A portion of meristem tissue (PO:0009013) that is the outer layer of an apical meristem (PO:0020144), or the outer layer of a plant embryo proper (PO:0000001), and gives rise to a portion of epidermis (PO:0005679). [source: ISBN:047124529, POC:Laurel_Cooper]
Comment
A protoderm may or may not arise from independent initial cells (PO:0004011). Some researchers also refer to it as an epidermis in a meristematic state.