Spirogyra algal strands. Light micrograph of filaments of ISpirogyra sp. i green algae. This alga forms slimy threads known as water-silk or mermaids' tresses in stagnant or slow moving water. The filaments are made up of elongated cells (one whole cell seen at the centre of each filament) that secrete a mucilaginous substance that binds adjacent filaments. Each cell contains chloroplasts arranged in spiral bands (yellow). The chloroplasts are the site of photosynthesis, the process that synthesises carbohydrates from carbon dioxide and water using sunlight.