Life Cycle of Polypodium | Pteridophyta | Botany

In the article we will discuss about the life cycle of polypodium which belongs to pteridophyta division.

The Sporophyte of Polypodium:

The stem of Polypodium is a peren­nial, sparingly bran­ched, erect or creep­ing rhizome, which when destitute of fronds (leaves) has the appearance of some kind of sea polypus. Usually, the greater part of the rhizome is sub­terranean, but it grows obliquely up­wards and its apex rises a little above the surface of the ground and bears the leaves.

This erect or ascending region is known as the caudex, often clothed with dense growth of scales, epidermal hairs or adventitious roots. The older portion of the stem shows the presence of persis­tent leaf-bases of the dead fronds, and when this portion dies the branches become disconnected and continue to develop as separate new plants.  

The primary root dies early, all subsequent roots are adven­titious and arise from lower side of the rhizome. Each root has a root cap and root hairs.

The leaves, often called fronds, are either simple, entire or pinnatifid or pinnately compound and when young show characteristic circinate ptyxis. Annually, rosette of leaves unfold from the caudex portion of the rhizome and these ultimately die at the close of the growing season.

The petioles (stipes) are short or slightly longer, tufted and are often clothed with numerous brown hairs known as ramenta, which are characteristic of ferns. These hairs are also present on the rachis. The veins of the leaf-segments are either simple or forked and ultimately bear groups (sori) of asexual reproductive organs (sporangia) and are, therefore, soriferous.

A cross-section of the rhizome shows two distinct regions, the cortex and the stele. The cortex is bounded on the outside by an epidermis consisting of a single layer of narrow cells with thick walls.

The cortex is differentiated into two regions:

(a) Hypodermis consisting of a few layers of thick-walled parenchyma,

(b) General cortex consisting of many layers of parenchyma cells with or without intercellular spaces; this tissue is mainly concerned with the storage of food and water.  

The vascular bundles vary in number, some of which are large and others are small and are generally found near about the centre of the stem. Each vascular bundle is discrete and hadrocentric and is surrounded by a typical endo­dermis, lying internal to which there is a single layer of pericycle; the protoxylem is exarch.

The phloem consists of sieve cells and ‘conjunctive’ parenchyma, and the xylem consists of tracheids varying in width with thick lignified walls and ‘conjunctive’ paren­chyma like that of phloem. There is pith in the centre of the rhizome. Leaf-trace bundles are present in the cortex.

Early in summer, the foliage leaves bear on their under surface many greenish brown structures, called sori, which are developed directly over the veins so that the foliage leaves are now spoken of as sporophylls. Thus, in Polypodium, there is no differentiation into foliage leaves and sporophylls, both the functions of photosynthesis and spore-production are taken over by all the leaves.  

The sori are generally round, large or small, and sometimes are mixed with hairs. They are generally terminal on the veinlets or at their bases forming a regular row on each side. Each sorus is naked and consists of a group of sporangia which arise from a tissue of the leaf, called placenta.

Each sporangium develops by the leptosporangiate method. The fully-formed sporangium is a lenticular capsule attached to the stalk. On the capsule there is a specialized, cutinized, incomplete cellular layer called annulus. Where the annulus stops snort, the cells are thin and form what is called stomium.  

Within the single-layered wall of each sporangium there are one or two layers of nutritive tissue, the tapetum, surrounding a mass of sporogenous cells, which ulti­mately form about sixteen spore mother cells.

From each cell, by reduction divi­sion, a spore-tetrad is formed. When mature, the spores are dark-brown in colour and morphologically alike,—hence Polypodium is homosporous. With reduc­tion division and formation of spores, the gametophytic or haploid generation begins.  

The Gametophyte of Polypodium:

When the sporangium ripens, it bursts at the stomium owing to the hygroscopic movement of the annulus; and the spores escape. Each spore has two coats- the outer coat is called exospore and the inner coat, endospore.

Under proper conditions of tempera­ture and moisture each spore germinates. On germination, the outer coat bursts and the inner one elongates and protrudes into a multicellular filamentous structure, which when fully developed forms a flat, green, heart-shaped body called prothallus.

The prothallus gives out from its under surface many delicate, uni­cellular, hair-like structures called rhizoids, by means of which the prothallus is not only attached to the substratum but it also draws nourishment therefrom. The prothallus is, therefore, an independent plant which can absorb raw food materials from the substratum and can carry on photosynthesis.  

The prothallus bears on its under surface, scattered among the rhizoids, especially on the thicker part of it, several multicellular bodies of two distinct kinds:

(1) Antheridia or the male sex organs, and

(2) Archegonia or the female sex organs.

Each antheridium is a spherical body and contains about thirty- two spermatozoid mother cells (spermatocytes), from each of which a spirally-twisted, multiflagellate spermatozoid or anthero­zoid is produced.

When mature, the antheridium opens at the top and the mother cells are liberated; the spermatozoids set them­selves free from the mother cells and begin to swim in the film of water already present on the surface of the prothallus.

Each archegonium consists of two parts:

(i) A basal swollen portion, called the venter and

(ii) An elongated upper portion, the neck.

It contains only one elongated neck canal cell, one ventral canal cell and an oosphere or ovum or egg.  

When the archegonium matures, its neck canal cell and ventral canal cell become dis­organized so that a thorough passage is established between the exterior and interior of the archegonium. This passage becomes filled with a mucilaginous substance containing malic acid.  

The spermatozoids, set free from the antheridium, move about by means of their flagella in the film of water already present on the surface of prothallus and are attracted towards the arche­gonium by the malic acid given out by it for the purpose.

The spermatozoids enter the canal, pass down the venter but only one fertilizes the ovum. This fertilized ovum by secreting a wall round itself becomes an oospore. With fertilization and formation of oospore, the sporophytic or diploid generation begins.  

It is to be noted that although the prothallus bears, as a rule, both kinds of sex organs, the antheridia and archegonia, cross-fertilization generally takes place, i.e., spermatozoids developed on one pro­thallus pass into the archegonia of another. This is necessary because antheridium and archegonium are not developed on the same prothallus simultaneously.

The New Sporophyte of Polypodium:

The oospore gradually forms an embryo consisting of foot, root, primary leaf and stem, and from this embryo ultimately a new sporophyte is developed. After the establishment of new sporophyte the prothallus dies down and thus it becomes independent.