Sporophyte

The plane of the first division of the zygote is longitudinal, unlike in other bryophytes where it is transverse. Early in its development, the embryo is

Fig. 5.9. Longitudinal section of an archegonium in Phaeoceros laevis with two cover cells (CC), six neck canal cells (NC), a ventral canal cell (V) and an egg cell (E) (reproduced from Renzaglia et al. 2009 with permission of Cambridge University Press).

divided into a basal region that produces the somewhat enlarged foot and an upper region that develops into a capsule that grows acropetally from a basal meristem. A stalk subtending the sporangium, like the seta in mosses and liverworts, is always lacking.

The peripheral cells of the foot form a haustorium that penetrates the surrounding gametophytic tissue to form the placenta, characterized by a large exchange surface between the two generations, favouring the efficient nourishment of the sporophyte (Fig. 5.11). A distinctive feature of the placenta of some hornwort species is the occurrence of abundant protein crystals, which likely derive from gametophytic cells and may be a source of amino acids for the developing sporophyte, and the presence of wall ingrowths only in the gametophyte.

The sporangium grows from the activity of a basal meristem. The cells deriving from the meristem follow two distinct developmental patterns. The axial cylinder composes the endothecium, which will form the columella.

Fig. 5.10. Section of antheridial chamber with 11 antheridia at different stages of development (reproduced from Renzaglia et al. 2009 with permission of Cambridge University Press).

The outer cylinder, or amphithecium, develops into the epidermis, the assimilative layer and, unlike in other bryophytes (except Sphagnum and Andreaea), also the sporogenous tissue. The basal meristem remains active throughout the life of the sporophyte, except in Notothylas. In the latter, the basal meristem functions for a limited period. The sporophyte, hence, remains small and is frequently retained within the protective tissue of the gameto-phyte (Fig. 5.1c). The continuous growth of the sporophyte, unique among land plants, results in a continuous and, hence, non-synchronized production of spores and an acropetal gradient of their maturation. Consequently, the different stages of spore development beginning with meiosis near the base of the sporangium, can thus be observed within a single sporophyte. In mosses and liverworts, all sporocytes undergo meiosis more or less synchronously and at the time of dispersal all spores have completed their maturation.

When young, the capsule is protected by a multilayered involucre, developed from the gametophytic tissue enclosing the archegonium (Fig. 5.11). The involucre typically ceases to grow when the first meiotic divisions occur in the sporangium. Through continuous basal growth, the sporophyte soon

Fig. 5.11. Longitudinal section of young sporophyte still enclosed within the protective gametophytic involucre, I. The prominent foot, F, consists of large ventral cells and smaller peripheral cells that interdigitate with gametophytic cells to form the placenta, Pl. A basal meristem, B, has begun to produce a columella, C, and assimilative tissue, As (reproduced from Renza-glia et al. 2009 with permission of Cambridge University Press).

Fig. 5.11. Longitudinal section of young sporophyte still enclosed within the protective gametophytic involucre, I. The prominent foot, F, consists of large ventral cells and smaller peripheral cells that interdigitate with gametophytic cells to form the placenta, Pl. A basal meristem, B, has begun to produce a columella, C, and assimilative tissue, As (reproduced from Renza-glia et al. 2009 with permission of Cambridge University Press).

ruptures the involucre near its apex and emerges from the gametophytic cylinder (Figs. 5.8, 5.14a). By contrast, in Notothylas, the sporophyte remains short, emerges only slightly from the involucre and, hence, completes its development while enclosed by the gametophytic protective tissue. Stomata occur in many, but not all species and open to a substomatal chamber when the epidermis is exposed above the involucre. The stomata are morphologically similar to those of vascular plants and defined by two guard cells (Fig. 5.12). These cells seem to lack an ability to open and close (Lucas & Renza-glia 2002) and rather than control water and gas exchange in relationship to photosynthesis, stomata may be essential for the dehydration and dehiscence of the sporophyte (see Section 1.4.2). The spore mass is indeed embedded

Fig. 5.12. Stoma in sporophyte epidermis of Leiosporoceros showing two guard cells surrounding a median pore (reproduced from Renzaglia et al. 2009 with permission of Cambridge University Press).

in mucilage that protects the developing spores by keeping the tissue hydrated; once the spores are mature, the mucilage must dry out for spores to be shed freely. Spores vary among species in size, colour, wall thickness and longevity. Yellow and brown spores with thick walls tend to be long-lived (up to 21 yrs in herbarium packets). Longevity may be accounted for by the oils filling the spores, with the oil functioning as nutrient storage and protection against desiccation. Green spores are restricted to tropical and subtropical genera. Their colour comes from the large chloroplast that is visible through the thin translucent wall. These spores lack oils and are short-lived. Features of the spore, especially colour and wall ornamentation, as well as of the pseudo-elaters, are central to species delineation in hornworts (Fig. 5.13).

Before sporogenesis, the cells of the sporogenous region undergo a division that will yield a spore mother cell (or sporocyte) and a pseudo-elaterocyte. Pseudo-elaters consist of one or more elongate diploid cells. Multicellular

Fig. 5.13. Variation in spore and pseudo-elater (El) shape in hornworts. (a) Tetrad of smooth spores in bilateral arrangement in Leiosporoceros dussii. (b) Papillose spore and thick-walled pseudo-elater in Folioceros appendiculatus. (c) Tetrad of spinose spores surrounded by short, smooth pseudo-elaters in Phaeoceros carolinianus. (d) Spores with striate-canaliculate ornamentation in Hattorioceros striatisporus. (e) Tetrad of spores with prominent mammilla in Phymatoceros phymatodes. (f) Spore with vermiculate surface with six depressions around a larger, central one in Phaeomegaceros fimbriatus (reproduced from Renzaglia et al. 2009 with permission of Cambridge University Press).

Fig. 5.13. Variation in spore and pseudo-elater (El) shape in hornworts. (a) Tetrad of smooth spores in bilateral arrangement in Leiosporoceros dussii. (b) Papillose spore and thick-walled pseudo-elater in Folioceros appendiculatus. (c) Tetrad of spinose spores surrounded by short, smooth pseudo-elaters in Phaeoceros carolinianus. (d) Spores with striate-canaliculate ornamentation in Hattorioceros striatisporus. (e) Tetrad of spores with prominent mammilla in Phymatoceros phymatodes. (f) Spore with vermiculate surface with six depressions around a larger, central one in Phaeomegaceros fimbriatus (reproduced from Renzaglia et al. 2009 with permission of Cambridge University Press).

pseudo-elaters are filamentous. The cell walls may be thin, evenly or spirally thickened (Fig. 5.13). Spores and pseudo-elaters are typically arranged in alternating layers inside the sporangium. Spore mother cells enter meiosis immediately following the isolation from the pseudo-elater mother cell. The latter typically undergoes repeated mitotic divisions; as a result, pseudo-elaters outnumber the spores in the mature sporangium. Pseudo-elaters are very similar to the elaters of liverworts. Both trace their cell lines to a shared ancestry with spore mother cells, but the pattern of cell divisions, including the interpolation of mitotic divisions prior to cytological differentiation, differs and is interpreted by some as indicative of analogy rather than hom-ology of these structures.

Sporophyte dehiscence occurs near the apex by splitting along one or two longitudinal lines with the valves sometimes remaining attached apically (Figs. 5.1, 5.8). Separation and dispersal of the spores is often facilitated by the twisting of the capsule wall and pseudo-elaters that disrupts the spore mass upon drying. Spores maintain their tetrahedral arrangement until nearly mature and, hence, often exhibit a conspicuous trilete mark that reveals the pole of the spore touching the three other spores within a tetrad. Except in Dendroceros, germination of the spore occurs after dispersal and is exosporic, with the germ tube protruding from the spore. The sporeling consists of a globose mass from which rhizoids emerge. Soon, an apical cell is differentiated and a thallus develops.

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