does not work on 
one plane, but provides for every case special means of protection 
and fresh weapons to carry on the struggle. Side by side with the 
thorn-bristling Zila spinosa we find the thickleaved, wax-coated 
Capparis spinosa, whilst near by are the hedgehog-like Astragalus 
and Fagonia, and the soft, fleshy, fibreless Mesembrianthemum. In 
marked contrast, too, are the Chenopodiaceae, a similar almost leafless 
everlasting-woody throughout, and one would think indestructible — 
and the delicate Parietaria with its thin and battist — like foliage. 
Among the life-destroying agencies of the desert, the omnipresent 
salt should be mentioned, and primarily — in the particular district 
1) Schweinfurth in Page May: Helwan. — London 1904. — Second edition. 
