FIGURE plant ancestry
Classification at the kingdom level is not straightforward
As explained earlier, kingdoms are at the top of the classification hierarchy. You may have heard of the plant kingdom and the animal kingdom. There are organisms that are clearly plants and organisms that are clearly animals, but there are many other organisms that do not fit neatly into these two major groups. In fact, no satisfactory criteria have been found that will allow all living things to be classified as either plant or animal. The table below shows a variety of arrangements that have been suggested for organising living things into kingdoms. Since the late 1960s the five kingdom system has become increasingly popular among taxonomists because they recognise that it helps to overcome the problems that occur if a smaller number of kingdoms is used.
Classification is one of the many fields of biology that is now being helped by greater understanding of the structure of DNA. It is now possible to draw up a 'family tree' for the various sorts of living things based on the amount of similarity in their nucleic acids. DNA comparisons show that amoebas do not fit neatly with the animals and that, although green plants can be grouped together, if this criterion is used, some algae and fungi are all mixed up. No wonder that in the past there have been so many different opinions about how algae and fungi should be classified.
However, our purpose here is to put living things into groups so that when we refer to a group of organisms, such as arthropods or angiosperms, you will immediately know the general characteristics of those organisms. For such a purpose it is useful to describe the groups of organisms that are not plants and not animals.
How many kingdoms? A variety of classification systems.
The two kingdom system
PLANTS: Bacteria, Cyanophytes, Algae, Slime moulds, Fungi, Bryophytes, Tracheophytes
ANIMALS: Protozoa, Multicellular animals
A variety of three kingdom systems
MONERA: Bacteria, Cyanophytes
PLANTS: Algae, Slime moulds, Fungi, Bryophytes, Tracheophytes
ANIMALS: Protozoa, Multicellular animals
PROTISTA: Bacteria, Cyanophytes, Protozoa, Slime moulds
PLANTS: Algae, Fungi, Bryophytes, Tracheophytes
ANIMALS: Multicellular animals
PROTISTA: Bacteria, Cyanophytes, Protozoa, Algae, Slime moulds Fungi,
PLANTS: Bryophytes, Tracheophytes
ANIMALS: Multicellular animals
A four kingdom system
MONERA: Bacteria, Cyanophytes
PROTISTA: Protozoa, Algae, Slime moulds, Fungi
PLANTS: Algae, Fungi, Bryophytes, Tracheophytes
ANIMALS: Multicellular animals
A five kingdom system
MONERA: Bacteria, Cyanophytes
PROTISTA: Protozoa, Yellow-green and golden brown algae
PLANTS: Green, red and brown algae, Fungi, Bryophytes, Tracheophytes
FUNGI: Slime moulds, Fungi
ANIMALS: Multicellular animals
Plants are living organisms whose cells have a cell wall made of cellulose. Plants have chlorophyll and other pigments that enable them to make their own food from simple, inorganic materials like carbon dioxide and water.
Multicellular plants are usually attached to one spot, they do not move around the way animals do. Plants live in most places - in fresh and salt water, on land, and even under the polar ice sheets. Most living material on Earth is plant material, and the largest living things are plants. Mountain ash trees (Eucalyptus regnans) in Victoria and Tasmania sometimes grow to over 100 m tall, and the giant redwoods of California (Sequoia and Sequoiadendron), though no taller, are the most massive organisms known.
The following descriptions of various kinds of living things follow the evolutionary pattern by starting with the simplest and moving up to the more complex. We describe the plants first, although of course complexity was increasing in animals at the same time as complexity in plants.
FIGURE green alga
FIGURE kelp farmers
FIGURE red coralline alga
FIGURE seaweed
FIGURE U. dotea
Algae (from the Latin alga, meaning seaweed) have a simple structure not differentiated into leaves, stems and roots. Although large algae may appear to have leaves and stems, their internal structure is simple and quite unlike that of true leaves and stems. Most live submerged in water, or are only exposed to air for short periods of time, such as when the tide goes out, and most dry out easily.
The best-known algae are the seaweeds of the rocky coasts, but others grow in fresh water and in damp places on land. They range from microscopic single-celled plants to the giant leathery kelps of cold waters, such as Macrocystis, which may be over 50 m long, and bull kelp, Durvillea. Algae are very diverse and are classed into several phyla. The green algae (Phylum Chlorophyta) are predominantly green and have similar photosynthetic pigments to the vascular plants. Many (and some of the other groups) are much smaller than seaweeds and some are microscopic.
Microscopic algae, which float or swim near the water's surface, are called phytoplankton and are important in the ecology of the sea. They may be so numerous that they colour the water yellow, red or green, depending on which type is most common - the golden algae and diatoms (Phylum Chrysophyte), the dinoflagellates (Phylum Pyrrophyta) or the green algae.
The brown algae (Phylum Phaeophyta) contain deep orange-yellow pigments as well as green pigment and look brownish.
The red algae (Phylum Rhodophyta) have red pigment as well as greenish pigments.
Most brown and red algae, and some green algae, are from 10 cm to a metre long. Some algae are remarkable for living in high temperatures in hot springs and others for living in snow. They are very important as the basis of food chains and are eaten directly by people in Japan and China, and also probably by you, in ice-cream. The kelp Macrocystis is harvested to provide extracts (alginates) that help to give the creamy texture to ice-cream and cosmetics.
For some other aquatic plants that-do not fit into the algal phyla see Chara.
Fossils of charophyte oospores have been found in rocks that are about 400 million years old (Silurian period), but charophytes flourished best about 100 million years ago (Cretaceous period), at the same time as many dinosaurs. Were they a favourite food of some dinosaurs? At all events, they have evolved to be some creatures' favourite food, since they store sugar in the cell vacuoles when the oospores are ripe. In this way the plant is made more attractive to herbivores (nowadays fish or birds such as ducks) so that when the oospores are ripe they can be dispersed in animal droppings. There is a parallel here with the sweet fruit of some angiosperms. However at other times Chara has an unpleasant smell and a tough cell wall to repel herbivores.
There is a lot of Chara and its relative, Nitella, in Australia. Most dams contain some. Recently, a Sydney University scientist who collected Chara from a farm dam near Dungog, NSW, estimated that the dam contained 10 tonnes of the plants. In some places Chara, which is often encrusted with lime, has laid down economically important deposits of calcium carbonate. These deposits are sometimes used to make bricks and cement or to improve soils.
Scientists find Chara fun; they enjoy watching the protoplasm streaming around inside the large cylindrical cells. So can you by finding some Chara and using a microscope that will magnify about 200 times or so. (Use transmitted light and look at the cells of the young shoots after mounting them in water.) The protoplasm moves steadily, smoothly, until the cell is bent, pricked or injured, when it suddenly stops. This is because the cell sends an electrical signal along its membrane from the injured spot and stops the streaming movement. If the injury is mild, streaming will start again in a few minutes, pick up speed, and soon be as fast as before.
The electrical signals that Chara sends, like the electrical signals sent by animal nerve cells, convey information to other parts of the cell or organism. The membrane also displays electrical changes when it is transporting nutrients. These changes help scientists to work out how molecules cross cell membranes and how cells generate electric currents.
FIGURE moss life cycle-2
FIGURE liverworts
Phylum Bryophyta (from the Greek bryon, meaning 'moss', and phyton, meaning plant').
Mosses and liverworts are small plants, often with stems and leaf-like structures and very fine root-like systems. They are found on rock, wood or soil that is damp.They are most common in wet forests, but some are rather hardy plants that can survive dry weather in an inactive or dormant condition. At one stage in their life cycle mosses produce spores in capsules that in many species are held on a stalk above the leafy parts of the plant.
A leaf of a flowering plant held up to the light usually shows a midrib running along the centre of the leaf and a network of fine strands, or veins, running through the rest of the leaf Under a microscope we can see that these strands consist of cylindrical bundles of cells, some of which look like long vessels and have woody walls. They are quite different from the rest of the leaf, and are called vascular bundles (from the Latin word vasculum, meaning 'small vessel'). Plants that have them are called vascular plants.
Further examination shows that the vascular bundles run from the leaves down through the stems and roots. Tests show that they carry water and nutrients through the plant. Some of the conducting cells are ridged like a windpipe (trachea), hence the name Tracheophyta. In addition to the flowering plants, many plants that do not flower also have vascular bundles. They are the pines, cypresses, other cone-bearers and ferns. These plants are all grouped in the Tracheophyte.
Most vascular plants have a body with a shoot system consisting of stems, leaves and reproductive structures above ground, and a complex root system below ground. Because vascular plants are the most abundant plants around us, and provide us with most of the plant material we use (timber, food, and so on) we will consider the various classes into which they are sorted.
FIGURE sporangia
FIGURE sporangia-2
FIGURE life cycle
Class Filicopsida (from the Latin filix, meaning 'fern').
Like the mosses, ferns reproduce by producing spores. At certain seasons small golden or brown areas develop on the undersides of the leaves (called fronds). Each small area is a group of minute spore cases on stalks, and each spore case contains large numbers of almost microscopic spores. When the spore cases are ripe, they open slowly and then fling the spores into the air.
So in ferns there are actually two types of plant, each of which leads a separate existence, alternating with one another from generation to generation. This alternation of generations is found in many plant groups, but the two generations are rarely as clearly separated from one another as they are in ferns.
Even though they reproduce so differently, some ferns resemble flowering plants. The bramble fern of the West Indies grows much like the common blackberry and even has thorns along its stem. Some ferns grow lie trees, some grow in water and others grow on other plants or as climbers. Although most ferns require moist conditions, some are found in dry places.
FIGURE diversity
FIGURE diversity-2
FIGURE gymnosperms
With greater knowledge of the structure and function of living things the way in which they are classified is gradually being refined.
At one stage in the development of plant taxonomy all those plants that produced seeds were grouped together as Spermaphyta, which was subdivided into two classes, the gymnosperms and the angiosperms. In Gymnospermae (from the Greek gymnos, meaning 'naked', and sperms, meaning 'seed') the ovules are on the upper surface or edges of open scales. As the seeds mature the scales may harden to form a cone, as in most conifers, or they may become fleshy, as in the cycads and ginkgo.
Taxonomists no longer use gymnosperms as a group in classification because the differences between the conifers, cycads and ginkgo seem to be as great as the differences between any one of these classes and the angiosperms. So conifers, cycads, the ginkgo and angiosperms are now all separate classes.
FIGURE seeds
Class Cycadopsida (from the Greek koikas, meaning 'palm-like plant').
Cycads grow in many warmer parts of the world. They have a crown of hard, tough leaves on top of a stout stem that is usually a metre or less tall. They produce their seeds in cones in the centre of the crown. The cones are large and rather fleshy, and although they appear edible they contain poisonous compounds or toxins. Some Aboriginal people know how to remove the toxins so that the starchy seeds and the pith from the stem can then be eaten.
There are only about 100 species of cycad in the world today, but fossil evidence shows that there used to be many more. They were the main food of dinosaurs
Class Ginkgopsida (from the Japanese ginkgo, meaning 'maidenhair tree').
The maidenhair or ginkgo tree, Ginkgo biloba, is the only present-day member of this class. For thousands of years it has been cultivated in gardens but in the natural state it occurs only in a small part of China. It has unique fan-shaped leaves, and the small male cones and plum-sized female cones are home on different trees. Unlike most cone-bearing trees, the ginkgo is deciduous, shedding its leaves in winter.
Fossils show that the ginkgo that survives today is a tiny remnant of a group of plants that was once quite common. Because it has remained unchanged for 200million years, it is called a living fossil (a term dubbed by Charles Darwin).
FIGURE bristlecone pine
FIGURE Algerian fir
FIGURE yew
FIGURE conifer parts
FIGURE cones
FIGURE pine-life-cycle
FIGURE pine-life-cycle2
Class Coniferopsida (from the Latin conus, meaning 'cone', and the Greek opsis, meaning 'appearance').
Conifers produce seeds on the scales of woody cones, hence their name. Timber from conifers is classed as softwood.
Extensive coniferous forests, valued for their timber and their ability to withstand long, cold winters, grow across North America, northern Europe and northern Asia. They include the pines, firs, spruces and cypresses, and, further south, the giant redwoods.
Comparatively few conifers grow in parts of the world with warmer climates, such as Australia. Forests of Murray (or cypress) pines (Callitris) grow in various parts of southern and eastern Australia. Other trees, such as the kauri (Agathis) and hoop pine (Araucaria), grow in forests mixed with flowering plants.
True pines are members of the genus Pinus, and nearly all come from the northern hemisphere. The Monterey pine from California, (Pinus radiata, has been planted widely for timber in the southern hemisphere and now grows wild in some places.
The wood and foliage of many conifers have a characteristic smell from the resins and oils in the wood and bark. The resin may help block wounds to the tree, thus preventing infections, and also help preserve the wood.
Gymnosperms
FIGURE ancestryCycads
FIGURE cycadsGinkgo
FIGURE ginkgoConifers
FIGURE conifers
