Polypodium hydriforme Ussow is a fresh-water form parasitic on the eggs of the sterlet.
These conditions of orchid-growing have undergone great changes of late years, and the plants are grown much as other stove and greenhouse plants in ordinary pots with composts not only of peat but of leaf-mould, and fibres from osmunda and polypodium ferns.
For potting or basketing purposes, or for plants requiring blockculture, the materials used are light fibrous peat, special leaf-mould, osmunda or polypodium fibre and living sphagnum moss, which supply free drainage for the copious supply of water required.
Thus there are a few 3 Ferns which climb, others are .8 water plants, while many, especially those which live as epiphytes, are more or less xerophytic. Some of the epiphytic forms (Polypodium quercifolium, Platycerium) have strongly dimorphic leaves, the sterile leaves serving in some cases to catch falling debris, and thus to provide the plant with soil.
Lastly, the symbiotic relation between the plant and ants is found in Ferns, the rhizome of Polypodium carnosum containing cavities inhabited by these insects.