III. PHYSICAL AND VEGETATIONAL ASPECTS OF THE SAHARA
149
tions of soluble salts in the soil. Moreover, the intensity of the heat and
of the sun's direct rays are much more harmful for the germinating young
plants than for the better protected adults.
The dissemination of plants is often facilitated by wind and animals.
Anemochores (composed of Gramineae, Compositae, Asclepiadaceae, and
Cruciferae) have fruits with hooks or equipped with tufts. Other complex
types of fruits are those of Aristida and of Er odium, which, after being
transported by the wind, fall and literally screw themselves into the soil
as a consequence of their configuration and their particular pilosity. The
Schouwia purpurea, once dry, is frequently blown off the plant and is rolled
great distances by the wind.
The zoochores are represented by a certain number of Gramineae, Compositae, and Boraginaceae, which have fruits with hooked denticles, which
cling to animals' fur. The seed of Neurada procumbens (Rosaceae) becomes attached to the feet of animals. The fruit, in the shape of a button,
has spines on the superior surface which attach to the animals' feet and
thus assure dispersion of the seed (Van der Pijl, 1972).
B. VEGETATION GROUPS
1. Generalities
a. Objectives. The first botanical objective is a thorough knowledge of
the flora. However, systematic lists are soon covered with notations about
the vegetation and the grouping of species. The major important points
are found in a number of major works (Maire, 1931, 1933, 1940,
1952-1968; Corti, 1942; Hernandez-Pacheco et al, 1949; Sauvage, 1949;
Guinochet, 1951; Guinet and Sauvage, in Joly et al, 1954; Guinochet and
Quézel, 1954;Quézel, 1954a,b; Leredde, 1957;Ozenda, 1958).
Another botanical objective is phytogeographic understanding—
domaines and sectors—fixed by the origin of the flora, its life cycle, the
climate, and the soil (Eig, 1931; Monod, 1939, 1945, 1957; Murat, 1944;
Ozenda, 1954; Quézel and Santa, 1962; Quézel, 1965).
b. Bioclimatic Stages. This concept is similar to the phytogeographic
subdivisions, but it uses mainly climate as a factor. Le Houérou (1962)
found two bioclimatic stages in south Tunesia, the upper and the lower
Sahara. Quézel (1965) found three principal stages in the Sahara: (1)
the upper stage of Saharan vegetation, the superior desert stage, composed
of the hyperarid zones practically devoid of vegetation; it represents only
one quarter of the surface of the Sahara; (2) a middle stage of Saharan
vegetation, the middle desert stage, where the annual precipitation is between 20 and 50 mm; it is characterized by diffuse vegetation; and finally
149
tions of soluble salts in the soil. Moreover, the intensity of the heat and
of the sun's direct rays are much more harmful for the germinating young
plants than for the better protected adults.
The dissemination of plants is often facilitated by wind and animals.
Anemochores (composed of Gramineae, Compositae, Asclepiadaceae, and
Cruciferae) have fruits with hooks or equipped with tufts. Other complex
types of fruits are those of Aristida and of Er odium, which, after being
transported by the wind, fall and literally screw themselves into the soil
as a consequence of their configuration and their particular pilosity. The
Schouwia purpurea, once dry, is frequently blown off the plant and is rolled
great distances by the wind.
The zoochores are represented by a certain number of Gramineae, Compositae, and Boraginaceae, which have fruits with hooked denticles, which
cling to animals' fur. The seed of Neurada procumbens (Rosaceae) becomes attached to the feet of animals. The fruit, in the shape of a button,
has spines on the superior surface which attach to the animals' feet and
thus assure dispersion of the seed (Van der Pijl, 1972).
B. VEGETATION GROUPS
1. Generalities
a. Objectives. The first botanical objective is a thorough knowledge of
the flora. However, systematic lists are soon covered with notations about
the vegetation and the grouping of species. The major important points
are found in a number of major works (Maire, 1931, 1933, 1940,
1952-1968; Corti, 1942; Hernandez-Pacheco et al, 1949; Sauvage, 1949;
Guinochet, 1951; Guinet and Sauvage, in Joly et al, 1954; Guinochet and
Quézel, 1954;Quézel, 1954a,b; Leredde, 1957;Ozenda, 1958).
Another botanical objective is phytogeographic understanding—
domaines and sectors—fixed by the origin of the flora, its life cycle, the
climate, and the soil (Eig, 1931; Monod, 1939, 1945, 1957; Murat, 1944;
Ozenda, 1954; Quézel and Santa, 1962; Quézel, 1965).
b. Bioclimatic Stages. This concept is similar to the phytogeographic
subdivisions, but it uses mainly climate as a factor. Le Houérou (1962)
found two bioclimatic stages in south Tunesia, the upper and the lower
Sahara. Quézel (1965) found three principal stages in the Sahara: (1)
the upper stage of Saharan vegetation, the superior desert stage, composed
of the hyperarid zones practically devoid of vegetation; it represents only
one quarter of the surface of the Sahara; (2) a middle stage of Saharan
vegetation, the middle desert stage, where the annual precipitation is between 20 and 50 mm; it is characterized by diffuse vegetation; and finally