Fig. 4

Developmental stages of the fern species Adiantum raddianum and Equisetum giganteum. (a–e) A. raddianum stages: Two sterile stages include Fd: fiddlehead and S: sterile expanded pinnae (a, b). Three fertile stages include: F0 leaves with unrolling pinnae and an indusium of ca. 250 µm (c), F1 Nearly fully expanded leaf and an indusium of ca. 800 µm (d), and F2 Fully expanded leaves and an indusium of ca. 2 mm. (e). Fi = False indusia, S: Sterile stage, F: Fertile stage. (f–o) Developmental stages of E. giganteum. SAMs producing photosynthetic leaves only in as thin and green at sterile stage S0 (f) and thick and brownish-green at sterile stage S1 (g) that vary in meristem size (k, l). Stage F0 has fully differentiated peltate fertile leaves (sporangiophores), that are covered and protected by the sterile leaf sheath and the spore mother cells have not undergone meiosis (h, m); Stage F1 is characterized by the exposure of half of the strobili above the sterile leaves, of ca. 1 cm and carry mature spores (i, n). Stage F2 has a fully exposed strobili with sterile leaves only at its base of ca 4 mm and mature spores and expanded elaters (j, o). Scale bars for c–e = 250 µm; f–j = 1 cm; Scale bars k–o = 2 mm. Scale bars in magnifications = 50 µm. Black arrowheads indicate the SAM, black arrowhead in insets of n, o indicate the elaters. Sl = Sterile leaves, Pl = Peltate spore bearing leaves, Sti = sporogenous tissue, S = Spores