Plant Systematics

An Integrated Approach, Fourth Edition

Gurcharan Singh

• Publisher: CRC Press
• Year: 2019
• ISBN:
  9781000586756
• Edition: 4
• Topic: Scienze biologiche
• Subtopic: Botanica

This fourth edition of Plant Systematics is completely revised and updated. It incorporates the updated International Code of Nomenclature for Algae, Fungi and Plants (Shenzhen Code, 2018), the new version of PhyloCode (Beta version of Phylocode 5, 2014), APweb version 14 (September, 2018), revised Angiosperm Phylogeny Group classification (APG IV, 2016), new Pteridophyte Phylogeny Group Classification (PPG I, 2016), besides the updates since the publication of third edition.

The book is a blend of classical fundamental aspects and recent developments, especially in the field of molecular systematics, cladistics and computer identification. Special attention has been given to information on botanical nomenclature, identification, molecular systematics and phylogeny of angiosperms. The complicated concepts of phylogeny, taxometrics and cladistics have been explained with a view to providing a comparison between these diverse but interactive fields of study. An attempt has been made to build upon a common example when exploring different methods, especially in procedures of identification, taxometrics and cladistics. The major systems of classification are evaluated critically. Discussion on major families of Pteridophytes, Gymnosperms and Angiosperms, especially those of major phylogenetic interest, form a major portion of this edition. The ebook includes nearly 500 color photographs set out in 36 pages covering plants from different parts of the world. In addition, 305 black & white illustrations have been included to provide a better understanding of the plants covered in the book.

Chapter 1

Plants, Taxonomy and Systematics

Taxonomy (or systematics) is basically concerned with the classification of organisms. Living organisms are placed in groups based on similarities and differences at the organismic, cellular, and molecular levels. Described species of organisms on earth were estimated between 1.4 million and 1.8 million (Stork, 1988; Barnes, 1989; Hammond, 1992). Wilson (1992) estimated that roughly 1.5 million species of described species comprised 73.1% Animals, 17.6% Plants, 4.9% Fungi, 4.1% protists and only 0.4% Bacteria. Hammond (1992) proposed a working estimate of 12.5 million species organisms on earth consisting of 9.6 m animals (1.2 m described), 300,000 plants (250,000 described), 990,000 Fungi (80,000 described), 400,000 protists (90,000 described) and nearly 400,000 bacteria (only 6000 described).

The United Nations Environment Programme’s Global Biodiversity Assessment published by Convention on Biological Diversity (2001) estimates the number of described species of living organisms as approximately 1.75 million, slightly more than ten percent of estimated 14 million species. Whereas nearly 90 percent of estimated plant species have already been described (270,000 of 300,000), only 12.5 percent of animals (1.3 m of 10.6 m), 4.7 percent of Fungi (70,000 of 1.5 m), 13 percent of Protoctists (80,000 of 600,00) and only 0.4 percent of Bacteria (4,000 of 1 m) have been described so far. The list grows every year. Subsequent publication by Chapman (2009) puts figures of described plant species as 310,129 (of 390,800 estimated) that includes 268,600 described angiosperms (estimated 352,000). The more recent estimates, however, put figures of flowering plants only as around 400,000 (Edwards, 2010 on Physorg.com). Best estimate by Census of Marine life, published by Mora et al. (2011) in PLoS Biology puts total number of Eukaryotic species on earth as 8.7 million, with 6.5 million species on land and 2.2 million in oceans, as against the earlier rough estimates between 3 million and 100 million. These 8.7 m species comprised 7.77 m animals (954,434 described), 611,000 Fungi (43,271 described), 298,000 plants (215,644 described), 36,400 Protozoa or protists (8,118 described) and 10,358 bacteria (described, predicted 9680). Only 1.2 m species of 8.7 m estimated have been described and catalogued. Furthermore, the study says a staggering 86% of all species on land and 91% of those in the seas have yet to be discovered, described and catalogued.

Pappas (2016), concluded that there may be 100 million to even 1 trillion species on this planet, based on scaling rules that linked the number of individual organisms to the number of total species. The list grows longer every year.

Christenhusz and Byng (2016) who have compiled a list of estimated number of genera and species in families recognized mainly in APG IV (2016) and other recent classifications estimate described and accepted number of plant species as ca 374,000, of which approximately 308,312 are vascular plants, with 295,383 flowering plants. Global numbers of smaller plant groups are as follows: algae ca 44,000, liverworts ca 9,000, hornworts ca 225, mosses 12,700, lycopods 1,290, ferns 10,560 and gymnosperms 1,079.

Most recent study by Brenden et al. (2017) estimates suggest that there are likely to be at least 1 to 6 billion species on Earth, and in contrast to previous estimates, rather than being dominated by insects, the new Pie of Life is dominated by bacteria (approximately 78%), protists 7.3%, plants 0.02%, Fungi 7.4%, and animals 7.3% (Figure 1.1). Classifying these organisms has been a major challenge, and the last few decades have seen a lot of realignments as additional ultrastructural and molecular information piles up. These realignments have primarily been the result of realization that the branches of the phylogenetic tree must be based on the concept of monophyly, and each taxonomic group, kingdoms included, should be monophyletic. Unfortunately, only a fraction of these estimates, just 1.5 million species have been described.

Before attempting to classify the various organisms, it is necessary to identify and name them. A group of individuals, unique in several respects, is given a unique binomial, and is recognized as a species. These species are grouped into taxonomic groups, which are successively assigned the ranks of genera, families, orders, and the process continues till all the species have been arranged (classified) under a single largest, most inclusive group. Classifying organisms and diverse forms of life is challenging task before the biologists.

Figure 1.1: Estimated number of species on earth and the described number of species (solid lower portion of bar) in major groups of organisms. The estimates in recent years have shifted from Animals to Bacteria, Fungi and Protists as dominant groups.

PLANTS AND KINGDOMS OF LIFE

Plants are man’s prime companions in this universe, being the source of food and energy, shelter and clothing, drugs and beverages, oxygen and aesthetic environment, and as such they have been the dominant component of his taxonomic activity through the ages. Before attempting to explore the diversity of plant life it is essential to understand as to what our understanding of the term Plant is, and the position of plants in the web of life. Traditionally the plants are delimited as organisms possessing cell wall, capable of photosynthesis, producing spores and having sedentary life. A lot of rethinking has resulted in several different interpretations of the term plant.

Two Kingdom System

The living organisms were originally grouped into two kingdoms. Aristotle divided all living things between plants, which generally do not move or have sensory organs, and animals. Linnaeus in his Systema naturae published in 1735 placed them under Animalia (Animals) and Vegetabilia (Plants) as two distinct kingdoms (Linnaeus placed minerals in the third kingdom Mineralia). Linnaeus divided each kingdom into classes, later grouped into phyla for animals and divisions for plants. When single-celled organisms were first discovered, they were split between the two kingdoms: mobile forms in the animal phylum Protozoa, and colored algae and bacteria in the plant division Thallophyta or Protophyta. As a result, Ernst Haeckel (1866) suggested creating a third kingdom Protista for them, although this was not very popular until relatively recently (sometimes also known as Protoctista). Haeckel recognized three kingdoms: Protista, Plantae and Animalia.

Two Empires Three Kingdoms

The subsequent discovery that bacteria are radically different from other organisms in lacking a nucleus, led Chatton (1937) to propose a division of life into two empires: organisms with a nucleus in Eukaryota and organisms without in Prokaryota. Prokaryotes do not have a nucleus, mitochondria or any other membrane bound organelles. In other words, neither their DNA nor any other of their metabolic functions are collected together in a discrete membrane enclosed area. Instead everything is openly accessible within the cell, though some bacteria have internal membranes as sites of metabolic activity these membranes do not enclose a separate area of the cytoplasm. Eukaryotes have a separate membrane bound nucleus, numerous mitochondria and other organelles such as the Golgi Body within each of their cells. These areas are separated off from the main mass of the cell’s cytoplasm by their own membrane in order to allow them to be more specialized. The nucleus contains all the Eukaryote cell DNA, which gets organized into distinct chromosomes during the process of mitosis and meiosis. The energy is generated in mitochondria. The exception to this rule are red blood cells which have no nucleus and do not live very long. Haeckel (1966) proposed a three kingdom classification recognizing Protista, Plantae and Animalis, the additional kingdom Protista including all single-celled organisms that are intermediate between animals and plants. Herbert Copeland (1938), who gave the prokaryotes a separate kingdom, originally called Mycota but later referred to as Monera or Bacteria. Copeland later on (1956) proposed a four-kingdom system placing all eukaryotes other than animals and plants in the kingdom Protoctista, thus recognizing four kingdoms Monera, Protoctista, Plantae and Animalia. The importance of grouping these kingdoms in two empires, as suggested earlier by Chatton was popularized by Stanier and van Niel (1962), and soon became widely accepted.

Five Kingdom System

American biologist Robert H. Whittaker (1969) proposed the removal of fungi into a separate kingdom thus establishing a five kingdom system recognizing Monera, Protista, Fungi, Plantae and Animalia as distinct kingdoms. The fungi like plants have a distinct cell wall but like animals lack autotrophic mode of nutrition. They, however, unlike animals draw nutrition from decomposition of organic matter, have cell wall reinforced with chitin, cell membranes containing ergosterol instead of cholesterol and have a unique biosynthetic pathway for lysine. The classification was followed widely in textbooks.

Six or Seven Kingdoms?

Subsequent research concerning the organisms previously known as archebacteria has led to the recognition that these creatures form an entirely distinct kingdom Archaea. These include anaerobic bacteria found in harsh oxygen-free conditions and are genetically and metabolically completely different from other, oxygen-breathing organisms. These bacteria, called Archaebacteria, or simply Archaea, are said to be “living fossils” that have survived since the planet’s very early ages, before the Earth’s atmosphere even had free oxygen. This together with the emphasis on phylogeny requiring groups to be monophyletic resulted in a six kingdom system proposed by Carl Woese et al. (1977). They grouped Archaebacteria and Eubacteria under Prokaryotes and rest of the four kingdoms Protista, Fungi, Plantae and Animalia under Eukaryotes. They subsequently (1990) grouped these kingdoms into three domains Bacteria (containing Eubacteria), Archaea (containing Archaebacteria) and Eukarya (containing Protista, Fungi, Plantae and Animalia).

Margulis and Schwartz (1998) proposed term superkingdom for domains and recognized two superkingdoms: Prokarya (Prokaryotae) and Eukarya (Eukaryotae). Former included single kingdom Bacteria (Monera) divided into two subkingdoms Archaea and Eubacteria. Eukarya was divided into four kingdoms: Protoctista (Protista), Animalia, Plantae and Fungi.

Several recent authors have attempted to recognize seventh kingdom of living organisms, but they differ in their treatment.

Ross (2002, 2005) recognized Archaebacteria and Eubacteria as separate kingdoms, named as Protomonera and Monera, respectively again under separate superkingdoms (domains of earlier authors) Archaebacteria and Eubacteria. He added seventh kingdom Myxomycophyta of slime molds under superkingdom Eukaryotes. Two additional superkingdoms of extinct organisms Progenotes (first cells) and Urokaryotes (prokaryotic cells that became eukaryotes):

Superkingdom Progenotes*……..first cells now extinct

Superkingdom Archaebacteria

Kingdom Protomonera…..archaic bacteria

Superkingdom Eubacteria

Kingdom Monera……..bacteria

Superkingdom Urkaryotes* …prokaryotic cells that became eukaryotes

Superkingdom Eukaryotes …cells with nuclei

Kingdom Protista……..protozoans

Kingdom Myxomycophyta…..slime molds

Kingdom Plantae…………plants

Kingdom Fungi…………..fungi

Kingdom Animalia………..animals

Patterson and Sogin (1992; Figure 1.2) recognized seven kingdoms but include slime molds under Protozoa (Protista) and instead establish Chromista (diatoms) as seventh kingdom. Interestingly the traditional algae now find themselves distributed in three different kingdoms: eubacterial prokaryotes (the blue-green cyanobacteria), chromistans (diatoms, kelps), and protozoans (green algae, red algae, dinoflagellates, euglenids).

Cavalier-Smith (1981) suggested that Eukaryotes can be classified into nine kingdoms each defined in terms of a unique constellation of cell structures. Five kingdoms have plate-like mitochondrial cristae: (1) Eufungi (the non-ciliated fungi, which unlike the other eight kingdoms have unstacked Golgi cisternae), (2) Ciliofungi (the posteriorly ciliated fungi), (3) Animalia (Animals, sponges, mesozoa, and choanociliates; phagotrophs with basically posterior ciliation), (4) Biliphyta (Non-phagotrophic, phycobilisome-containing, algae; i.e., the Glaucophyceae and Rhodophyceae), (5) Viridiplantae (Non-phagotrophic green plants, with starch-containing plastids). Kingdom (6), the Euglenozoa, has disc-shaped cristae and an intraciliary dense rod and may be phagotrophic and/or phototrophic with plastids with three-membraned envelopes. Kingdom (7), the Cryptophyta, has flattened tubular cristae, tubular mastigonemes on both cilia, and starch in the compartment between the plastid endoplasmic reticulum and the plastid envelope; their plastids, if present, have phycobilins inside the paired thylakoids and chlorophyll c2. Kingdom (8), the Chromophyta, has tubular cristae, together with tubular mastigonemes on one anterior cilium and/or a plastid endoplasmic reticulum and chlorophyll c1 + c2. Members of the ninth kingdom, the Protozoa, are mainly phagotrophic, and have tubular or vesicular cristae (or lack mitochondria altogether), and lack tubular mastigonemes on their (primitively anterior) cilia; plastids if present have three-envelop membranes, chlorophyll c2, and no internal starch, and a plastid endoplasmic reticulum is absent. Kingdoms 4–9 are primitively anteriorly biciliate. A simpler system of five kingdoms suitable for very elementary teaching is possible by grouping the photosynthetic and fungal kingdoms in pairs. It was suggested that Various compromises are possible between the nine and five kingdoms systems; it is suggested that the best one for general scientific use is a system of seven kingdoms in which the Eufungi and Ciliofungi become subkingdoms of ...