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.

The leaf surface, or phylloplane, is inhabited by various microorganisms. Micromycetes are typical organisms of the phylloplane that are able to exert negative effects on plants. Seventeen species of micromycetes were indicated in the phylloplanes of G. gnemon and G. montanum. Micromycetes form biofilms on the surface of the upper epidermis. In the lower epidermis, they mainly colonize cork warts and destroy their cells. Collapsing cells and the micromycetes are isolated from the living tissues of leaves by layers of densely arranged cells. During changes from season to season, the number of disappearing micromycete species in the upper epidermis is approximately equal to the number of appearing new species. The total number of micromycete species in the phylloplane of the lower epidermis is reduced in winter. Structural organization, biodiversity, seasonal changes in the phylloplane and the influence of its micromycetes on leaf cells differ in the upper and lower epidermis of Gnetum leaves.

The leaf epidermis of four species of Gnetum L. and four species of the Jurassic Bennettitales (Nilssoniopteris Nathorst and Ptilophyllum Morris) were studied. In addition to the description of qualitative characters, the analysis of various quantitative characters was carried out using statistical methods. Coefficients of variation for quantitative characters of the epidermal structure in Gnetum range from 6.4 to 24.0%, in Ptilophyllum from 15.7 to 63.5%, in Nilssoniopteris from 18.0 to 39.9%. The sinuosity of tangential cell walls of the epidermal cells in the upper and lower epidermis is the stable character in both groups of plants (Cv ≤ 18.0%). In Ptilophyllum and Gnetum, the length of stomata demonstrates a low level of common variability (Cv ≤ 16.8%). A significant range in the coefficients of variation in both Gnetum and the studied genera of Bennettitales is typical for the number of epidermal cells per 1 mm2 of the upper and lower epidermis (17.5% ≤ Cv ≤ 31.9%), the area of the epidermal cells of the upper and lower epidermis (21.2% ≤ Cv ≤ 63.5%), and the number of stomata per 1 mm2 of the epidermis (29.3% ≤ Cv ≤ 39.9%). The similarity in the correlation structure of epidermal characters is manifested in the correlation between sinuosity of tangential cell walls of epidermal cells, the number of stomata per 1 mm2 of the epidermis, and size of epidermal cells, as well as between the length of the stomata and the number of epidermal cells and also between the stomatal index and the number of cells in epidermis. In Gnetum, the ratio of the number of differentiated stomata correlates with the number of aborted stomata per 1 mm2 of epidermis. In N. angustifolia and P. caucasicum, the number of stomata correlates with the number of papillae per 1 mm2.