Specialized Forms

From a basic type of gametophyte, somewhat like that just described, a number of highly specialized forms have evolved that are characteristic of certain genera. Ribbonlike and strap-shaped gametophytes are known especially among tropical rainforest ferns such as Vittaria, Grammitis, and Hymenophyllum and are usually irregularly and extensively branched, forming large masses of intertwining ribbons. Some of these are actually more abundant than their corresponding sporophytes in certain localities (e.g., the Appalachian Mountains, where “pure cultures” of gametophytes totally lacking sporophytes have commonly been found). Filamentous (threadlike) gametophytes are known in the genera Trichomanes (Hymenophyllaceae) and Schizaea (Schizaeaceae).

Tuberlike gametophytes occur in several groups—e.g., the families Ophioglossaceae (all members), Schizaeaceae (Actinostachys), and Gleicheniaceae (Stromatopteris). All are nongreen underground plants that have close associations with fungi and are therefore assumed to be mycoheterotrophic (i.e., dependent on fungi for nutrition from organic material in the soil). They commonly occur 5 to 10 cm (2 to 4 inches) deep in the ground.

In heterosporous ferns the endosporous gametophytes are much reduced. The male gametophyte in the microspore is made up of the equivalent of one antheridium and its complement of sperm. The female gametophyte, although considerably larger in size, is equally reduced in a morphological sense, the greater space within the megaspore being filled by stored nutritive materials and tissues formed around the base of the female sex organs.