No abstract

The ontogenesis and structure of the periderm were investigated in two dicotyledonous plants: <em>Acer negundo</em> L. and x <em>Fatshedera lizei</em> Guillaum. In Fatshedera lizei periderm is produced by the monolayer of phellogen proper, whereas in <em>Acer negundo</em> sometimes cells in the immediate neighbourhood of phellogen proper, particularly the phelloderm participate in the formation of periderm. In Acer negundo at the boundary with the next annual phellem increment typical phelloid cells are present. Their thick walls contain pectins, cellulose and lignin, but do not contain suberin. The presence of phelloid was not detected in <em>Fatshedera lizei</em>. The walls of the phellem cells with horseshoe shaped thickenings of the inner tangential walls contain, beside pectins, cellulose and lignin, also a thin suberin layer directly on the primary walls. Thus, they are specifically shaped thickwalled cork cells. It is suggested to refer to the suberin layer jointly with the thick cellulose-lignin layer of the walls of thickwalled cork cells by the term "secondary wall". The term "tertiary wall (lamella)" is reserved for thin wall straw of the type of' "membranogenic stratum" and "warty stratum". In <em>Acer negundo</em> the structure of the suberin layers in the thick ceil walls of cork proper is cornplex.

The ontogenesis and structure of phelloid cells was studied in the cork tissue of <em>Viburnum opulus</em> L. Phelloid was found to form as the result of division of various cells of the phellogen zone (from phellogen proper to subepidermal cells). There are quantitative differences in the phelloid which forms characteristic thickenings in its inner tangential walls in one- and may-year-old stems. These thickenings contain pectin compounds, cellulose and lignin but no suberin. Preliminary observations in the electron microscope did not allow identification of phelloid cells earlier than in the period when thickening of their walls begins. The strong contrast staining of protoplasts in these cells is noteworthy.