Observations of the spore, gametophyte and young sporophyte of Pteridium caudatum (L.) Maxon using scanning electron microscopy.

This paper describes the development of the sexual phase of the invasive fern, Pteridium caudatum, from spore germination to young sporophyte formation. Spores samples for gametophyte cultures were taken from various sporophytes and then sown on mineral agar with Thompson's media. Gametophytes were maintained under fluorescent light on a 12h light, 12h dark cycle at 24-25°C. Developmental phases were fixed in FAA-sucrose solution and processed for observation with the scanning electron microscope. Spores are trilete and germination takes place on the second day after sowing; germination is of the Vittaria-type. Adiantum-type prothallial development was observed. The differentiation of a two-dimensional thallus begins 5 days after germination maturation of adult gametophytes occurs about 30 days after sowing. Adult gametophytes are heart-shaped, bisexual and glabrous. Antheridia are formed by three cells: basal, annular and opercular cell with a pore. Archegonia have a neck of 4-cells. The young sporophyte becomes visible within 8 weeks after spores are sown. The taxonomic significance of the gametophyte morphology is discussed.

Ultrastructural and cytochemical studies on oogenesis of the fern Pteridium aquilinum.

Egg development in Pteridium aquilinum var. latiusculum was studied using ultrastructural and cytochemical methods to examine structural features influencing fertilization in leptosporangiate ferns. Ultrastructural observations indicate a separation cavity is first formed above the egg during oogenesis with a pore region persistently connecting the egg and the ventral canal cell. The egg envelope is formed by deposition of amorphous materials in the separation cavity on the outer surface of plasmalemma. The egg envelope was not formed across the pore region; instead, a fertilization pore was formed. During oogenesis, the egg nucleus produced extensive evaginations containing osmiophilic bodies. Cytochemical experiments revealed that the egg envelope displays strong periodic acid-Schiff reaction indicative of polysaccharides, with negligible Sudan black B staining for lipids, suggesting that the egg envelope is composed principally of polysaccharides, and not lipids. The present manuscript provides new insights into egg structure and development of Pteridium, including discovery and characterization of the fertilization pore and observations on the chemical nature of the egg envelope, thus contributing to the understanding of the cytological mechanism of the sexual reproduction of ferns.

The structure and development of haustorial placentas in leptosporangiate ferns provide a clear-cut distinction between euphyllophytes and lycophytes.

This light and electron microscope study revealed that leptosporangiate ferns have highly distinctive gametophyte-sporophyte junctions characterized by sporophytic haustoria, the absence of intraplacental spaces and degenerating cells, and the early appearance of wall ingrowths in both generations. Other notable cytological features are highly pleomorphic plastids and mitochondrial aggregates in the gametophytic placental cells. Close similarities with the gametophyte-sporophyte junctions in Tmesipteris and major differences from those of homosporous lycophytes are in line with the placement of psilophytes and ferns in the same clade and distance both from lycophytes. A smooth interface between the two generations in Azolla suggests a clear-cut discontinuity between homosporous and heterosporous ferns, although this is the only heterosporous fern investigated to date. Similarities between the gametophyte-sporophyte junctions of leptosporangiate ferns and hornworts, when balanced against differences between them, are considered more likely the result of parallel evolution rather than homology.