Lower groups

The lower groups of organisms, that is, those which reproduce without the means of seeds, include a wide range of species, from the simplest unicellular bacterium to complex algae and fungi. In modern classification, these groups of organisms are not considered plants. Most of the groups have little more than this one reproductive feature in common. Instead of seeds they employ such diverse methods of reproduction as binary fission,

conjugation, and spore formation. But despite the success and reduced risks of asexual and nonspermatophytic reproduction, most plants are spermatophytes.

The primary division of living organisms in modern classification systems is into prokaryotes and eukaryotes. Prokaryotes have relatively simple cells without true nuclei and other cell bodies, whereas eukaryotes have complex ceils with nuclei and organelles, such as mitochondria and Golgi bodies. The prokaryotes consist of bacteria and blue-green algae; eukaryotes comprise all other cellular organisms. Blue-green algae are the earliest identifiable living organisms, having been found in schist that is about 3.5 billion years old.

Bacilli are rod-shaped bacteria, shown here (left) magnified 700 times; each individual microorganism is only about 5 micrometers long. The diagram (right) illustrates the main features of a generalized bacterium. Not all bacteria have flagella, but in those that do it is the whiplike lashing of this appendage that propels the organism.

Bacteria

Most bacteria (class Schizomycetes) are unicellular microscopic organisms. Almost all of them are heterotrophic, which means that they feed off living and decaying organisms as parasites and saprophytes, rather than autotrophic, which means that they manufacture their own food by the process of photosynthesis, as many other plants do. They occur in every possible environment and in large quantities—one drop of liquid can contain 50 million bacteria, one ounce of average soil contains more than 30 billion.

Heterotrophic bacteria include the parasitic pathogens that cause cholera, syphilis, botulism, and many other diseases. The pathogens

are, however, outnumbered by the bacteria that are beneficial to us. Most nonpathogenic bacteria are saprophytes and are responsible for the decomposition of dead plants and animals, restoring essential mineral elements to the ecosystem, and preventing waste accumulation and pollution. Some live inside our bodies and assist digestion. Others are used in such industrial processes as the fermentation of alcohol to acetic acid in vinegar production and the making of yogurt and cheese.

Some saprophytic bacteria play an important role in the production of nitrogen. In well-aerated soil that is not very acid, bacteria such as Nitrosomonas convert ammonium to nitrite, which is itself converted by another bacterium, Nitrobacter, to nitrate. Nitrate is taken up by plant roots more easily than is ammonium. Nitrogen is also fixed in the root nodules of certain plants by the bacterium Rhizobium, which is of vital importance to agriculture. Other bacteria, such as Pseudomonas, reduce the nitrates in the soil to nitrogen and nitrous oxide, which are released into the atmosphere.

Autotrophic bacteria include those that photosynthesize, using light to split hydrogen sulfide rather than water. These bacteria, examples of which are Chlorobium and Rho-domicrobium, do not evolve oxygen as other photosynthetic organisms do. Other autotrophic bacteria are chemosynthetic; they are found, for example, near submarine fumaroles on the Mid-Atlantic Ridge and in areas of the Pacific Ocean Ridge near the Galapagos Islands, where they oxidize hydrogen sulfide to obtain energy.

Bacteria are usually identified and classified by cell shape, size, grouping, and flagellar arrangement. Streptococci are spherical and occur in groups; Bacillus can occur singly or in chains of rodlike cells; Spirillum is found singly and, as its name suggests, is spiral in shape.

Bacterial cell structure

A generalized bacterial cell consists of a cell bounded by a cell wall and moved by the whiplike action of flagella. The cell wall contains polysaccharides, proteins, and lipids, but not cellulose. Gram-positive bacteria (so-called because they retain the purple color of Gram’s stain) have weak walls with few amino acids. Examples are Staphylococcus (which causes boils) and Streptococcus (which, among other things, causes sore throats). They are, therefore, sensitive to certain antibiotics such as penicillin. Fortunately, most pathogens are of this type. Gram-negative forms have strong walls—they include Salmonella (a species of which causes typhoid) and Escherichia (the source of gastroenteritis).

Inside the cell, the hereditary material consists of a single loop molecule of DNA (the genophore) attached to the cell membrane, but not bounded by an envelope to form a nucleus, as in eucaryotes. Other tiny DNA fragments known as plasmids also occur in the cell and can become incorporated into the genophore when they are concerned with sexual conjugation. Plasmids can be used to transfer genes from one species to another (even from a bacterium to a higher plant or animal cell) for genetic engineering. The cytoplasm contains ribosomes for protein synthesis, and storage inclusions. These organelles are much smaller than those of eukaryotic cells. Prokaryotes also do not contain chloroplasts, as do most other plant cells.

Cell division is another method of bacterial reproduction. The organism’s DNA consists of a closed chromosome loop anchored to the cell wall (A). Replication of the DNA (B) results in two loops (C). The cell then elongates at the middle (D) until the loops are separated (E), and then forms a “neck” (F), which eventually divides the original cell into two new ones (G).

Bacterial reproduction

Reproduction in bacteria takes various forms. Some develop into endospores, which contain a “resting” cell called a spore. Most spores can witth stand extreme changes in their environment, such as heating, freezing, or drying. They germinate when conditions again become favorable.

Some bacteria multiply by very rapid cell fission (Pseudomonas, for example)—one fission can take place every 20 minutes. Others, such as Clostridium, produce dormant survival spores called endospores, which resist boiling and can cause food poisoning, such as botulism. Sexual reproduction in the form of conjugation is also exhibited by some bacteria. In this process, DNA passes in one direction from one bacterial cell to another via filamentous structures on the cell wall called pili. Foreign DNA can also enter a cell from the environment (transformation) or when transferred by a bacterial virus (transduction).

Euglena spirogyra is a photosynthetic microorganism, which propels itself using a long flagellum (at left side of the cell in this photograph). This organism, whose characteristics lie between those of plant and animal, swims toward light, which it detects with a photoreceptor located near the base of the flagellum.

Cyanobacteria

The cyanobacteria, formally called blue-green algae (class Schizophyceae), are a successful and ubiquitous group of photosynthetic procaryotes. They occur in all freshwater habitats and in the sea, in the soil, as slime and gelatinous growths on rocks and manmade surfaces, as the algal partner in some lichens, and in extreme environments such as hot springs with temperatures up to 185° F. (85° C).

The cyanobacterial cell is typically prokaryotic; it differs, however, from a true bacterial cell in that photo synthetic pigments occur on the internal membranes, although they are not delimited into chlorophyll a (which is green), the yellow xanthophylls, blue phycocy-anin, and red phycoetythrin. Different combinations of these pigments result in organisms that are blue-green, blue, black, purple, brown, red, and yellow.

Cyanobacteria usually form groups of spherical orcoccoid cells, or filaments. Some filaments have complex cell types and specialized morphology, such as branching, aggregation into colonies, and three-dimensional cell division to produce simple tissues. The three main cell types are vegetative cells, akinetes, and heterocysts. Vegetative cells multiply by fission, and no sexual process is known. Akinetes are thick-walled resting spores. Fletero-cysts are concerned with nitrogen-fixation, a procaryotic attribute of vital ecological and economic importance; for example, the fertility of rice paddyfields depends upon the fixation of nitrogen by cyanobacteria rather than the application of nitrogenous fertilizers.

A recently discovered prokaryotic alga is Prochloron, which lives symbiotically inside marine tunicates (animals like the sea-squirt) on reefs. Prochloron1 s cell structure is similar to that of a cyanobacterium, but its photosynthetic pigments include chlorophyll b as in green algae and higher plants. Prochloron could, therefore, be a key organism in the evolutionary history of land plants.

Rockweed (Fucus) is a common seaweed that grows on the shore between high- and low-water marks. The leathery leaflike blades (below) contain small cavities, in which antheridia produce male sperm and oogonia produce female eggs (above). As these sex cells are released into the water, the flagellate sperm swim to the eggs and combine with them to form zygotes, from which new mature plants grow.

Eukaryotic algae

Algae form a heterogeneous collection of photosynthetic lower organisms ranging from single cells, multicellular colonies, and filaments to highly organized plant bodies such as seaweeds. Algae are mainly aquatic but also occur on soil, moist rocks, trees, and snow; they also occur as epiphytes and zoophytes.

In most species cellulose is contained in the cell walls. All species have no true vascular system, little tissue differentiation, and no morphological parts, such as stems, roots, or leaves. The giant seaweeds, such as giant kelp Macrocystis sp.) have trumpet cells, which form tubes down the stems that transport synthesized sugar alcohols and other foods, much like the phloem system of higher plants.

The sexual reproduction of algae is unique in that the whole organism may form the gamete, or the gametes are reproduced in uni- or multicellular gametangia in which every cell becomes a gamete. The number of algal divisions varies depending on the classification system used, but in all there are at least six divisions. Among these groups, the green algae (Chlorophyta) contain motile single cells with two or four equal smooth flagella (Chlamydo-monas, for example), colonial forms such as Volvox, microscopic filaments such as Spiro-gyra, and macroscopic seaweeds like Ulva, the sea lettuce. The pigments of this group are identical to those of mosses, ferns, and flowering plants—not surprisingly, because the land plants are believed to have evolved from the green algae. Sexual reproduction in chloro-phytes varies from simple fusion of identical swimming gametes (isogamy) to advanced systems involving eggs and sperm (oogamy). Several genera have complex life cycles involving alternation of generations and are the forerunners of such systems in land plants.

The chrysomonads of the division Chryso-phyta are golden-brown algae. These cells each have two unequal flagella (one with stiff hairs, the other smooth and bearing a photoreceptor), and an eyespot within a chloroplast. This arrangement is called heterokont organization. The cells are naked or are covered in siliceous scales or live inside cases (loricas) of cellulose or chitin. In many species the cells join together to form elaborate spherical or dendroid motile colonies.

In contrast, Prymnesiophytes, which are also members of the division, have two equal smooth flagella and a unique third coiling appendage (the haptonema), which may function as a chemoreceptor; the cells are covered with delicate organic scales that may become calcified to form disks and spines. Vast deposits of such flagellates in prehistoric seas have resulted in the formation of deep layers of chalk as in the White Cliffs near Dover, England, and on the French coast of the English Channel, as well as the vast Niobrara Chalk formations of Kansas.

Diatoms (Chrysophyta) are characterized by a boxlike covering composed of richly patterned silica; they are either elongate or circular, and the ceils may be single or joined in chains, stars, or zigzags. Fossil deposits of diatom skeletons (diatomaceous earth) form the basis of household scouring powders and toothpaste, as well as industrial filters and insulation material. One particular type— kieselguhr—absorbs about three times its own weight of the explosive nitroglycerin to form the more easily handled explosive dynamite.

Dinoflagellates (Pyrrophyta) are brown and have a complex symmetry and an armored cuticle (periplast) of cellulose plates; some are bioluminescent and give off flashes of light when disturbed. Conyaulax, for example, blooms to produce red tides; shellfish that eat these blooms accumulate a nerve toxin, which can be lethal to anyone who eats it.

Most of the other main algal divisions consist of the macroscopic thalli—the seaweeds. The red algae (Rhodophyta), which are feathery and leafy seaweeds, have no flagella or motile stages and only one chlorophyll pigment, a. They have, however, a predominance of the red pigment, phycoerythrin. The brown algae (Phaeophyta) comprise the bulk of the intertidal and coastal seaweeds, including the wrack Fucus and the giant kelps. Most of the vast array of species of brown and red seaweeds have complicated life cycles involving two or three phases.

Many of these algal species constitute the marine and freshwater phytoplankton that serve as a primary food source for zooplankton and larger filter-feeders, such as sponges, mollusks, and certain fish. Their potential and value in food and energy production is tremendous. Together the algal groups, and phytoplankton in particular, are responsible for almost half of the annual global fixation of carbon (estimated at about 23 X 109 tons of carbon per year).

Algae are not very important economically, compared with the higher plants. Certain kinds are eaten as food, especially in Wales where it is called laver bread, and in Japan, where nori, as it is called, is specially cultivated, dried, and eaten wrapped around rice. However, red algae provide agar and brown algae produce alginates. These jellylike substances are extracted and used in food canning; for making emulsions for use in paint, cosmetics, ice cream, medicines, and photographic film; and as a medium for growing microorganisms in the laboratory.

Nostoc is a cyanobacterium, here enlarged about 200 times, whose filamentous strands are held in fluid-filled sacs. It belongs to the group called Cyano-phyta and, despite its name, may be black or red, as well as blue or green.