MODERN BOTANY - P. RAVEN - 1990

SECTION IV. DIVERSITY

CHAPTER 10. CLASSIFICATION OF LIVING ORGANISMS

Major Groups of Organisms

In Linnaeus's time, all organisms were considered to be either plants or animals. Animals grew only until adulthood, were motile, ingested food, and respired. Plants were non-motile, and it was believed that they did not respire or ingest food, and could grow indefinitely.

As new groups of organisms were discovered, they were classified as either plants or animals. Thus, Fungi and Bacteria came to be considered plants, and Protozoa — animals. Eventually, organisms similar to *Chlamydomonas*, a motile green alga that moves about yet produces its own food, were discovered. Such creatures are difficult to classify as either animals or plants, and by the 1930s the traditional division of living things into two kingdoms had practically become a historical curiosity.

Class="center">Fig. 10-6. A. The lichen *Pseudocephalaria atuhraspis* on a tree trunk. B. A hoverfly (*Syrphus*) killed by an infection of the zygomycete fungus *Empusa muscae*. C. A coral fungus of the family Clavariaceae. D. An agaric fungus of the genus *Marasmius* with dew drops, growing in a rainforest in Peru. E. The puffball *Scleroderma aurantium*

Fig. 10-7. Plants. A. The peat moss *Sphagnum* (Division Bryophyta) forms extensive bogs in cold and temperate Regions of the globe. B. *Marchantia*, the best-known thalloid liverwort of the same division; a widely distributed genus growing on damp soil and rocks. C. The running clubmoss, *Lycopodium digitatum* (division Lycophyla). D. The field horsetail, *Equisetum sylvaticum* (division Sphenophyla). E. The bulbiferous fragile fern, *Cystopteris bulbifera* (division Pterophyta). F. The dandelion *Taraxacum officinale* and G — the cactus *Mammillaria microcarpa* from the dicotyledons (class Dicotyledones, division Anthophyta). H. The maned barley (*Hordeum jubatum*) and I — an orchid of the genus *Cymbidium* representing monocotyledons (class Monocotyledones, division Anthophyta). J. The sugar pine (*Pinus lambertiana*) and the incense cedar (*Calocedrus decurrens*) belong to conifers (division Coniferophyta)

Unfortunately, no universally acceptable alternative has been proposed, and scientists have not reached a consensus on how many kingdoms to recognize and which organisms to include in each kingdom. Furthermore, the old division into plants and animals is still widely reflected in the Organization of academic departments, research projects, and textbook structures (including the one you are reading). All groups traditionally classified as plants or studied in botany courses are covered in this book.

Prokaryotes

As discussed in Chapter 2, it is now evident that the most profound boundary in the living world is between PROKARYOTES AND EUKARYOTES (Fig. 10-8). Prokaryotes, or bacteria, include many subgroups, among them cyanobacteria (formerly known as "blue-green Algae"). They constitute the KINGDOM MONERA and differ from eukaryotes far more than plants and animals differ from each other.

Fig. 10-8. Electron micrographs. A. The prokaryotic cyanobacterium *Anabaena*. B. A Introduction/5.html">Eukaryotic Cell from a sugar beet (*Beta vulgaris*) leaf. Note the more complex Structure of The Eukaryotic Cell, featuring a prominent Nucleus and METABOLISM/14.html">Chloroplasts

Bacteria (discussed in detail in Chapter 11) lack membrane-bounded cell Organelles, microtubules, or complex flagella with the characteristic (9+2) structure typical of eukaryotes. Their genetic material is concentrated in a single circular DNA molecule not associated with Proteins. Although some Mechanisms of Genetic recombination are known in bacteria, it occurs infrequently, and when it does, it proceeds without sexual processes. Bacteria also differ biochemically from eukaryotes in many respects.

Table 10-1. Biological Classification. Note how much information can be conveyed about an Organism when its position in the system is known. The descriptions provided here are not exhaustive Definitions of the various categories, but rather give some insight into them

Viruses

As will be shown in Chapter 12, viruses are segments of DNA or RNA that have acquired The ability to utilize the machinery of other Cells for self-Replication. They form their own protein coat, which protects them from destruction when moving from one host to another.

Eukaryotes

All eukaryotes possess a nucleus clearly demarcated by a double membrane (the nuclear envelope). Inside, there are complex Chromosomes in which DNA is bound to proteins. These chromosomes undergo regular mitotic division. Eukaryotic flagella and cilia feature a characteristic (9+2) microtubule structure (described in Chapter 2). The latter are also found in the Cytoplasm, which is intricately compartmentalized by membranes. Eukaryotic cells of all types contain organelles, including Mitochondria. In eukaryotic organisms, particularly plants, single-membrane-bound vacuoles, or tonoplasts, are also frequently present.

In addition, many eukaryotes are characterized by two key features absent in bacteria: integrated multicellularity and sexual reproduction. Although bacterial cells sometimes remain attached after division to form filamentous or even three-dimensional aggregates, protoplasmic connections between individual cells are typically lacking in such cases; consequently, it is not possible to speak of emergent Properties of the filament or mass as a whole. In plants, the protoplasts of adjacent cells are connected by plasmodesmata piercing The Cell walls. Animals lack cell walls, and their cells are separated primarily by their Plasma Membranes.