Salicyclic acid is not absorbed by the skin, but it rapidly kills the cells of the epidermis, without affecting the immediately subjacent cells of the dermis.
In many Laminariaceae the thallus also grows regularly in thickness by division of its surface layer, adding to the subjacent permanent tissue and thus forming a secondary meristem.
In a second type they are situated at the ends of tracheal strands and consist of groups of richly protoplasmic cells belonging to the epidermis (as in the leaves of many ferns), or to the subjacent tissue (the commonest type in flowering plants); in this last case the cells in question are known as epithem.
The passage from this tegumentary layer to the subjacent longitudinal muscular one is gradual, no membrane separating them.
From the ventral surface of the collar nerve-tube numerous motor fibres may be seen passing to the subjacent musculature.