The outermost layer of the tissue external to the periderm is called the rhytidome.
The periderm is a protective layer of tissue that forms during secondary growth in the stems, branches, and roots of most dicotyledons and gymnosperms. As a plant grows and develops, it often needs to replace its epidermis (the outermost layer of cells in its primary structure) with a more robust protective tissue. This is where the periderm comes in. The rhytidome then consists of all the tissues outside the functioning periderm.
The rhytidome is essentially what we commonly recognize as bark on trees. It's composed of layers of dead periderm and other tissues that accumulate over time as the plant continues its secondary growth. Because the rhytidome includes everything outside the periderm, its composition can vary greatly depending on the plant species and its age.
In summary:
- Periderm: A protective tissue that replaces the epidermis in older stems and roots.
- Rhytidome: The outermost layer composed of dead periderm layers and associated tissues, lying external to the functioning periderm.
