Eukaryotes are organisms whose cells contain functional units separated from the general cytoplasm by membranes.
The photosynthetic apparatus is enclosed in a chloroplast, aerobic respiration takes place in a mitochondrion and the chromosomes are confined in a nucleus.
Unicellular, filamentous, colonial or thalloid plants which live mostly in fresh or salt water; occasionally on very moist surfaces on land. They range from unicellular forms such as those which cause ‘greening’ of aquaria to huge seaweeds many metres long.
They lack the internal conducting and supporting tissue subsequently developed by the higher plants. If they have internal structure, it is made up of largely unspecialized cells.
Sexual reproductive structures are mainly unicellular but if multicellular an outer wall of sterile cells is not formed as in higher plants. Asexual sporangia are also usually unicellular but if multicellular, all of the cells form spores.
This group is known as the green algae and constitutes one of the largest groups of algae, in terms of numbers of genera and species.
They are also noteworthy in that there are a number of features of their photosynthetic pigments, metabolism and protoplasmic structure which are very similar to those of mosses, liverworts and higher plants. It is possible therefore that they were on the main line of evolutionary development which led to those plants which were the first colonizers of the land.
- Spherical single cells: Caryosphaeroides
- Spherical single cells and in colonies: Glenobotrydion
- Single cells of desmid form: Paleoclosterium
- Filamentous: Palaeogeminella
- Coenocytic (large multi-nucleate structures): Courvoisiella.
- Coenocytic, with calcareous encrustation, some so abundant as to be rock-forming: Rhabdoporella, Vermiporella, Beresella, Selotonella, Palaeoporella, Primocorallina, Coelosphaeridium, Dimorphosiphon, Pianella, Ortonella.
- Branched with enlarged final segments: Caulerpites
- Colonial, producing hydrocarbon deposits: Botryococcus
- Thalloid: Parka, Pachytheca
- Large colonies: Gleocapsamorpha
- Small colonies: Pediastrum
- Main axis with branches in whorls; highly characteristic, spirally marked reproductive bodies: Trochiliscus, Clavator, Aclistochara, Echinochara.
These are unicellular algae, some of which (the diatoms) secrete about themselves highly ornamented silica structures that resemble old-fashioned pill boxes, the lids of which are larger than the bases.
At certain geological times Chrysophyta were so prolific in lakes and seas that enormous deposits up to 1000m thick formed from their empty silica shells. The latter are much used in various abrasives and also for providing the solid parts of sticks of gelignite.
Other members of this group (the coccoliths) secrete minute calcareous plates on to the external surfaces of their cells. These plates are of great geological importance in that huge deposits of these plates collected on the sea floor to form the familiar chalk of the south of England.
- Unicellular, secreting two silica shells (diatom structure): Cyclotella, Cymbella, Pyxidicula, Triceratium, Raphoneis, Delphineis
- Unicellular, secreting minute calcareous plates (coccolith structure): Cyclococcolithus, Arkhangelskiella.
Most of this group are unicellular organisms but there are some filamentous types. We shall only deal with the unicellular ones.
At the present day these are very important members of the marine phytoplankton and are more commonly known as the dinoflagellates. They have thick resistant walls and many somewhat bizarre shapes involving spiny projections. Sudden local increases in the populations of certain species may lead to the death of fish and other animals due to the release of toxic substances. As fossils they are very important for stratigraphic determination when used in conjunction with spores.
These are the so-called brown algae which are well known as seaweeds.
They adopt many different shapes according to the species. Straps, blades, cords and long ribbons are commonly encountered forms. Many anchor themselves to the sea bed but they are in no sense rooted, they cannot absorb substances through the root-like structures.
They are of quite a large calibre as plants go, their internal structure is not of the type that we shall see in the plants that colonized the land. It is a rather loose aggregation of relatively simple cells that hold together sufficiently well to maintain the integrity of the plants.
These are the largest thalloid plants known. Some have air bladders that lift them up from the sea bed. Their collapsed appearance on shores at low time is transformed on the return of the tide.
These are the red algae, another group which are mainly known as seaweeds. They are not as common as the brown seaweeds but are almost always to be found amongst them on seashores.
Many have become preserved as fossils because they deposit calcium carbonate in their cell walls. Enough calcium carbonate may accumulate in this way to form nodular masses of limestone which are important geologically. Solenopora is one fossil genus that makes limestone.