Plant Classification (Biol 1300 Unit 1) PDF

**Biol 1300 Unit 1** **PLANT CLASSIFICATION** **HISTORY OF PLANT CLASSIFICATION** Organisms have been classified since Ancient Greek times, if not earlier. To develop a workable classification system, two key questions must be addressed: 1. How similar must entities be to be grouped together?...

**Biol 1300 Unit 1** **PLANT CLASSIFICATION** **HISTORY OF PLANT CLASSIFICATION** Organisms have been classified since Ancient Greek times, if not earlier. To develop a workable classification system, two key questions must be addressed: 1. How similar must entities be to be grouped together? 2. How should similarity be defined? Classification aims to define natural groupings, name these units, and organize them into an ordered system. Early systems were "artificial," based on similarities not necessarily related to evolutionary relationships, such as grouping root crops or plants believed to cure headaches. They focused on appearance and medicinal properties. **Theophrastus**, a disciple of Aristotle, created the first plant classification system around 2300 years ago. He identified key characteristics to distinguish natural plant groups, recognizing families like Pea, Grass, Sunflower, and Mustard. His works were used until the 17th century in folk taxonomies. **The Age of Herbals** (1470-1670), is a time where these early works were expanded, following the invention of the printing press. Herbal manuscripts listed illustrations of plants and their medicinal properties, using common names. However, common names posed problems: the same name could refer to different species, or one species could have multiple names, and different languages like Greek or Latin often used different common names. Due to communication problems with common names, a standard, **Latin-based system of nomenclature** was developed. Initially, a polynomial nomenclature was used, where a generic designation was followed by a descriptive Latin "phrase name." This system was cumbersome, up to a paragraph in length and later simplified by Carl Linnaeus \[Uppsala University, Sweden\]. His **binomial system**, introduced in *Species Plantarum* (1753), is still used today. In this system, the species name consists of a generic name and a species epithet, such as *Picea glauca* for white spruce, Manitoba\'s provincial tree (\"*Picea*\" is the generic name, \"*glauca*\" the species epithet). The system follows rules from the International Code of Botanical Nomenclature, including capitalization, italicization, and the use of authority designations: The first letter of the generic name is capitalized, but not the species epithet. The binomial name is italicized (or underlined). The species name consists of both parts; all members of a genus can be referred to (e.g. Picea, when referring to all spruces). If a name is used several times in a manuscript, a \"short form\" of the generic name (P. glauca) is generally used. An \"authority\" designation often follows the binomial name, e.g. *Picea glauca* (Mo-ench) Voss; this summarizes the nomenclatural history of the scientific name. **PLANT SPECIES** Plant classification is based on the species concept. A **biological species** is defined as a group of individuals that can interbreed with one another, but not with individuals of another species. Otherwise, offspring will not be fertile. Several problems may arise when applying this definition to plants: It is impossible to apply this definition in practice. Spatially separated populations of the same species may not easily interbreed. Some plant species may form fertile hybrids when crossed. Conversely, some species never interbreed with one another (e.g. dandelion which use agamospermy, a form of asexual reproduction). This suggests that plants do not need to interbreed, undermining the biological species concept. Plant taxonomists therefore use morphological form to classify and describe species (morphology in biology is the study of the form, structure, and size of organisms, including animals, plants, and microorganisms). But even this is problematic: Large changes in morphology may result from simple genetic changes. Plants are notoriously \"plastic\"; their morphology depends on the environmental conditions under which they are grown. This includes light levels (influence height and overall growth), touch (which disrupts growth), and so on. Examples: - potentilla plants are particularly sensitive to light - Fruits and vegetables are also very different domestically than in the wild. - Ranunculus aquatilis influenced by water exposure Evolution is constantly altering morphological form. - Rafflesia, with the world′s largest flower, has evolved to produce a \"corpse\" smell to attract insects - South African Parasitic Plant has evolved parasitic tendencies to siphon nutrients from other plants - Titan Arum, is very tall flowering plant, up to 3 meters - Duckweed is an invasive species from Asia, looks like little green \"specks.\" It is this plant's design that has enabled its spread. It also has the world′s smallest fruit. - Victoria Water Lily, has maximized its surface area to allow light absorption - Baobab trees in Madagascar have evolved to have large, thick trunks - Socotra Dragon Trees are Sub-Saharan and can grow up to 12 meters - Giant Senecio grow on Mt. Kilimanjaro and have evolved to survive in foggy weather - Azarella compacta look like a cushion and are in the South American Andes - Colocynth lives in North African deserts like Libya - All these species reflect the diversity in morphology that evolution yields **EVOLUTIONARY RELATIONSHIPS OF FLOWERING PLANTS** Morphologically based classifications are known as phenetic classification systems. In this approach, many characters are used, and conservative characters (i.e. characters that do not vary with environmental conditions) are emphasized. Thus, highly variable or \"plastic\" plant characteristics such as leaf size and shape are not used; more morphologically conservative floral characters are generally favoured. The publication of Charles Darwin\'s *Origin of Species* (1859) increased interest in phylogenetic approach to classification, in which evolutionary relationships are emphasized. Implementation of a phylogenetic classification requires: Information on the ancestral relationships of flowering plants. Plants are poorly preserved in the fossil record, but recent advances in molecular biology have greatly increased our knowledge of the phylogenetic relationships of plants. Deciding which characters are \"primitive\" (evolved early), and which are \"advanced\" (evolved more recently). The lack of a good fossil record makes this task difficult, but advances in molecular biology in the early 21st century have provided invaluable information on evolutionary relationships within the plant kingdom. In the 19th century two competing theories regarding flowering plant evolution were given: ENGLER, Adolf (Germany, 1844-1930): the earliest evolved flowers were small, structurally simple, and wind-pollinated (e.g. willow and poplar flowers). HOOKER, Joseph D. (England, 1817-1911): The earliest evolved flowers were large with numerous floral parts, i.e. sepals, petals, stamens and pistils (e.g. magnolia and buttercup flowers). This theory hypothesizes that evolution resulted in: (a) reduction in the number of floral parts; (b) fusion of petals, and development of an irregular corolla; (c) separation of the sexes (unisexual plants or flowers). The Hooker system (slightly modified by Bessey, Cronquist and others) is accepted today; between 300 and 350 flowering plant families are recognized. Molecular biology studies of plant genomes have largely confirmed the evolutionary validity of the Hooker-Bessey-Cronquist system, although many refinements have been made. **FLOWERING PLANTS: MONOCOTS AND EUDICOTS** Traditional plant classification recognizes two major flowering plant (Angiosperm) groups: the monocotyledons (or monocots) and the eudicotyledons (or eudicots). This division is based on the number of cotyledons (\"seed leaves,\" the embryonic leaf in seed-bearing plants, one or more of which are the first to appear from a germinating seed) found in the seed: monocots have a single cotyledon, whereas eudicots have two cotyledons. Several additional morphological features distinguish these two groups (although there are exceptions): MONOCOTS Plant Form: Herbaceous, never woody (some are \"tree-like\") Floral Parts: Three or multiples \[3,6,9..\] Leaves: Linear, sheathing the base, parallel-veined. Root System: Primary root is short-lived, fibrous root system developed. EUDICOTS Herbaceous or woody (\"true\" trees and shrubs). Four, five or multiples \[4,8,10..\] Broad, not sheathing, net veined. Primary root often persists, forming a taproot. A group of flowers and leaves Description automatically generated Recent genetic investigations have revealed that, from an evolutionary perspective, the division of the flowering plants into monocot and eudicot groups is too simplistic. A third group, known as the magnoliid Angiosperms, evolved prior to the divergence of the monocots and eudicots. There are five economically important plant families in the magnoliid group: Magnolia (Magnoliaceae), which includes a number of horticultural and timber species; Laurel (Lauraceae), which includes cinnamon, avocado, bay leaves, and some timber species; Soursop (Annonaceae), which includes a number of tropical fruits such as soursop, custard apple and cherimoya; Pepper (Piperaceae), which includes black pepper; and Nutmeg (Myristicaceae), which includes nutmeg and numerous tropical timber species. The Soursop, Pepper and Nutmeg families are restricted to tropical regions.