Abstract
Stachys aegyptiaca Pers. is a desert calcicolous chasmophyte. Its growth in the Egyptian Desert is confined to the limestone formations of the Northern Arabian Desert, Sinai and the Isthmic Desert. The plant lives under harsh climatic conditions: high summer temperatures with mean maxima above 35°C and low rainfall ranging from 9.4 mm/annum at El-Tor to 87.3 mm/annum at Hemme.The vegetation dominated by Stachys aegyptiaca is a pioneer stage in the development of the wadi bed ecosystem. Other chasmophytes are the main associates of the Stachys community. The studied plant has been recorded among the associates of numerous plant communities.Runoff, accelerated by the rugged topography in areas where the plant grows, and the stoniness of the substratum contribute to securing ample water supply to the plant.The daily march of the transpiration rate was investigated in two seasons. It has been clear that there is an obvious regulation of transpiration in summer.The osmotic pressure of the plant sap was determined at regular monthly intervals throughout one year. The average osmotic value is 17 atm, ranging from 13.8 atm in April to 19.3 atm in November.
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