Structure and development 511 Gametophyte

The vegetative body of hornworts is a flattened bilaterally symmetric thallus (Fig. 5.1). Although the thallus lacks organized external appendages, the margin can be deeply incised, giving the appearance of irregular leaves emanating from a thickened midrib in some taxa (Fig. 5.1b). In some cases,

Fig. 5.1. Variation of thallus morphology in hornworts. (a) Typical thallus in rosette in Phaeoceros. Note the horn-like sporophyte that dehisces along one or two vertical lines. (b) Deeply incised thallus margins in Dendroceros, giving the appearance of irregular leaves emanating from a thickened midrib. (c) Notothylas orbicularis, a vigorous plant with mature and immature sporo-phytes, which are reduced in size owing to the terminate development of the basal meristem (reproduced from Schuster 1966-1992 with permission of the Field Museum of Natural History, Chicago).

Fig. 5.1. Variation of thallus morphology in hornworts. (a) Typical thallus in rosette in Phaeoceros. Note the horn-like sporophyte that dehisces along one or two vertical lines. (b) Deeply incised thallus margins in Dendroceros, giving the appearance of irregular leaves emanating from a thickened midrib. (c) Notothylas orbicularis, a vigorous plant with mature and immature sporo-phytes, which are reduced in size owing to the terminate development of the basal meristem (reproduced from Schuster 1966-1992 with permission of the Field Museum of Natural History, Chicago).

the margin may be recurved downwards, forming narrow capillary spaces that may function to hold excess water. The growth of the thallus results from the activity of the single-celled apical meristem, which is located in thallus notches and covered by mucilage secreted by epidermal cells (Fig. 5.2). The geometric shape of the apical cell (Fig. 5.3) determines the growth form of the gametophyte, which can appear as either a rosette (Fig. 1.3) or a ribbon (Fig. 5.4), and also to a lesser extent of the thickness of the thallus (i.e. tapering from the centre to abruptly thinned from the midrib outwards). The thallus lacks conspicuous internal differentiation (Fig. 5.14c), except for the presence of cavities borne through schizogeny, which are essential for symbiotic cyanobacteria. Most parenchyma cells may synthesize, store and secrete mucilage, a carbohydrate that may be essential for water retention.

Fig. 5.2. Longitudinal section of growing thallus notch overarched by mucilage, M, and showing rectangular apical cells, AC, with well-developed nucleus N and chloroplast, P (reproduced from Renzaglia et al. 2009 with permission of Cambridge University Press).
Fig. 5.3. Wedge-shaped segmenting along four cutting faces (a) and hemi-scoid apical cells cutting along three faces (b) resulting in a rosette (Fig. 1.3) or ribbon-like (Fig. 5.4) growth form (reproduced from Renzaglia et al. 2009 with permission of Cambridge University Press).

The upper surface of the thallus harbours the sex organs and, in some cases, small dissected flaps or lamellae (Fig. 5.5). Their function is unknown. The lower surface of the thallus bears smooth unicellular rhizoids that are typically unbranched, except at their tips in Megaceros.

Fig. 5.4. Ribbon-shaped growth form of Dendroceros showing swollen central midrib and lateral wings. Note the presence of pore-like mucilage clefts, C, and unicellular rhizoids, R (reproduced from Renzaglia et al. 2009 with permission of Cambridge University Press).
Fig. 5.5. Small dissected flaps on the dorsal thallus in Phaeomegaceros squamuligerus.
Fig. 5.6. Surface view of mucilage cleft in ventral thallus epidermis (reproduced from Renzaglia et al. 2009 with permission of Cambridge University Press).

Upon closer examination, the lower (and rarely also the upper) epidermis reveals small pores defined by two kidney-shaped cells, that very much resemble the stomatal guard cells, as seen in the sporophyte (Fig. 5.6). Like the guard cells in the sporangial wall, these cells lack the ability to open and close the pore; in fact the pore, once formed, remains open. The pore leads to a small chamber that may hold globose colonies of Nostoc embedded in mucilage. The 'stomata' are hence called mucilage clefts. Stomata are a necessity only in plant bodies covered by a cuticle, which would limit free gas exchange over the surface of the plant. Hornworts may have a cuticle, but probably not a thick and effective one considering that water is still absorbed over the whole surface of the thallus. Furthermore, the function of the stomata complex relies on the pore opening into an air chamber and a lax parenchyma to allow for the diffusion of carbon dioxide and oxygen. Schizogenous cavities are frequent in hornworts and some are formed above the epidermal pore. They are, however, filled with mucilage and, hence, are inadequate for gas exchange. Whether mucilage clefts are homologous to stomata, as argued by Schuster (1984c), has been debated, and rejected by some researchers, in part on the basis of the functional (and therefore probable ultrastructural) differences (Renzaglia et al. 2007). The guard cells of early polysporangiophytes also resemble those of modern taxa but in some species the stoma is not connected to a substomatal chamber (Edwards et al. 1998), raising the possibility that stomata did not have the same function as in extant tracheophytes. The possibility remains that mucilage clefts in the gametophyte of hornworts are stomata that have been co-opted from the

Fig. 5.7. Upper epidermal thallus cells of Megaceros cf. vincentianus, each containing a single, large chloroplast with abundant starch and pyrenoid, Py (reproduced from Renzaglia et al. 2009 with permission of Cambridge University Press).

sporophyte to fulfil a new function, in which case their evolutionary history is connected. Indeed, the pore serves not for gas exchange but as the entry point for filamentous Nostoc, which will form the endophytic colonies characteristic of all hornworts (see Section 5.1.4).

The photosynthetic cells, including the epidermal cells of the gametophyte, typically possess a large solitary chloroplast. Numerous, small plastids characterize only a few taxa, such as Megaceros. In either case, a single chloroplast occurs in the cell at the time of cell division. Rubisco, the enzyme responsible for carbon fixation and, hence, essential to photosynthesis, is often concentrated in a discrete region of the chloroplast forming what is called the pyrenoid (Fig. 5.7). This feature is unique among land plants but known from algae, including the Charophycean ancestor to embryophytes.

Most hornworts are monoicous, with individual thalli producing both types of gametangia. Dioicy is less common and sexual dimorphism between unisexual thalli weakly pronounced. Gametangia are produced at the dorsal thallus midline (Fig. 5.8). Archegonia are shaped like a vase, with a slightly swollen venter holding the single egg and a short neck, capped by two to four cover cells; at maturity the central canal cells of the neck disintegrate and the cover cells are shed (Fig. 5.9). The female sex organs develop from an epidermal-initial and remain mostly sunken at maturity with only the apex of the neck protruding from the surface (Fig. 5.14b). Sperm cells are protected by a unistratose jacket subtended by a short stalk. The development of antheridia is similar to that seen in other bryophytes, except that the initial is borne immediately below the epidermis. Male gametangia always develop within a chamber and typically occur in aggregates, with all antheridia derived

Fig. 5.8. Antheridia and mature sporophyte in Phaeoceros laevis thriving on steep, wet rocky slopes of humid laurel forests in La Palma, Canary Islands A: antheridia; C: capsule; I: involucre. Note the yellowish apex of the capsule from which yellow spores, S, are released, while new spores are still being produced at the green base of the capsule (photo A. Vanderpoorten). See plate section for colour version.

Fig. 5.8. Antheridia and mature sporophyte in Phaeoceros laevis thriving on steep, wet rocky slopes of humid laurel forests in La Palma, Canary Islands A: antheridia; C: capsule; I: involucre. Note the yellowish apex of the capsule from which yellow spores, S, are released, while new spores are still being produced at the green base of the capsule (photo A. Vanderpoorten). See plate section for colour version.

from a single initial (Fig. 5.10). The epidermal cell above the antheridial initial gives rise to the two-layered roof of the antheridial chamber. A schizogenous cavity is formed following the separation of the two initials early in gametangial ontogeny. At maturity, the location of the endogenous antheridia is revealed by the convex ceiling of the chamber on the thallus surface.

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