Structural organization of epidermis in leaves of extant species of Gnetum L. (Gnetales) and Middle Jurassic Bennettitales

An attempt to assess the identification and evaluation of correlations between the epidermal characters of leaves in Bennettitales and the comparison of the results with the structural organization of leaves of extant plants has been carried out. Epidermis of four species of Gnetum L. and four species of Jurassic Bennettitales (Nilssoniopteris Nathorst and Ptilophyllum Morris) was studied. The coefficients of variation for epidermal characters Gnetum range from 6.4 to 24.0 %, Ptilophyllum — 15.7–63.5 % and Nilssoniopteris — 18.0–39.9 %. In both groups of plants, the sinuosity of the tangential cell walls of the epidermal cells in the upper and lower epidermis is a stable character (Cv ≤ 18.0 %). In the Ptilophyllum and Gnetum, the length of stomata demonstrates a low level of variability (Cv ≤ 16.8 %). A significant range of the coefficients of variation in both Gnetum and Bennettitales show the number of epidermal cells per 1 mm2 of the upper and lower epidermis (17.5 % ≤ Сv ≤ 31.9 %), the area of the epidermal cells in the upper and lower epidermis (21.2 % ≤ Сv ≤ 63.5 %) and the number of stomata per 1 mm2 of epidermis (29.3 % ≤ Сv ≤ 39.9 %). Similarities in the correlation structure of the epidermal characters are revealed in correlations between sinuosity of the tangential cell walls of the epidermis, the number of stomata per 1 mm2 and their size; the length of stomata and the number of epidermal cells; stomatal index and the number of epidermal cells of the epidermis. In Gnetum, the number of differentiated stomata correlates with the number of aborted stomata. In N. angustifolia and P. caucasicum, the number of stomata correlates with the number of papillae per 1 mm2. Similarities in correlational structure of epidermis in Nilssoniopteris, Ptilophyllum, and Gnetum could be ecological adaptations or ontogenetic characters, such as the development of stomata that has been described by other researchers.