INTRODUCTION
As regulators of gaseous exchange in plants, stomata have developed a specific structure and unique physiology, both functionally coordinated to result in movement of guard cells (GCs) (Meidner and Mansfield, 1968; Galatis and Mitrakos, 1980; Palevitz, 1982; Sack, 1987). Stomata are ubiquitous in the epidermis covering all above‐ground plant parts of primary structure, but, as a rule, they are absent from the rhizodermis (Esau, 1965; Fahn, 1982). However, stomata have been reported on the rhizodermis of the primary root of a few plant species (Tietz and Urbasch, 1977; Lefebvre, 1985; Christodoulakis and Psaras, 1987; Tarkowska and Wakowska, 1988).
Ceratonia siliqua (carob tree; Leguminosae) is a Mediterranean evergreen sclerophyllous species. The morphology and anatomy of the leaf bud, leaf, stem and root of this species and of other xeromorphic Mediterranean shrubs were studied in detail by Christodoulakis (1992). Stomata were noted to be scattered on the primary root of germinating seedlings of C. siliqua, above the root hair zone. The orientation of these stomata did not necessarily follow that of the rhizodermal cells. In all cases described to date, the stomata have been open and most of them have been suspended over a discrete substomatal cavity (Christodoulakis and Psaras, 1987). Although the occurrence of stomata on the root is an interesting and peculiar phenomenon, there is as yet no information on the structure, development and fate of these stomata, or their functional significance.
In this paper the structure and development of root stomata of C. siliqua are investigated. The probable function of these stomata during their short existence on the root is also discussed.