Anatomical studies on the genus Euphorbia L. Saudi Arabia (Subgenera: Triucalli, Ermophyton, Esula and Chamaesyce)
by Aldhebiani, A. and Jury, S.
in International Research Journal of Plant Science (ISSN: 2141-5447) Vol. 4(6) pp. 168-191, June, 2013
The genus Euphorbia is the largest in Saudi Arabia, even though no anatomical study has been done intensively.
In this study the epidermis, the stomata and the venation patterns have been investigated. The shape of the epidermal cell in Euphorbia species in Saudi Arabia varies: polygonal, rectangular, undulate or elongated. Moreover, the cell shape relies on the cell location on a leaf, i.e. the middle region, the margin, the apex or above the vein. Furthermore, in some cases both leaf surfaces have the same cell shape but more often they are unlike. Hairs are generally simple, unbranched and with a warty ornamentation on their surface. Papillae occur only in one species E. hypericifolia .
The most common stomata type is anomocytic, while the rare type is actinocytic, recorded only in E. helioscopia. Stomata of more than one type (have been encountered on the same leaf surface as in E. scordiifolia and E. hirta.
Venation patterns vary from one-veined, three-veined to those with four or more veins.
A stoma consists of the stomatal aperture and the pair of guard cells that form it. Stomata usually tend to be on the lower surface only. But in some cases this distribution varies from species to species and depends on whether the plant is a xerophyte or a mesophyte. They might be superficial or sunken. Stomata sometimes are surrounded by specialized epidermal cells which are called subsidiary cells. These subsidiaries differ from unmodified epidermal cells in shape, size and staining properties (Baranova, 1992; Metcalfe and& Chalk, 1950; Stace, 1965). On the other hand, the arrangement of subsidiary cells, where present, is of the greatest interest to the taxonomist. This variation is used to define the different types of stomata. Occasionally species have several types of stomata on one leaf, while some have only one type for the species (Stace, 1984). In addition, Van Cotthem, (1973) pointed out that those morphological stomata types can provide not only diagnostic characters but also very valuable taxonomic ones or even phylogenetic clues. Metcalfe and Chalk,(1950) had established some terms to replace the representative ‘family’ name proposed by Vesque, (1889).
Anomocytic was substituted for the ranunculaceous type; anisocytic replaced cruciferous, diacytic the caryophyllaceous and finally paracytic for the rubiaceous. The tetracytic type which can be found in most of the monocotyledons was added by Metcalfe, (1960). Later, Stace, (1965) proposed the term cyclocytic for the narrow ring of four or more subsidiary cells surrounding the stomata. Metcalfe and Chalk, (1950) have named and defined the actinocytic type as stomata surrounded by a circle of radiating cells. Three more types were introduced by Van Cotthem, (1970), hexacytic, epicytic and hemiparacytic. And some intermediate types were added by Payne, (1970) who described the helicocytic and allelocytic types in relation to mesogenous forms of anisocytic, paracytic and diacytic patterns. Stace, (1989) lists 35 types of stomata in vascular plants. Closely related families are distinguished by the presence of a specific type of stomata; such as Acanthaceae and Scrophulariaceae separated by the presence of diacytic stomata in the former as against anomocytic in the latter. Moreover, some stomatal types are distinctive of certain families: for example, Ranuculaceae has the anomocytic type, Brassicaceae the anisocytic, Caryophyllaceae diacytic, Rubiaceae paracytic and finally Poaceae has the graminaceous type (Singh, 2004). According to Metcalfe and Chalk (1950), the mature stomata of Euphorbiaceae, are anomocytic, paracytic and anisocytic. They are usually confined to the lower leaf surface, more rarely on both surfaces of the lamina. Paracytic stomata were reported by Tognini, (1897) which are mesogenous in development in E. variegata and Ricinus communis. On the other hand, according to Raju and Rao (1977), stomata in the Euphorbiaceae show considerable variation. They found that the woody taxa have predominantly the paracytic stomata type, while the anisocytic stomata are characteristic of the herbaceous Phyllanthoideae. Moreover, they indicated that Chamaesyce has a high percentage of anomocytic stomata (Raju and Rao, 1987). Finally, a considerable diversity of stomatal types were found in Euphorbia by Kakkar and Paliwal, (1974). They reported that the common type of stomata in Euphorbia species is the anomocytic, even though stomata of paracytic and anisocytic have been also observed.