Some hydathodes are active glands, secreting the water they expel from the leaf.
The protoplasm appears to be able also to bring about thc change without secreting any enzyme.
The epithelial layer consists of (1) so-called " indifferent " cells secreting the perisarc or cuticle and modified to form glandular cells in places; for example, the adhesive cells in the foot.
The cells concerned, like all secreting organs, have abundant protoplasm with large nuclei, and sometimes, in addition, part of the cell-wall is modified as a filter.
Shell conical; foot secreting a ventral calcareous plate; animal fixed.