138
CLAUDE J. GRENOT
seeds of Anabasis aretioides unfold their embryos in 10 minutes. Ten hours
later, their roots are anchored in the soil and the spread green cotyledons
are capable of photosynthesis. But germination is equally a function of
certain edaphic characters. Seeds of Anabasis aretioides germinate generally on the surface of permeable soil, soil free of chlorides and one which
can retain a sufficient amount of water. The plants, widely scattered, are
often spaced nearly 50 meters apart. They give no indication of being a
cover of vegetation. This necessary spacing between each plant occurs by
mutual elimination and progressively through competition. The more resistant vegetation exists as a consequence of minute adaptations which give
the desert flora a unique physiognomy.
In spite of the plasticity of the Saharan plants, a "pasture" is composed
of only a few species, and one of these predominates. It is the same for
trees, in that, over vast stretches of land, one species of tree will dominate
the landscape. It seems that climate, rather than the nature of the soil,
dictates this localization. In this physical environment where the struggle
for life is hard, the smallest accumulation of salt or chalk may render plant
life impossible.
Natural selection and evolution have created the types of plants which
are adapted to the rigors of an arid climate. Arid region plants—the xerophytes—can withstand the dryness, extreme heat, and intense sunlight. In
the Sahara, soil surface temperatures sometimes exceed 70°C. Desert
plants are also resistant to sand storms. However, not all of the plants
of this region are necessarily adapted to eremic conditions. Some plants
survive under a general scarcity of water, but these must, especially, be
able to cope with the extreme irregularity of the hydrologie regime of the
desert. There are often periods of several years without a drop of rain.
Hence, the periods of plant activity are very short. Between such times
of activity, the plants must remain in the dry air and soil, unable to benefit
from heat and light which only aggravate the dryness.
Plants use various means to subsist during the long dry spells. Certain
plants exhibit a shortened period of development, a stratagem ensuring
maximum use of a minimum of water. Some desert plants may suppress
all of the aerial parts during the dry period and remain as a seed or as
a subterranean organ (for example, a bulb or a rhizome). Other plants,
by contrast, maintain the aerial portion but expose anatomical and functional devices which assure them a better supply of water or a reduction
of transpiration. The ephemeral and the permanent types of vegetation
are distinguished below.
b. Ephemer als. These constitute a temporary vegetation consisting of annuals and geophytes. Annuals appear shortly after a rainfall and develop
with surprising rapidity (see Fig. 17). The blanket of vegetation denoted
CLAUDE J. GRENOT
seeds of Anabasis aretioides unfold their embryos in 10 minutes. Ten hours
later, their roots are anchored in the soil and the spread green cotyledons
are capable of photosynthesis. But germination is equally a function of
certain edaphic characters. Seeds of Anabasis aretioides germinate generally on the surface of permeable soil, soil free of chlorides and one which
can retain a sufficient amount of water. The plants, widely scattered, are
often spaced nearly 50 meters apart. They give no indication of being a
cover of vegetation. This necessary spacing between each plant occurs by
mutual elimination and progressively through competition. The more resistant vegetation exists as a consequence of minute adaptations which give
the desert flora a unique physiognomy.
In spite of the plasticity of the Saharan plants, a "pasture" is composed
of only a few species, and one of these predominates. It is the same for
trees, in that, over vast stretches of land, one species of tree will dominate
the landscape. It seems that climate, rather than the nature of the soil,
dictates this localization. In this physical environment where the struggle
for life is hard, the smallest accumulation of salt or chalk may render plant
life impossible.
Natural selection and evolution have created the types of plants which
are adapted to the rigors of an arid climate. Arid region plants—the xerophytes—can withstand the dryness, extreme heat, and intense sunlight. In
the Sahara, soil surface temperatures sometimes exceed 70°C. Desert
plants are also resistant to sand storms. However, not all of the plants
of this region are necessarily adapted to eremic conditions. Some plants
survive under a general scarcity of water, but these must, especially, be
able to cope with the extreme irregularity of the hydrologie regime of the
desert. There are often periods of several years without a drop of rain.
Hence, the periods of plant activity are very short. Between such times
of activity, the plants must remain in the dry air and soil, unable to benefit
from heat and light which only aggravate the dryness.
Plants use various means to subsist during the long dry spells. Certain
plants exhibit a shortened period of development, a stratagem ensuring
maximum use of a minimum of water. Some desert plants may suppress
all of the aerial parts during the dry period and remain as a seed or as
a subterranean organ (for example, a bulb or a rhizome). Other plants,
by contrast, maintain the aerial portion but expose anatomical and functional devices which assure them a better supply of water or a reduction
of transpiration. The ephemeral and the permanent types of vegetation
are distinguished below.
b. Ephemer als. These constitute a temporary vegetation consisting of annuals and geophytes. Annuals appear shortly after a rainfall and develop
with surprising rapidity (see Fig. 17). The blanket of vegetation denoted