In this article we will discuss about:- 1. Occurrence of Angiopteris 2. Sporophyte of Angiopteris 3. Gametophyte 4. Phylogeny.
Occurrence of Angiopteris:
The family Angiopteridaceae has seven genera and nearly 145 species. All are tropical plants occurring in humid forests. Of the seven genera Marattia and Angiopteris are widely distributed.
Angiopteris flourishesl in the tropical forests and is represented very well in the eastern hemisphere. The number of species in Angiopteria is variable. While some people-recognise only one variable species (A. evecta) others recognise over a hundred species. A. evecta is a common species found in India.
Sporophyte of Angiopteris:
Morphology of the plant:
The sporophytic plant body consists of an upright, tuberous, conical, fleshy rhizomatous stem. The stem is quit thick and inhabit the plant resembles a tree fern (Fig. 129). The stem is often called a caudex or trunk which may be a foot or two in height and almost the same girth.
The upper surface of the rhizome bears a crown of graceful, stately leaves. The leaves are 5-6 metres long in a luxuriously growing plant, with a petiole as thick as a man’s arm. The leaves are typically bi-pinnately compound. The venation is of the open dichotomous type (Fig. 130). Pinnae are glabrous (smooth).
It is very typical of Angiopteris and other marattiaceous ferns to have a pair of thick fleshy stipules at the base of the leaf. When the leaves fall off, these persist with the leaf bases and form a protective armour around the stem.
The base of the petiole and the stipules together appear like a horse’s hoof. In some species (A. evecta), at the base of the pinnae and the petiole there are swollen articulations (joints) which are apparently the regions of abscission. The pinnae are long (more than 10 – 12 cm long), dorsiventrally flattened and have a long drawn tip. The margin is serrate.
Roots are produced from the under surface of the rhizome at the base of each leaf. While leaves are deciduous, roots are perennial. They are thick and have a mycorrhizal association. The branching of the root may or may not be profuse.
Roots are endogenous in origin and pierce the stipules during the course of growth. Root hairs are peculiar in being multicellular. At the margins of the pinnae on the abaxial surface are borne, the sori. There is no distinction into foliage leaves and sporophylls. The sori occupy a near terminal position on the dichotomously branched veins.
A cross section of the stem shows an outer epidermis, a broad cortex and a central region of vasculature. The cortex is wholly parenchymatous. Here and there, mucilage ducts are seen in the cortex. In young stems an endoderm is may be seen at the inner face of the cortex.
The vascular cylinder of the first formed part of a stem is protostelic. At the level of the third or fourth leaf the stele changes into a siphonostele and then to a dictyostele. There are many bundles arranged in several concentric rings.
These concentric circles represent the funnel shaped zones of anastomosing bundles. Mattenius (1986) who made a detailed study of vasculature in Angiopteris observes. “The vascular bundles form funnel shaped zones with the lower ends in the stem, and their upper portions continued into the leaves as leaf traces. Segments of the outer zone pass into the leaves as leaf traces and the gaps thus left are filled by segments of the next inner zone”.
According to Campbell (1911), the concentric, vasculature of the stem is built up by the union of leaf traces.
A very weak secondary development may occur in some cases. Tracheids have scalariform pitting.
The petiole shows much the same structure as in the stem. In the hypodermal region there is a sclerenchymatous band. The leaf trace may be ‘C’ shaped. Lamina shows a prominent midrib. There is a palisade parenchyma below the upper epidermis and a loose spongy parenchyma above the lower epidermis. Leaves are generally hypostomatic.
A transverse section shows an epidermis, cortex and stele. The cortex has parenchyma as well as sclerenchyma. There is a conspicuous endodermis encircling the stele. Stele is an exarch, polyarch, actinostele. In the young roots a mycorrhizal fungus is present.
Sporophyte reproduces vegetatively as well as by spore production. Dormant buds are formed on the rhizome where the stipules join the petiole. When the stipules are ultimately shed from the plant, the buds develop into new individuals.
Spore Producing Organs:
Spores are produced in sporangia which aggregate to form the son. The sporangia as has already been said, lie at the back of the veins. The sporangia in the sori are arranged in two linear rows. They may exhibit a certain degree of lateral fusion.
But this is not sufficient to call it a synangium. The synangial nature in Angiopteris is not as evident as in Marattia. In fact, many people regard the sporangia of Angiopteris completely independent. There is no typical induciuim. Rarely indicial hairs may be present.
The first indication of a sporangial development is the accumulation of cytoplasm in certain epidermal cells lying in localized regions on the abaxial surface of the pinnae. This forms the receptacle, from which sporangia develop. There is usually a single sporangial initial, which undergoes periclinal division to form upper and lower cells (Fig.134a).
While the outer cell contributes to the jacket, the inner cell gives rise to the archesporium. However a part of the jacket (basal portion) is derived from the surrounding cells. Hence the development is of the eusporangiate type. A tapetum differentiates from the innermost wall layer (Fig. 134b).
A nearly mature sporangium has a multilayered jacket enclosing a mass of spore mother cells, which undergo reduction division and produce haploid spores. Spore output varies from 1450-7500, per sporangium. All spores are of the same type. All the sporangia in a sorus mature together (simple sorus).
Dehiscence of the Sporangium:
In a mature sporangium there is an arch of cells in the wall, with slightly thickened end walls. This (annulus) extends from side to side across the top of the sporangium. By the shrinking of these cells and those in the wall of the sporangium away from the centre of the sorus, the wall is torn open towards the centre. Spores come out in a mass. They are wind dispersed.
Gametophyte of Angiopteris:
Structure and Germination of Spores:
Angiopteris is homosporous. The spores are minute and usually tetrahedral in shape. They germinate a few days after shedding. During the first month there is considerable enlargement in the size of the spore together with the appearance of chlorophyll (Eames 1964). The first division of the spore is transverse.
Of the two cells, the smaller one forms the first rhizoid. The outer wall of the spore ruptures facilitating the exit of the rhizoid. The larger cell forms a cluster of cells in which is differentiated an apical cell. Soon a plate of cells is formed. Subsequent growth by marginal divisions results in the formation of a thalloid structure.
The Mature Pro-thallus:
The pro-thallus is a deep green, aerial (surface living), lobed or cushioned, thalloid structure (Fig.135). There is a definite growing point. The thallus has dorsiventral symmetry. Rhizoids are produced from the ventral surface and help in anchorage and absorption. In spite of the presence of chlorphyll, a mycorhizal association is always present. To what extent this association is helpful to the thallus is doubtful.
All the cells in the pro-thallus are parenchymatous.
Prothalli of Angiopteris are monoecioius, with archegonia towards the cushioned region and antheridia scattered all over. In structure and development antheridia are similar to those of Ophioglossum. Antherozoids are multi-ciliate.
The archegonia also are similar to those of Ophioglossum but have a shorter, broader neck. The neck is almost sunk in the gametophylic tissue. There is a single, (bi-nucleate) neck canal cell. The venter canal cell is also large and conspicuous. Fertilisation is similar to that in other pteridophytes.
The first division of the zygote is transverse to the long axis of the archegonium. According to Eames (1964) quadrant divisions are obscure and the young organs are not definitely traceable to these. Lang (1923) has reported the development of suspensor from the epibasal half in some individual cases of A. evecta.
Thus, the embryogeny may be endoscopic. In any case it is evident that the typical embryonal parts like stem, root and cotyledon arise from the hypo-basal half of the embryo. No definite foot is formed. Thus as a contrast to ophioglossaceae, the embryo has a reverse orientation. The primary root is formed only after the embryo has attained a considerable size. Only one sporophyte is normally formed on a pro-thallus.
Phylogeny of Angiopteris:
Angiopteris resembles both Ophioglossum and true ferns. While in habit and general appearance it is close to the latter, in important structural features it appears to be closer to the former.
Pinnately branched leaves showing circinate venation recall what is seen in true ferns. The presence of stipules shows resemblance to Ophioglossum. But it has to be pointed out that the stipules of Angiopteris are thick and paired unlike those of Ophioglossum.
Anatomically Angiopteris occupies a position intermediate between Ophioglossum and true ferns. While the bulk of soft tissue present in the stem is suggestive of Ophioglossum, the presence of some mechanical tissue is indicative of true ferns.
In eusporangiate development, and output of spores Angiopteris is definitely like Ophioglossum. But the soral organisation not seen in Ophioglossum, is a true fern character.
Embryogeny bears many resemblances to Ophioglossum. Within the family, Angiopteris with no synangial arrangement is regarded as primitive when compared with Marattia and others which show a synangial organisation.