CHAPTER 8. THE FIVE KINGDOMS
PROTEROPHYTIC AEON (2600 - 1500 MYA):
DEVELOPMENT OF KINGDOM PROKARYOTAE
During the Proterophytic aeon, Kingdom Prokaryotae developed. Here are the bacteria and the cyanobacteria that used to be known as blue-green algae. The prokaryotes are organisms that lack membrane-bounded organelles in their cells. In other words, these cells lack nuclear envelopes, plastids (which make starch), mitochondria (that function in respiration), and advanced flagella (whip-like hairs used for movement). Most of the organisms get their energy through absorption of nutrients, but some genera are photosynthetic or chemosynthetic. Reproduction is predominantly asexual by means of fission, whereby the organism divides into two or more parts. (Some of the species do have a primitive form of sexual reproduction. For instance, there are various forms of conjunction, where a sexual exchange of genetic materials occurs.) In this simplest of kingdoms different phyla began to develop variations that would have important consequences for the development of more advanced kingdoms, namely the fungi, plants, and animals.
Once the key processes such as photosynthesis and metabolism were worked out, more complicated forms of life could develop in the first kingdom, prokaryotae. The prokaryotes are divided into two main groups, the Archaebacteria and the Eubacteria. The archaebacteria contain two primitive phyla, the methanogenic bacteria, respoonsible for marsh gases and much of natural gas, and halophilic and thermoacidophilic bacteria. The Eubacteria comprise the rest of the phyla (with one exception). These phyla can also be divided into two main groups: the Firmicutes and Gracilicutes. The Firmicutes are gram-positive (take a stain) and are not photosynthetic. This group contains the fermenting bacteria. The Gracilicutes are gram-negative and many are photosynthetic. Many of these bacteria are involved in diseases, like syphilis, yaws, and cholera. Others help break down waste matter, while still others have developed photosynthesis. This last group includes the cyanobacteria. The reader may be familiar with this phyla since many modern-day cyanobacteria take in nitrogen from the air and fix it (incorporate it) into organic compounds necessary for plant growth. The oldest evidence for this group goes back as far as 2000 million years ago.
Many people are not aware of the extreme importance of bacterial for life. A common misconception is that the food chain of life starts with the plants. Plants, however, were the last kingdom to develop. The importance of bacteria can be demonstrated by the fact that in the sea zooplankton feed on the protoctists, which feed on bacteria. In turn zooplankton provide more of the earth's oxygen than all of the land forests put together.
Bacteria are also absolutely crucial for recycling material so that life can continue. Bacteria make the entire food chain possible by breaking organic compounds down into inorganic compounds that can be taken up by other living organisms.
M-2 Halophilic and Thermoacidophilic Bacteria
M-14 Fermenting Bacteria
M-9 Nitrogen-fixing Bacteria
M-12 Chemoautotrophic Bacteria
Phototrophic and Respiring Bacteria
M-6 Anaerobic Phototrophic Bacteria --
Purple nonsulfur bacteria
green and purple sulfur bacteria
M-7 Cyanobacteria (Blue-green Algae)
PROTEROZOIC AEON (1500 - 580 MILLION YEARS AGO):
DEVELOPMENT OF THE EUKARYOTES
This is an important period because four kingdoms arose to accompany the prokaryotes. These other kingdoms include the protoctists, fungi, plants, and animals. It is difficult to tell exactly when each kingdom arose, but we can use fossil evidence to indicate the earliest date possible.
Kingdom Protoctista has twenty-seven phyla, and contains such familiar organisms as amoebas, kelp, and algae. In years past, these organisms would have been forced into the animal or plant kingdom, always accompanied by vigorous debates about the differences between plants and animals at this low level of evolution. The creation of five kingdoms has gone a long way to avoid these seemingly indeterminable debates.
The term protozoa is an old one that went back to the division of all organisms into plants and animals. Traditionally the protozoa are one-celled animals. These are now included in the kingdom protoctista. These organisms lack any organized nervous network. Therefore, they do not have a nervous system (Hickman et. al. 1990:242).
Around 1800 million years ago proto-eukaryotes first developed. An eukaryote is an organism that has membrane-bounded organelles in its cells. The developing eukaryotes may have obtained protomitochondria and protochloroplasts by actually incorporating entire, free-living organisms into their own bodies. This resulted in the establishment of a relationship wherein both organisms benefited. Kingdom Protoctista arose some 1500 million years ago with the oldest fossils going back to 1200 million years ago. This kingdom is defined by exclusion. It contains all members not classified as a member of one of the other kingdoms. Therefore, it constitutes a heterogeneous assemblage of unicellular, colonial, and multicellular eukaryotes. In this kingdom are found the algae (green, brown, and red algae, diatoms, euglenoids, etc.) and the slime molds. Some of the eukaryotes (the algae) were photosynthetic, and, in the long run, much more energy efficient than the cyanobacteria. Around one billion years ago single-celled organisms aggregated into small multicellular communities.
The ancestry of fungi is not well understood. This kingdom of five phyla is believed to have begun around 1200 million years ago. It is possible that the true fungi descended from conjugating protoctists, perhaps from the rhodophytes or the gamophytes. Kingdom Fungi contains both the mushrooms and bracket fungi. These organisms have traditionally been grouped with plants, but it is now overwhelmingly certain that they are an independent line. They do not have plastids or photosynthetic pigments, but rather absorb their nutrients from either dead or living organisms. It has been suggested that plant-fungus associations made it possible for plants to become truly terrestrial for the fungi could have transported nutrients to plants and prevented them from drying out (Margulis and Schwartz, 1988:155).
Plants include the ferns and fern allies that develop from spores and the seed plants which develop from seed rather than a spore. The seed plants are divided into gymnosperms and angiosperms. The gymnosperms contain the conifer plants that do not have what we commonly know as the flower. The conifers have cones or related reproductive systems rather than flowers. The angiosperms are the flowering plants.
Botanists divide the angiosperms into two large groupings: dicots and monocots. The dicots developed first, followed by the monocots. There are many differences between the two groupings. In the embryo, the dicotyledons have two cotyledons (i.e., the first leaves sent up by the germinating seed). The monocots have one cotyledon. The dicots are either herbaceous or woody. (An herb is an annual or perennial that dies completely or dies back to the ground at the end of the growing season because it lacks the firmness resulting from secondary growth.) Since the dicots dominated the land, many of the monocots became small and took to the water. They often dominated the water's around lakes and ponds. Dicots usually have annual growth rings that can be used to determine the age of dicot trees. Monocot trees, such as palms, are fibrous, not woody, and have a soft inner part. There are no annual growth rings. Monocot leaves have veins that are usually parallel to each other, and their flower parts, such as the sepals and petals, usually occur in threes or multiples of threes.
The most primitive flowers are those belonging to the magnolia subclass. Some of the orders in the Magnolia subclass are magnolia, laurel, water lily, butter cup, and poppy. These flowers developed in association with the long-established beetles. Beetles were plentiful when the proangiosperms appeared and may have been the first animal pollinators. The accidental beetle pollination provided a more efficient method of pollination than chance pollination by wind. The insects which we usually think of as pollinators, had not yet appeared when the first angiosperms evolved. For instance, butterflies and moths did not develop until 70 million years ago.
A major theme of this book is that life is evolving chemistry. As evolution continued, life forms became even more complicated chemically. Many of the chemical changes for animals also occurred. This especially took places in the evolution of neurotransmitters that accompanied the development of the nervous system. This particular chapter section deals with the chemical bases of the behavior of plants. The emphasis is on the impact of hormones on plant life.
Hormones and other chemicals control the plant's every activity. These chemicals in turn are regulated by the plant's genetic program, which establishes the limits within which the organism develops. In its nucleus, every living plant cell has its own copy of the complete genetic program.
Different regions, tissues, and organs produce various plant hormones with no central control. The hormones direct the plant system by directing the "attention" of the cell to specific portions of the genetic program that need to be turned on or off at that particular time in order for development to proceed correctly.
The amount and kinds of hormones produced depends on the interaction with environmental conditions (many of which are also chemical in nature). In other words, the physiological condition of the plant (such as the amount of sugar, water, and minerals reaching each hormone-producing cell) affects the amount of production of a given hormone. The amount of hormone, in turn, affects the amounts of other hormones produced.
There are many different hormones and chemicals that work to control a plant's life. Among the most important hormones are the auxins, gibberellins, cytokinins, abscisic acid, ethylene, and the not yet discovered, but already named, florigen. Each of these chemicals will be discussed in terms of how they affect the life cycle of a flowering plant in the temperate zone of hot summers and cold winters.
A flowering plant usually starts its life as a seed. The healthier seeds germinate in the spring as the night length lessens. The plant knows that the night is lessening because the pigment phytochrome measures night length and thus functions as a growth regulator. This chemical is a protein that exists in two interconvertible forms: one absorbs red light and the other absorbs far-red light. At night, the far-red absorbing form spontaneously reverts to the red-absorbing form. The amount of the far-red type converted to the red-type makes it possible for the plant to "measure" the night length. So, in a sense, phytochrome lets the seed "know" if it is spring or not.
For development, the seed has to break its dormancy and start using its food reserves. To begin growing, the seed breaks dormancy through the release of ethylene, the only plant hormone that is a gas. Also involved in breaking dormancy is gibberellic acid. This hormone starts the consumption of the seed's stored food.
The growing seed sends up its seed leaves and starts it roots on a downward growth pattern. But how does a seed know which way is up and which way is down? The growing plant orients itself to the earth, a process known as geotropism. Some physiologists think that plants sense gravity through statoliths, which are particles that are heavy enough to sink to the bottom of the plant cells. Through this process, the plant can orient itself and
direct auxin transport to the necessary places. Auxin causes greater growth on the side where it is concentrated. For example, greater concentration of auxins on the lower side of the stem causes faster growth in this section resulting in turning the tip of the stem upward.
In order to take maximum advantage of the sun, the growing plant needs to orient its seed leaves towards the sun, a process known as phototropism. Phototropism depends on a pigment (a flavoprotein) that absorbs blue and violet light. On the side facing the sunlight there is more activation of the flavoprotein. This causes more auxin to flow to the lighted side and, hence, the plant grows toward the sun.
Plants need energy for growth. This energy comes from the crucial process of photosynthesis. This in turn is regulated by pigments that absorb light. The most important pigment is chlorophyll "a" which has a highly, light-absorbing chemical. Many accessory pigments, such as chlorophyll "b" and carotenoids, absorb light and pass the energy to chlorophyll a. The plant continues to grow and sends out many leaves in order to capture more sunlight. Cytokinins prevent early senescence (aging) of leaves. These hormones are also responsible for the production of leaf galls in response to insect invasions. (Insects induce leaf galls by such activities as laying their eggs on leaves.)
In order to reproduce, the flowering plant produces elaborate reproductive structures known as flowers. These are designed to appeal to various animal pollinators. Flowering involves processes deeply connected with hormones. Florigen, the universal flowering hormone, has yet to be discovered.
Chemicals also help insure that a plant will breed only with another of its species. For instance, there are chemicals in pollen grains that interact with chemicals in the stigma (that, is the head of the ovary tube) to insure that flowers accept the right pollen grains and reject the wrong ones.
After fertilization, hormones produced by the embryo seed cause the ovary to enlarge and develop into fruit. Cytokinins cause the plant to translocate nutrients to the developing seed, thereby building up the food reserves necessary in spring for seed germination.
Phytochrome measures the coming of fall. As fall appears, fruits start to ripen. Ethylene gas produced by one fruit actually speeds up the ripening of adjacent fruits. This gas also works in conjunction with abscisic acid to cause the formation of a layer (known as the abscission layer) in the fruit stalk that will cause the ripened fruit to fall from the tree.
As autumn proceeds the leaves begin to fall. Plants shed their leaves so that during winter they do not lose water through these structures. Cytokinins promote cell division in mature cells, while ethylene controls leaf aging or senescence. At the same time the plant produces abscisic acid and reduces auxin synthesis, thereby inducing dormancy in stem tissues and buds. Abscisic acid promotes the formation of the abscission layer in the leaf stalk that blocks the transmission of water to the leaves, thereby causing the leaves to fall. Now the flowering plant is ready for winter. With the coming of spring, the cycle starts all over again.
The fifth kingdom, that of the animals, will be discussed in the next chapter.
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