  The formation of vegetation patterns in the arid and the semi-arid climatic
zones is studied. Threshold for the biomass of the perennial flora is shown to
be a relevant factor, leading to a frozen disordered patterns in the arid zone.
In this ``glassy'' state, vegetation appears as a singular plant spots
separated by irregular distances, and an indirect repulsive interaction among
shrubs is induced by the competition for water. At higher precipitation rates,
the diminish of hydrological losses in the presence of flora becomes important
and yields spatial attraction and clustering of biomass. Turing-like patterns
with characteristic length scale may emerge from the disordered structure due
to this positive feedback instability.
