Darwin's works on dimorphic flowers and the fertilization of orchids gave powerful support to this statement.
In the case of a dimorphic flower, such as Primula, four modes of pollination are possible, two distinguished by Darwin as legitimate, between anthers and stigmas on corresponding levels, and two so-called illegitimate unions, between anthers and stigmas at different levels (cf.
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.
There is considerable diversity among them as regards external form, the majority having dorsiventral aerial shoots with dimorphic leaves (fig.
There are even dimorphic forms among the Syllids where the sexes are, as in many Polychaets, separate.