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KINGDOM: Plant PHYLUM: Lycopodiophyta CLASS: Isoetopsida ORDER: Isoetales FAMILY: Isoetaceae GENUS: Isoetes Ali Javey
Isoetes, the only genus of the family Isoetaceae, is a member of the phylum Lycopodiophyta. Exceptional morphological and anatomical characters of Isoetes put this genus of small vascular heterosporuos seedless plants in an isolated line of evolution: it is the nearest relative of the ancient tree lycophytes. Isoetes, also know as quillworts, may be found in aquatic, amphibian, or terrestrial environments, but they are found most commonly in aquatic habitat. So far, 85 species of quillworts have been identified in both the tropical and temperate areas. Isoetes are characterized by their secondary growth, ligulate microphylls, spirally arranged leaves, endosopy, and heterosopy. These evergreen grass-like plants have underground stems and roots. The only visible above-ground part of Isoetes is the leaf which is spirally arranged. Each quill-like leaf is a potential sporophyll. Each fertile leaf has a single sporangium located at its base which can produce megasporangia or a microsporangia depending on the position of the leaf. The microsporophylls are located nearer the base (center) of the plant. A tongue like structure known as the ligule is present just above the sporangium of each leaf. The function of ligule is still not known. The leaf is usually small, 2-50 cm long and 5-15 mm wide. A single unbranched vascular bundle runs through the vertical axis of the leaf, surrounded by four air chambers. The young gametophyte forms inside the sporangia. Spores are not separate, but are released as the sporangia decay. Even though all the details of the life cycle of quillworts are not known, the overall life cycle of this hetersporous fern-ally resembles the one in Selaginella. The sporophyte of Isoetes has a short and fleshy underground stem, two or three lobes, known as corm. The corm is covered by a cortex layer. The roots arise from grooves between the lobes of the corm and branch dichotomously. The corm could be divided into two layers for structural analysis. The upper layer of the corm is simple in structure and has a protostele running through it’s center. The lower layer is more complex and consists of up to six bilateral lobes. At the core of the lower layer runs the primary protostele which is continuos with the protostele of the upper layer. A layer of cells known as primary thickening meristem surrounds the primary xylem. This layer is used for food storage. Since the karyological data are very essential in classifying the organisms and finding their evolutionary path, it is important for scientists to know and study the number of chromosomes and chromosome mapping of each species of quillworts. Most Isoetes have 11 chromosomes, the exceptions include: Isoetes histrix which has 10 chromosomes, and Isoetes pantii and Isoetes muricata which have 12 chromosomes each. In some species, several levels of ploidy are known. The highest recorded number of chromosomes in a somatic cell of Isoetes is 132 in Isoetes novo-granadensis. The leaves of some species
of quillworts lack stomata and have thick cuticles which disables them
from carrying any gas exchange with the atmosphere. These plants instead
of obtaining carbon from the CO2 of the atmosphere, they take up the carbon
required for photosynthesis from the sediment in which they grow. This
in fact could be very essential in balancing the carbon cycle. Most of
the CO2 of the atmosphere come from the abundant carbon of the soil. There
are very few plants; however, that could use these inorganic carbons of
soil to transfer them into organic compounds such as proteins and nucleic
acids, and one of these plants is Isoetes.
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