III. PHYSICAL AND VEGETATIONAL ASPECTS OF THE SAHARA
137
in the Sahara, but their number decreases as one gets closer to the tropical
regions. The Zygophyllaceae are also represented by a large number of
Saharan species; seven genera and about 30 species are present. Many of
these play an important part in the physiognomy of the desert. Vegetation
of the hamadas of the septentrional and central Sahara is characterized by
Fagonia glutinosa (Grenot, 1968b).
The desertic environment has produced a general impoverishment of
the vegetation which varies greatly from one family to another. This has
not been compensated for by the development of typically Saharan
families. In fact, no endemic families exist in the Sahara. Most of the families are represented by only one or two genera, and most of the genera
by one or two species. These species are not necessarily the same from
the central Sahara to the meridional Sahara. Few genera (Euphorbia,
Aristida, Fagonia) occur with the same frequency from one end of the
desert to another.
The generic coefficient, a ratio of the number of genera to the number
of species, is very high, nearly 0.70. This high coefficient is characteristic
of an impoverished flora (Corti, 1942; Guinet and Sauvage, in Joly et al,
1954).
4. The Biology of Desert Plants
Are the effects of dryness leading directly to propitious "adaptations"
permitting the survival of certain plants in an arid environment? This type
of direct influence hypothesis follows the theories of Lamarck (Oppenheimer, 1961). But this hypothesis is not proved by actual facts. The facts
can be more aptly explained by the Darwinistic theory that adaptations
to the dryness are a result of natural selection on heterogeneous populations.
Tanezrouft and Ténéré are particularly abiotic regions of the Sahara.
Aside from some special cases, vegetation does exist, but its density is directly related to the ambient humidity. Animal life depends largely on
microclimates; these are often a function of the vegetation.
a. Adaptation to the Environment. In the desert, the cover of vegetation
is discontinuous and very irregular. The plants occur in areas where there
is adequate water and protection. Consequently, vegetation has a tendency
to be concentrated in the more favorable areas which are often scarce and
very localized and where the humidity factor is higher than in other areas
(Killian, 1934;Boyko, 1951;Binet, 1955).
The quantity of water provided by rainfall is important for the onset
of germination, but the prevailing atmospheric conditions are equally important for survival. Thus rainfall, temperature, and germination are
closely related. In the Sahara, as in other desert regions, plant seeds germinate with considerable speed. Saturated with water after a rainshower,
137
in the Sahara, but their number decreases as one gets closer to the tropical
regions. The Zygophyllaceae are also represented by a large number of
Saharan species; seven genera and about 30 species are present. Many of
these play an important part in the physiognomy of the desert. Vegetation
of the hamadas of the septentrional and central Sahara is characterized by
Fagonia glutinosa (Grenot, 1968b).
The desertic environment has produced a general impoverishment of
the vegetation which varies greatly from one family to another. This has
not been compensated for by the development of typically Saharan
families. In fact, no endemic families exist in the Sahara. Most of the families are represented by only one or two genera, and most of the genera
by one or two species. These species are not necessarily the same from
the central Sahara to the meridional Sahara. Few genera (Euphorbia,
Aristida, Fagonia) occur with the same frequency from one end of the
desert to another.
The generic coefficient, a ratio of the number of genera to the number
of species, is very high, nearly 0.70. This high coefficient is characteristic
of an impoverished flora (Corti, 1942; Guinet and Sauvage, in Joly et al,
1954).
4. The Biology of Desert Plants
Are the effects of dryness leading directly to propitious "adaptations"
permitting the survival of certain plants in an arid environment? This type
of direct influence hypothesis follows the theories of Lamarck (Oppenheimer, 1961). But this hypothesis is not proved by actual facts. The facts
can be more aptly explained by the Darwinistic theory that adaptations
to the dryness are a result of natural selection on heterogeneous populations.
Tanezrouft and Ténéré are particularly abiotic regions of the Sahara.
Aside from some special cases, vegetation does exist, but its density is directly related to the ambient humidity. Animal life depends largely on
microclimates; these are often a function of the vegetation.
a. Adaptation to the Environment. In the desert, the cover of vegetation
is discontinuous and very irregular. The plants occur in areas where there
is adequate water and protection. Consequently, vegetation has a tendency
to be concentrated in the more favorable areas which are often scarce and
very localized and where the humidity factor is higher than in other areas
(Killian, 1934;Boyko, 1951;Binet, 1955).
The quantity of water provided by rainfall is important for the onset
of germination, but the prevailing atmospheric conditions are equally important for survival. Thus rainfall, temperature, and germination are
closely related. In the Sahara, as in other desert regions, plant seeds germinate with considerable speed. Saturated with water after a rainshower,