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Two desert wadies of contradictory ecological conditions were studied. The water factor seems to be the most critical ecological factor in determining the type of vegetation as well as its richness. In Wadi Gimal, of extremely dry conditions (both climatic and edaphic) a typical xerophytic community dominated by Zilla spinosa, Pulicaria undulata and Cassia italica was identified, together with a less xeric (but more tolerant to salinity) community of Tamarix aphylla — Limonium axillare exist in the deltaic part of the wadi near the Red Sea coast. In Wadi El‐Miyah, of fairly rich water resources, 3 communities were identified the species composition of each reflects different degrees of aridity which is related to local physiography in the wadi coarse. These communities are: 1. Community of Salsola delileana — Francoeuria crispa, 2. Community of Acacia raddiana — Pergularia tomentosa and, 3. Community of Zygophyllum coccineum — Pulicaria undulata.

Three shrubby, succulent, spiny Euphorbia (Euphorbiaceae) species from western Saudi Arabia are described and illustrated as new: Euphorbia madinahensis Fayed & D.Al-Zahrani, Euphorbia saudiarabica Fayed & D.Al-Zahrani and Euphorbia taifensis Fayed & D.Al-Zahrani. Diagnostic characters, descriptions and taxonomic comments on each are given, and they are compared with closely related species. The geographical distribution of the three new species is shown.

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