In this article we will discuss about the structure and reproduction in Dryopteris.
Structure of Dryopteris:
The sporophytic plant body is differentiated into roots, rhizomatous stem and leaves (Fig. 7.111 A).
The primary root is ephemeral, and is replaced by a large number of adventitious roots growing from the leaf bases. The roots are small and branched.
The T.S. of root shows an outer piliferous layer, a cortex and a central stele. The cortex is differentiated into an outer parenchymatous zone and an inner sclerenchymatous zone. The stele is protostelic with exarch and diarch xylem.
The stem is rhizomatous — partly horizontal and partly erect with the apex rising just above the soil surface. The rhizome is covered with scales and adventitious roots.
Anatomically, the rhizome shows an outer single-layered epidermis, a few-layered thick brown sclerenchymatous hypodermis and a broad parenchymatous cortex with a centrally arranged dictyostelic vascular bundles consisting of many meristeles (Fig. 7.112A). The meristeles are arranged in rings and are broken by leaf gaps.
Each meristele is rounded or elliptical with mesarch xylem surrounded by phloem. Each stele is bounded by an endodermis of two layers, of which only the outer layer is distinct and the inner layer merges with the pericycle of one or two layers. Xylem is made up of tracheids and some vessels which are formed by the dissolution of the end walls of tracheids.
The leaves are borne acropetally at the apex of the rhizome. When young, the leaves are spirally coiled and show circinate ptyxis that is typical to true ferns. The leaves (fronds) are uni-pinnately compound with a long rachis. The leaf bases persist when the older leaves perish.
All leaves are fertile, bearing sori on the ventral (abaxial) surface of pinna (Fig. 7.111 B). The base of the rachis is paleaceous i.e., covered by numerous brown scales called ramenta.
Anatomically, the rachis is traversed by horse-shoe shaped vascular bundles (Fig. 7.112B). The lamina is isobilateral and amphistomatic. Mesophyll cells comprises of spongy parenchyma. A concentric vascular bundle with distinct bundle sheath is present in the midrib.
Reproduction of Dryopteris:
Dryopteris reproduces by means of spores.
Dryopteris is a homosporous fern. The sori are borne in two rows on two sides of the median vein of a pinnule in-between the margin and the midrib (Fig. 7.113A). Each sorus is covered by a kidney-shaped indusium (Fig. 7.113B). A number of stalked sporangia arise from the central placental tissue. Sori are of mixed type i.e., sporangia arise without any order of development.
Development of Sporangium:
The development of sporangium in Dryopteris is of leptosporangiate type, like that of Pteris.
Structure of a Mature Sporangium:
Like Pteris, the sporangium of Dryopteris is comprised of a long stalk and a terminal capsule (Fig. 7.113C). The single-layered jacket of the capsule is differentiated into a thick-walled vertical annulus, a thin-walled radially arranged stomium and large parenchymatous cells with undulated walls.
Dryopteris is homosporous, so the capsule contains isospores which are structurally and functionally alike.
The spores are triangular in shape with trilete aperture. The outer spore wall, exine, is either smooth or variously ornamented.
The sporangum dehisces transversely along the stomium due to the shrinkage of annular cells (Fig. 7. 113B). The spores are dispersed through air to a moderate distance.
The spores may remain viable for a long time and germinate after falling on a suitable substratum. Initially, the exine bursts and the intine along with inner contents comes out in the form of a germ tube and subsequently, by a transverse division, the germ tube forms. The further development of gametophyte is similar to that of Pteris.
The prothallus is heart-shaped, monoecious and protandrous (Fig. 7.114). Antheridia appear first and are confined to the basal central regions among the rhizoids. Archegonia develop near the apical notch.
The development of antheridium is like that of Pteris. The antherozoids are multiflagellated and coiled like that of Pteris.
The development of archegonium in Dryopteris is similar to that of Ophioglossum.
A mature archegonium of Dryopteris consists of a 5-7 celled projecting, curved neck, a binucleate neck canal cell, a ventral canal cell and an egg.
The mechanism of fertilisation in Dryopteris is similar to that of Pteris.
New Sporophyte (Embryo):
The development of embryo in Dryopteris is like that of Pteris. In the young embryo, the root and cotyledon grow more rapidly than the shoot. The root pierces the prothallus and establishes the sporeling in the soil. Later, the first leaf develops. Generally, only one sporophyte is formed from a gametophye.
The life cycle of Dryopteris is shown in Fig. 7.115.