ИСТИНА

The collection of cycads (order Cycadales) in the Main Botanical Garden RAS (MBG) includes 8 genera and 30 species belonging to 3 families – Cycadaceae, Zamiaceae and Stangeriaceae. A comparative analysis of leaflet venation of the modern and fossil cycad genera has been done on the basis of living plant collection of MBG and data on fossil plants fragments obtained from Geological Institute RAS. Modern species of Cycas are characterized by the only unbranched main vein. The fossil genus Paracycas had the same branching. Leaflets of the modern Stangeria genus also have a distinct main vein, but separation of this genus into a monotypic family is based on the dichotomous branching of side veins of leaf segments, as in ferns such as Marattia. A similar type of branching has been discovered in a fossil genus Mesodoscolea from Lower Cretaceous of the southern Argentina. A presence of many parallel veins, sometimes dichotomous, is characteristic for modern representatives of Zamiaceae as well as for fossil genus Pseudoctenis. However, fossil genera, such as Aldania, Ctenis and Heilungia represented more diversity in morphology and leaf venation.