THE plant body of ferns andis differentiated into , and , but in the lower plants such differentiation is not found. The simple plant body of these types is called a thallus, and the plants are placed in the division of the plant kingdom called the Thallophyta, which includes the plants known as the Algae, Bacteria, Fungi and Lichens. Seaweeds and many green fresh-water weeds are Algre ; the plants from this group are distinguished from the fungi by the possession of chlorophyll or some other similar pigment, which enables them to build up their body substance from simple raw materials by means of light energy absorbed by the chlorophyll or other pigment. Moulds, mildews, mushrooms, toadstools and rusts are fungi and have no chlorophyll, but live a parasitic or saprophytic existence. A parasite lives on material obtained from another living organism, while a saprophyte obtains its nutriment from non-living organic matter. The lichens are Thallo-phytes in which the thallus consists of a fungus and an alga which live together for their mutual benefit, the fungus supplying water and mineral salts while the alga makes food by .
Many of the green Algae, such as Protococcus, are unicellular, while others, for example Spirogyra, consist of a single row of cells forming a filament. Cell structure becomes more complicated in the seaweeds, some of which are the largest of living plants. In the brown seaweeds the green colour of the chlorophyll is masked by the presence of brown pigments, while red pigments similarly obscure the green colour in the red seaweeds. Generally the red seaweeds are found in the deep coastal water beyond the low tide mark, while the brown seaweeds extend shorewards from the red seaweeds, and are typically found between the tide marks.
Diatoms are unicellular alga; and are found in fresh water and in the sea. They form a large part of the plankton—organisms floating or swimming in the surface layers of the oceans—and serve as food for the animals in the plankton. A common diatom found in fresh water is Navicula. Like all diatoms, it has a cell wall impregnated with silica and composed of two halves fitted together like a box with a removable lid overlapping the sides of the box. In the cytoplasm are brown chromatophores which absorb light energy for photosynthesis. Diatoms reproduce by binary fission.
Diatoms form the basis of food chains supporting a number of edible fish, e.g. the herring, which provide a valuable source of food for Man. Fossil deposits of their shells are mined and used to absorb nitro-glycerine in the manufacture of dynamite used in blasting operations.
The fine transverse markings seen on the shells of many diatoms are used to test the resolving power of microscopic lenses.