Plants of the desert

The vegetal life of the desert comprises annual, ephemeral and perennial species.
Annual plants
Annual plants are all those plants, mostly herbaceous, having a life cycle of less than a year, such as, for instance, the Panicum turgidum which is an evergreen plant in moister alluvial soils, while in dry areas it becomes a deciduous plant, i.e. a plant that loses its leaves.
Ephemeral plants
Ephemeral plants are those plants that are born only after occasional rains and reproduce and die before a new drought comes, and they typically have therefore an extremely short life cycle, for instance the Alyssum alyssoides.
Perennial plants

Perennial plants must instead be able to survive in dry conditions, by minimising the loss of water during the hottest periods. These plants are normally xerophytic, which means they have leathery leaves coated in a waxy film and closed stomas to reduce transpiration and evaporation. Many plants have a reduced foliar surface or have turned their leaves into thorns, especially to avoid losing fluids, as well as to protect themselves from herbivores. These plants can trap great amounts of fluids within their tissues. This is the case of the succulents, such as the American cactus (Cactaceae) and the African and Asian euphorbia (Euphorbiaceae), which are ecological homologous species. In both cases, these plants have turned their leaves into thorns and the photosynthesis has been moved to the trunk tissue which, for this reason, always looks green because of the presence of the photosynthetic pigment.
In the desert, perennial plants generally grow very slowly because of the unfavourable environment in which they live and live very long to make up for regenerative problems. An impressive example is the Welwitschia mirabilis, a primitive plant related to the conifers, endemic of the Namibian desert, which has an extremely slow growth rate and lives to a ripe old age: the oldest specimen ever found is 1,500 years old. This plant has a small, woody trunk from which only two ribbon-like leaves sprout and grow slowly and relentlessly along the sandy ground. At ten years of age, the leaves measure just a few centimetres. Both ends of the plant come undone, so that it looks like it has a lot of leaves. Green female flowers and red male flowers grow in the middle. Plants growing in oases are different and are called phreatophytic because they have extremely long roots that absorb moisture from the water table. This is the case of the date palm (Phoenix dactilifera) growing in the Saharan and Middle Eastern oases.

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