In this article we will discuss about the life cycle of isoetes.
The Sporophyte of Isoetes:
Isoetes suggests the appearance of a tufted grass, which consists of a very short, two- or three-lobed, tuberous corm densely covered by rosette of overlapping bases of stiff awl-shaped leaves. From the lower surface of the corm many dichotomously branched roots develop.
The leaves are unique in structure, being arranged in a close spiral on the upper surface of the corm. Each leaf is a sporophyll, the outermost and lowermost leaves being sterile; successively within these are sporophylls with mature and immature sporangia. In this sense the entire plant body suggests the construction of a strobilus.
Each leaf consists of a basal sporangium-bearing region and a terminal foliage region. The awl-shaped foliage portion of the leaf contains within it four cylindrical air chambers, which may be transversely septate at intervals.
At the junction of the blade and basal portion of the leaf, there appears on the adaxial side a ligule being socketed in a small pit. Below this ligule the broad sporangium-region occurs. There is a single flattened sporangium on the adaxial face of each leaf. This sporangium is situated within a depression in the leaf-base and is more or less completely or incompletely shut off from the outside by a curtain of tissue (velum) arising just below the ligule.
The morphology of the corm has been interpreted variously. It has been described as an erect rhizome; as a stock; as a stem; as a stem combined with a stigmarian type of rhizophore; and as an upper leaf bearing part, the stem, and a lower root-bearing part, the rhizomorph. The corm is so much compressed that the vascular cylinder appears to be a little more than a vascular plate and is a protostele.
The xylem elements, however, do not completely fill up their region and the phloem elements are not completely recognizable. Anomalous secondary growth takes place by means of cambium, which is situated outside the phloem. The cambium cuts off both xylem and phloem elements inside, while secondary cortex outside.
Morphology of the Axis:
The axis consists of an upper leaf-bearing part—the stem, and a lower rhizophore portion from basal part of which roots come out, vertical and transverse sections of two-lobed as well as three- lobed stocks reveal their real nature. The central cylinder is protostelic with a core of mingled tracheids and parenchyma.
The vascular system is a vertical cylinder at the stem portion and spade- shaped at the rhizophore portion of the corm. The entire axial structure resembles somewhat a chopper. Occasionally dichotomy is found both in the axis and rhizophore. Externally the division of the corm into stem and rhizophore can be seen in some cases, as a transverse constriction near the middle, sometimes the axis is deeply two or three lobed.
The correct interpretation of the morphological nature of the axis of Isoetes is difficult, because the external form does not give any indication of the true morphological nature. The axis being much shortened both leaves and roots are brought together.
Different views have been just forwarded by different workers to explain the morphology of the axis. The axis has been called a stock, interpreted as a stem, and as a stem, combined-with stigmarian type of rhizophore. Many workers applied the terms ‘corms’ and ‘rhizophore’ which are considered not inaccurate on the light of recent anatomical researches. The exact interpretation rests upon the anatomy of the vascular system.
Isoetes is heterosporous. Both microspores and megaspores are produced in different types of sporangia, micro-sporangia and mega-sporangia, borne on micro-sporophylls and mega-sporophylls respectively. The method of development of both types of sporangia is somewhat alike in early stages.
Within each sporangium a large mass of sporogenous tissue (up to 10,000—15,000 cells) is developed. At this stage the differences resulting in heterosporous conditions start. In those sporangia, which are destined to become micro-sporangia, differentiation of the sporogenous tissue begins. Some of the cells of the sporogenous tissue form alternating plates of sterile cells (trabeculae) across the sporangium with fertile cells which function as microspore mother cells.
In those sporangia which are to become mega-sporangia, the trabeculae appear to be more massive and out of the thousands of sporogenous cells, only 42-75 megaspore mother cells are fertile, others performing nutritive function. Reduction division of the spore mother cells follows and from each four spores are produced with the initiation of the gametophytic or haploid generation.
The total output of microspores in a single microsporangium is 15,000-300,000 and that of megaspores in a single mega-sporangium is 150-300. At maturity, the sporangia are indehiscent in most cases and both types of spores are ultimately liberated by the death and decay of the sporophylls.
The Gametophytes of Isoetes:
Male Gametophyte:
The microspores, after liberation, germinate immediately and form male gametophytes within a few days. The nucleus of each microspore divides and the microspore is divided asymmetrically into a small prothallial cell and a large antheridial initial cell. The prothallial cell remains undivided, but the antheridial initial divides repeatedly and ultimately forms a so-called rudimentary antheridium, consisting of an outer jacket layer of four cells and four inner spermatozoid mother cells.
Each spermatozoid mother cell is ultimately metamorphozed into a large, spirally coiled, multiflagellate spermatozoid. The prothallial cell and the four jacket cells disintegrate and the spermatozoids are ultimately liberated by the decay of the spore wall.
Female Gametophyte:
Each megaspore germinates and forms a female gametophyte and its mode of development resembles that of Selaginella with the following differences:
The germination starts with a series of free nuclear divisions and a conspicuous central vacuole is never formed in the protoplast (as in Selaginella) in any stage of development. The nuclei gradually become more numerous in the apical portion of the multinucleate gametophyte and an apical tissue is formed at this stage.
The lower multinucleate portion of the gametophyte ultimately becomes cellular, but this process may be considerably delayed till the development of an adult embryo. The first-formed apical prothallial tissue is exposed by cracking of the spore wall along the tri-radiate ridge and is devoid of chlorophyll.
The gametophyte does not protrude through the megaspore wall, but may produce numerous rhizoids. The archegonium is broad and short, and consists of a single uninucleate neck canal cell, a ventral canal cell and an egg.
When the archegonium attains maturity the neck canal cell and the ventral canal cell disorganize. The multiflagellate spermatozoids swim towards the archegonium in dew or rain water and one of them ultimately fertilizes the ovum. The fertilized ovum on secreting a wall round itself becomes an oospore. With fertilization and formation of oospore, the sporophytic or diploid generation begins.
New Sporophyte of Isoetes:
The oospore gradually gives rise to an embryo possessing a massive foot, a root, the stem and a leaf (cotyledon) with a ligule. It is a peculiar feature that the embryo of Isoetes has no suspensor.
Of the various parts of the embryo the stem appears to be a belated member. From this embryo the Isoetes plant is derived.
Systematic Position of Isoetes:
Isoetes differs so markedly from other pteridophytes that its systematic position has long been a matter of dispute. The genus shows a heterogeneous assemblage and characters common to several groups. Its multiflagellate sperm character led Campbell to suggest that it should be included in the Filicinean series.
According to certain affinities with the spermatophytes in the character of the embryo, in the histology of the fully developed sporophyte, in the structure of the root as well as in the leaves with sheathing leaf bases surrounding the stout bulb-like stem.
Though the large multiflagellate sperm character, absence of the suspensor in the embryo, the general habit and the highly specialized leaves together find no parallel in any of the groups of pteridophytes, yet the presence of simple exarch vascular organization, the ligulate leaves and the solitary sporangium on the adaxial face of the sporophylls led majority of the pteridologists of recent years to include it in the Lycopodian series.
Moreover, the presence of stigmaria-like rhizophores, roots with the same distinctive structure as stigmarian rootlet, a vestigeal parichnos in the leaves, trabeculation of sporangia, the regular formation of 4 megaspores in the mega-sporangia are definitely Lepidodendroid characters.
Though the heterosporous habit, ligulate character and the development of the female gametophyte are of much resemblance to Selaginella, yet it differs markedly from Selaginellales, Lycopodiales and Lepidodendrales in its sperm character, its root system ‘and its large chambered sporangia. These characters themselves definitely justify its position in a separate order under the Lycopods.
In the modern system of classification Pleuromeiaceae has been placed together with Isoetaceae under the Order Isoetales. The genus Pleuromeia has so many characters in common with Isoetes that it definitely justifies its inclusion in the Order Isoetales.
These are:
(a) The leaves are of the same type and shape with broad bases and ring-like flange,
(b) Reniform microspores, and
(c) Large sporangia.
Moreover, the modern interpretation of the stock of Isoetes, i.e., it is a telescopic of the rhizophore, and stem of Pleuromeia also places it near Isoetales. The abaxial nature of the sporangia of Pleuromeia seems to be misinterpretation.
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