There are even dimorphic forms among the Syllids where the sexes are, as in many Polychaets, separate.
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.
Such plants are called dimorphic. Other plants are timorphic, as species of Lythrum, and proper fertilization is only effected by combination of parts of equal length.
In many fern-like plants of this period the fronds were dimorphic, the fertile leaves or pinnae having a form quite different from that of the vegetative portions.
There is evidence that the stem in some species was a climbing one; the pinnate leaves, arranged on the stem in a two-fifths spiral, were dimorphic, the sterile fronds resembling some forms of (From a drawing by Mrs D.
Darwin's works on dimorphic flowers and the fertilization of orchids gave powerful support to this statement.
- Rotifera are unisexual, with the sexes dimorphic. The ovary is, as in many Platyhelminthes, duplex; one part, the germary, being an organ for the production by cell multiplication of the germ-cells or eggs.
Richardsoni, a name that correctly applies only to whole-coloured examples, for this species too is dimorphic. Even its proper English name 4 is disputable, but it has been frequently called the Arctic gull or Arctic skua, and it is by far the commonest of the genus in Britain, and perhaps throughout the northern hemisphere.