Chara, commonly known as stonewort, is a fascinating green alga that exhibits characteristics resembling those of higher plants. It belongs to the family Characeae and is often found in freshwater habitats, where it plays a significant role in the ecosystem. The presence of calcium and magnesium carbonate coverings on its surface gives it a stony appearance, hence the name stonewort. In this article, we will delve into the life cycle of Chara and explore its structural affinities, shedding light on its unique biological and ecological features.
Systematic Position of Chara
To understand the biology of Chara, it's essential to know its classification within the broader context of algae and plants. The systematic position of Chara is as follows:
- Class: Chlorophyceae
- Order: Charales
- Family: Characeae
- Genus: Chara
This classification places Chara within the green algae, specifically within the Charales order, which is known for its complex structural organization and reproductive processes.
Occurrence and Habitat of Chara
Chara is a widely distributed genus with approximately 188 species identified globally. Of these, around 30 species are found in India, including notable species such as C. braunii, C. gracilis, C. nuda, and C. hatei. Chara species typically thrive in habitats characterized by hard water, low oxygen levels, and high concentrations of calcium. These conditions are often found at the bottoms of freshwater environments, such as pools, lakes, and slow-moving streams. Some Chara species are adapted to marine conditions, while others, like C. baltica and C. fragilis, can survive in unusual habitats like salty water and hot springs, respectively.
Structural Affinities of Chara
Chara exhibits a macroscopic plant body that can grow up to 10–30 cm in length, with some species reaching heights of 90 cm to 1 meter. The plant body is well-branched and consists of a main axis and rhizoids. This structural organization gives Chara a resemblance to higher plants, making it a subject of interest for studying the evolutionary links between algae and terrestrial plants.
Main Axis and Rhizoids
The main axis of Chara is long, erect, and divided into nodes and internodes. The internodes are elongated, cylindrical, and unicellular, while the nodes consist of clusters of cells arranged in a specific pattern. At each node, there is a pair of central cells surrounded by 6–20 peripheral cells. The nodes have a plate of cells, while the internodes consist of elongated cells. Rhizoids, which are thread-like, multicellular, and branched structures, help attach the plant body to the substrate, anchoring it in the muddy bottoms of aquatic environments.
Development of Structures from Nodes
From the nodes of the main axis, four types of structures develop: branchlets (or primary laterals), axillary branches, stipulodes, and cortex.
Branchlets
Branchlets grow in whorls at the nodes of the main axis and branches of unlimited growth. These branchlets also possess nodes and internodes, similar to the main axis. At the nodes of branchlets, secondary laterals, which are hair-like, and reproductive structures such as sex organs can develop.
Axillary Branches
Axillary branches emerge from the older nodes of the main axis and also consist of nodes, internodes, and whorls of branchlets. These branches contribute to the plant's ability to spread and colonize its aquatic habitat.
Stipulodes
Stipulodes are unicellular outgrowths that develop at the lowermost nodes of branchlets. These structures arise from certain peripheral cells and vary in number depending on the species. For example, C. braunii typically has stipulodes in a single circle, while C. baltica may have them in two circles.
Cortex
The cortex is a layer of cells that surrounds the long internodal cells of the axis in most Chara species. This layer develops from adjacent nodes and provides additional structural support. The cortical cells cover the internode partially, with coordinating filaments arising from the upper and lower nodes covering the respective halves of the internode. Some species of Chara, such as C. corallina and C. nuda, lack this cortical covering, a condition referred to as ecorticate.
Cellular Structure of Chara
The cellular structure of Chara is another aspect that highlights its affinities with higher plants. The nodal cells of Chara possess a four-layered cellulosic cell wall, a central nucleus, dense protoplasm, and numerous discoid chloroplasts that lack pyrenoids. Unlike internodal cells, nodal cells have poorly developed central vacuoles but contain several small vacuoles scattered within the cytoplasm.
Internodal Cells
Internodal cells in Chara are unique in their structure. These cells are bound by a cellulose-based cell wall and have a covering of calcium carbonate deposits. The interior of these cells is characterized by a large central vacuole and numerous discoid chloroplasts without pyrenoids. Internodal cells are multinucleate, with young internodal cells initially possessing a single nucleus. A notable feature of these cells is cyclosis, a process where the cytoplasm circulates within the cell, aiding in the distribution of nutrients and organelles.
Reproductive Strategies in Chara
Chara exhibits both vegetative and sexual reproduction, each contributing to the survival and proliferation of the species in various aquatic environments.
Asexual Reproduction
Asexual reproduction in Chara occurs through several structures that facilitate the growth of new individuals without the involvement of gametes. The following are the primary mechanisms of asexual reproduction in Chara:
Amylum Stars
Amylum stars are star-shaped structures rich in starch that develop near the lower nodes of Chara. These structures have the potential to give rise to new plants upon detachment from the parent plant.
Bulbils
Bulbils are small, bulb-like structures that form at the lower nodes or rhizoids of Chara. When these bulbils become detached from the main plant body, they can develop into new individuals.
Protonema-like Branches
Protonema-like branches are structures that resemble the protonema stage seen in the life cycle of mosses. These branches develop on certain nodes and can produce new plants, contributing to the vegetative propagation of Chara.
Sexual Reproduction
Sexual reproduction in Chara is oogamous, meaning it involves the fusion of a small, motile male gamete (antherozoid) with a larger, non-motile female gamete (egg). Most species of Chara are monoecious, meaning they have both male and female reproductive organs on the same plant.
Male and Female Sex Organs
The male sex organ in Chara is called the antheridium or globule, while the female sex organ is known as the oogonium or nucule. Both of these reproductive structures are visible to the naked eye and are located on the adaxial side of the branchlets.
Antheridium (Globule)
The antheridium is a spherical structure with a yellow-orange or red color. It consists of eight shield cells in its walls, with numerous antherozoids contained within. The antherozoids are the male gametes responsible for fertilization.
Oogonium (Nucule)
The oogonium, located above the antheridium on the node, is a small, green, oval structure. It is attached to the node through a pedicel and contains a single large egg cell surrounded by reserve food. The oogonium is encased in a sheath of spirally twisted cells known as the tube cells, which protect the egg cell.
Early Development of Sex Organs in Chara
The development of sex organs in Chara begins with the division of an adaxial peripheral cell at the lower nodes of the primary lateral. This cell divides periclinally to produce an outer antheridial initial and an inner cell that undergoes further divisions to form three cells. These cells differentiate into the internodal cell, the antheridium basal node, and the antheridial initial, respectively. The antheridial initial gives rise to the antheridium, while the other cells contribute to the formation of surrounding structures.
Development of Antheridium
The antheridial initial undergoes a series of divisions, starting with a transverse division that produces a basal pedicel cell and a terminal antheridial mother cell. The pedicel cell elongates to form the antheridial cavity, while the antheridial mother cell undergoes further divisions to produce an octant of cells. These cells divide further to create a structure with eight outer cells and eight inner cells. The outer cells develop into shield cells, the middle cells into rod-shaped manubrial cells, and the innermost cells into primary capitulum cells. The primary capitulum cells give rise to secondary capitulum cells, which form antheridial filaments containing numerous biflagellate antherozoids.
Development of Oogonium
The development of the oogonium begins with the division of the upper peripheral cells of the basal node of the antheridium. These cells act as oogonial initials and undergo transverse divisions to produce a short filament of three cells: the pedicel cell, the median cell or nodal cell, and the oogonial mother cell. The oogonial mother cell further divides to produce a lower small stalk cell and an upper large oogonium. The surrounding sheath cells, which arise from the nodal cell, form a protective layer around the oogonium, with the upper cells giving rise to the corona.
A mature oogonium is characterized by a single large egg cell at its center, surrounded by a wall composed of five helically twisted cells. These sheath cells, collectively known as tube cells, provide structural support and protection to the developing egg cell. The tube cells also contain a ring of five small cells, referred to as the corona, located at the apex of the oogonium.
Fertilization and Zygote Formation
Fertilization in Chara occurs when a biflagellate antherozoid from the antheridium swims to the oogonium and penetrates the wall to reach the egg cell. The fusion of the male and female gametes results in the formation of a diploid zygote. The zygote develops a thick wall, becoming a resting spore or oospore, which eventually germinates under favorable conditions to produce a new Chara plant.
Post-Fertilization Changes in Oogonium
After fertilization, the oogonium undergoes several changes, with the egg cell developing into a diploid zygote. The wall of the oogonium becomes thick and heavily pigmented, leading to the formation of a resting spore. The spore remains dormant until conditions become favorable for germination, at which point it gives rise to a new Chara plant, completing the life cycle.
Conclusion
Chara is a remarkable alga with a complex life cycle and structural affinities that bridge the gap between algae and higher plants. Its reproductive strategies, both vegetative and sexual, enable it to thrive in various aquatic environments, while its structural features, such as the main axis, rhizoids, and sex organs, provide insights into its evolutionary significance. By studying Chara, researchers can gain a deeper understanding of the evolutionary processes that have shaped the diversity of life on Earth, particularly the transition from aquatic to terrestrial habitats.
