Spirogyra is a water weed commonly found floating at the surface ofand ditches, and appears as a tangled mass of green threads which is buoyed up by bubbles of oxygen produced by and entangled among the threads. If the mass of weed is felt between the fingers it is found to be slimy to the touch. Examined under the microscope, a single filament is seen to consist of a number of cylindrical cells joined end to end. Each cell is about yjnj- in. long, and has a fairly thick cellulose wall covered with a mucilaginous layer which makes the plant feel slimy. The cell wall is lined with a layer of cytoplasm which encloses a central space or vacuole containing cell sap. The nucleus cither lies in the lining layer of cytoplasm or is suspended in the middle of the vacuole by a number of slender cytoplasmic threads running from the lining of cytoplasm. In the nucleus there is a round granule called the nucleolus, which consists of denser material than the surrounding nuclear material. Embedded in the cytoplasm is a chloroplast, though in some species two or more are found. Each chloroplast is a narrow spiral band in which colourless patches of protein nature, called pyrenoids, occur at intervals. If the plant has been exposed to light these pyrenoids are surrounded by starch grains, and it is supposed that they are in some way connected with the formation of starch.
The Spirogyra filament increases in length by division of cells. The nucleus of the cell divides, and the cell becomes divided into two by the formation of a cross wall. This new wall begins as a ring-like shelf on the inside of the wall and gradually grows farther into the cell, until it extends right across it. Then each daughter cell slowly grows in length to the normal size.
When parts of a plant body become separated to give rise to new individuals without any obvious change in the protoplasm the plant is said to reproduce in a vegetative manner. Sometimes Spirogyra reproduces in this way by the splitting up of the filaments into short lengths of a few cells each. These pieces then grow into new plants.
Between February and June, Spirogyra frequently reproduces by a sexual method , which takes place between two filaments which are lying side by side in contact throughout their whole length. First, protuberances are put out from the opposite cells of each filament. These protuberances as they grow push apart the two filaments, and, where they touch, the cell wall is dissolved away so that the cells become joined by a narrow tube. Meanwhile the cytoplasm in the cells of each filament has separated from the walls and contracted into a dense mass, called a gamete, in which the chloroplast cannot be distinguished. Then the gametes of one filament pass through the connecting tubes into the cells of the other filament, where the two gametes fuse. The nuclei of the gamates unite, and the whole mass of protoplasm becomes a rounded body containing oil drops and surrounded by a thick brown wall. This cell, which is the result of the fusion of the gametes, is the zygospore. Since the two which fuse together are structurally alike, we term them isogametes ; but as one is active and the other passive, they are physiologically different, and may be termed male and female respectively. After a resting period the wall of the zygospore is burst and a young Spirogyra filament grows forth.
All the cells of a Spirogyra filament are similar in structure, and each can carry out the functions of nutrition, breathing, growth and reproduction, and is therefore capable of living an independent life.
Although multicellular, a Spirogyra filament can be regarded as a collection or colony of unicellular organisms which do not become separated from each other after cell division but remain together within a mucilaginous sheath.
Practical Work on Spirogyra
Examine a mass of Spirogyra filaments in water. Rub it between the fingers and note the slimy feeling.
Examine with the microscope filaments mounted in water, and note the chloroplasts, pyrenoids and nucleus. If iodine solution is drawn under the coverslip, the nucleus becomes clearer, the pyrenoids appear brown and the starch grains around them are stained dark blue.
Stain with iodine Spirogyra filaments which have been kept in the dark for two or three days. Are starch grains present ?
Examine prepared slides of Spirogyra filaments in conjugation.