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Fig. 27. C is tanche tinctoria, a parasitic plant attached to the root of a Tamarix
(nebka of Kheneg Tlaia).
north Sahara (Killian and Faurel, 1936). The Chenopodiaceae of the
Sahara and the salt marshes of the temperate regions are biologically alike.
The establishment of high osmotic gradients in the tissues of certain
plants (had, guetaf, and zeita), is probably one of the factors responsible
for the construction of voluminous organs rich in secondary formations
and strong and resistant to low temperatures. However, the rain decreases
the osmotic pressure on the scleromorphs (Killian, 1935).
Salt tolerance appears to be an efficient means of resistance for desert
plants. Many plants have a high salt concentration in their sap; this is
an excellent means of reducing the vapor tension of tissue water and enables the accumulation of the water vapor during low atmospheric humidity, thus protecting them against the arid environment.
h. Dissemination. In an arid environment, the natural propagation of
plants by seed has certain problems : irregularity of rains and drought, destruction by mechanical action and by animals, and very strong concentra-