Study Guide: Taxonomy and Phylogeny
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Life on earth exists in great abundance and diversity. Over eons of geological time, living organisms have evolved through the process of natural selection. They have adapted to their environment to occupy every available niche. Over long periods of time conditions in the environment may have changed. Organisms must adapt to those conditions in order to survive. Organisms compete with other organisms for available resources. This can lead to adaptive changes over time. Living things are found all over the earth, in cold climates and hot ones, on land and in water, in wet areas on land and dry ones. In a particular area, the conditions can change seasonally or daily. The organisms that are found in a particular area with a particular set of conditions have had to adapt to their habitat in order to be successful. The result is biodiversity. It is also true that over long periods of time, challenges in the environment may have been so great that large groups of organisms may not have been able to successfully adapt. Great extinction events have occurred.
Biologists need to be aware of the diversity of living organisms on the earth. As a practical matter, they must be able to identify particular species. They must be able to communicate information about living organisms to other biologists in their own area or perhaps all over the world. If biologists discover what they believe to be a new species, it must be studied to verify that it is unique. This is done by carefully determining its characteristics and comparing it to previously identified related species. Because of increasing human populations, increased demand for resources, pollution, and climate change biodiversity is threatened. To be able to assess these threats to living organisms, we need a way to keep track of the abundance and diversity of living organisms. In order to do all of this each organism must be named, described, and categorized. The science of naming and classifying organisms is known as taxonomy. The taxonomic system is universal. It is used all over the world.
Taxonomy is part of the larger field of systematics, which in addition to identification and nomenclature includes the grouping of organisms according to their evolutionary relationships. Taxonomy differs from classification. Classification identifies a class as a group of organisms that have an essential feature or features in common. Systematists recognize that species evolve over time and attempt to form groups that include the most recent common ancestor of the group and its descendants. Inclusion in the group is based upon common descent, not the possession of an essential characteristic. Taxonomists have the role of giving every organism a name, identifying new species based on similarities and differences from previously defined species, and grouping organisms based on their morphological, physiological, developmental, genetic, biochemical, behavioral, and ecological characteristics. Because of the great diversity of living organisms there are many organisms that have not been discovered, named, and described. Biologists estimate that up to 95% of protozoa, more than 50% of terrestrial arthropods, and 10% of vertebrates have not been described (Convention on Biological Diversity https://www.cbd.int/gti/importance.shtml
Relationship of Taxonomy to Other Fields
A taxonomist must be familiar with many different fields in Science and biology including:
Paleontology examines the fossil record of living organisms providing tangible evidence of their appearance and structural characteristics at an earlier stage of evolution. Stratigraphy, Geology, and dating techniques based upon rates of radioactive decay provide evidence needed to establish a timeline useful in constructing phylogenetic trees.
Comparative anatomy examines similarities and differences in the structural characteristics of organisms to determine their evolutionary relationships. Comparative anatomy is the source for some of the most convincing forms of evidence for the evolution of living organisms. Organisms with similar anatomical features are considered to be closely related evolutionarily and are presumed to share a common ancestor. Such evidence of evolutionary relationships, based on studies of anatomical similarities and differences are important factors in determining and establishing classification of organisms.
Comparative physiology examines the similarities and differences in the functional diversity of living organisms. The objective is to determine the evolutionary relationships among the species. It attempts to explain how physiology influences adaptation of organisms to their environment. It also uses physiological information to elucidate phylogenetic relationships of organisms.
Developmental biology is the study of the growth and development of living organisms. Embryology examines development from the fertilized egg until birth. An example of the contribution of developmental biology to our knowledge of phylogenetic relationships of organisms is the discovery of the Hox gene complex (homeobox genes). This is a set of master genes that regulate the expression of a hierarchy of other that control the development of body parts along the front to back axis. They were first discovered in the fruit fly Drosophila, and since their discovery have been found in organisms ranging from fungi to vertebrates. Hox genes determine where appendages of the body such as limbs or wings develop (Kardong, 2006). The loss of forelimbs in Pythons is caused by loss of the expression of a Hox gene, the sonic hedgehog gene in the hind limb region.
The sequence of nucleotides in DNA can be used to examine phylogenetic relationships among species. The genetic sequence of a particular species can be compared to an outgroup to establish evolutionary relationships.
Protein Sequencing can also be used to compare species.
Aristotle and Classification of Animals
The first person to classify organisms was Aristotle. Aristotle carried out the first systematic studies of animals, examining over 500 species. Consequently, he founded the science of zoology. Aristotle gained first-hand knowledge of animals by performing dissections and made very accurate descriptions of their structure. Aristotle developed a system, which he used to classify animals. It is here that we find the origins of the methods of classification used by biologists today. Aristotle began to classify the animals that he studied on the basis of their characteristics. For example animals that have wings, feathers, beaks, and two fleshless legs were birds. Fish were cold-blooded, gilled, scaled, aquatic, and oviparous.
He divided animals into blooded and bloodless. These divisions were later replaced by vertebrates and invertebrates. Aristotle recognized a genus that consisted of quadrupedal, viviparous, animals that were covered with hair. This group corresponded to the mammals. Examples were man, lion, stag, horse, and so on. Aristotle recognized cetaceans as another genus within blooded animals. Today they are classified as mammals. Oviparous animals included birds, which were characterized by wings, feathers, beaks, and two fleshless legs. Oviparous quadrupeds corresponded to reptiles and amphibians. Fish are oviparous swimming creatures with fins (History of Animals part 5).
Among the bloodless animals, Aristotle identified those with hard shells, such as the oyster. Animals with soft shells included crustacea such as spiny crawfish, crabs, and lobsters. In another genus Aristotle grouped shell-less marine animals including the octopus, squid and cuttlefish, which today are recognized as molluscs. An additional genus of bloodless animals was made up of insects. Bees were considered a separate genus.
Theophrastus and Classification of Plants
Theophrastus was a student of Aristotle. He investigated plants, creating the Science of Botany. Theophrastus created the first classification of plants, dividing plants into four broad categories: trees, shrubs, subshrubs, and herbs. In his book, Historia Plantarum of ten volumes of which 9 survive, Theophrastus examined the structure of plants, their growth, and reproduction. He surveyed the varieties of plants around the world and compiled their uses.
Carolus Linnaeus (May 23, 1707 – January 10, 1778) was a Swedish botanist and natural historian who taught at the University of Uppsala. Linnaeus made two major contributions to the field of taxonomy:
1) He established conventions for the naming of living organisms using binomial nomenclature (the genus name followed by the species name), and
2) He developed a hierarchical system for classification of organisms. These became the universal standard for biological classification in the scientific world.
Linnaeus also developed an extensive system of taxonomy, which he published in his manuscript, Systema Naturae, which was published in 1735. It classified 7,700 species of plants and 4,400 species of animals. Linnaeus’s classification was based on physical characteristics. He recognized that the number and kinds of teeth could be useful in the classification of animals. He classified plants on the basis of the number and positions of their reproductive organs, the stamens and pistils. A plant’s class was determined by its stamens and its order by its pistils.
Two Kingdom Classifications
The Classification system by Carolus Linnaeus in 1735 recognized three kingdoms. He recognized two kingdoms of living organisms: Regnum Animale, the animal kingdom and Regnum Vegetabile, the plant kingdom. Linnaeus included minerals in his classification system, describing them in a third kingdom, Regnum Animale.
Kingdom Protista and Three Kingdom Classification
The existence of previously unknown single-celled microscopic organisms was discovered by Antonie van Leeuwenhoek, a Dutch businessman and scientist who designed and made early single-lensed microscopes. Leeuwenhoek was the first person to observe through a microscope swarms of these tiny microscopic creatures swimming in a drop of standing water that he had collected in his garden.
Initially, microscopic organisms were classified either in the plant kingdom or in the animal kingdom. However, as time went on biologists realized that many microscopic organisms could not be easily distinguished as either plant or animal. In order to try to solve this dilemma, in 1860 John Hogg proposed a third kingdom, Kingdom Protoctista, to include such organisms. Ernst Haeckel also proposed a third kingdom of living organisms in 1866, the Protista, for unicellular life forms.
Although the kingdom Protista was proposed in the 1800s it did not come into widespread use. Most textbooks used the two kingdom system. Bacteria, algae, and fungi were grouped with plants in the plant kingdom. Protozoa were included with multicellular animals in the animal kingdom.
Four-Kingdom Classification System
In 1938, Herbert Copeland introduced a four-kingdom classification system. He created an new kingdom called Kingdom Monera, which was made up of prokaryotic organisms, one celled animals that lacked a membrane enclosed nucleus. Cyanobacteria, previously known as blue-green algae, were included in the new kingdom.
Five-kingdom Classification System
Robert Whittaker introduced a five-kingdom classification system in 1969? The five kingdoms in Whittaker’s scheme included Kingdom Monera (Bacteria), Kingdom Fungi, Kingdom Protista, Kingdom Plantae, and Kingdom Animalia. Lynn Margulis also advocated a five-kingdom system. However, in her system she used the term protist to designate microscopic organisms, while the larger kingdom Protoctista included large multicellular eukaryotes, such as kelp and red algae. It also included slime molds.
In 1977, Carl Woese and his colleagues proposed the separation of prokaryotes into two groups, the Eubacteria (((later called the Bacteria) and the Archaebacteria (later called the Archaea) based on the structure of ribosomal RNA. This produced a six-kingdom system that included the Kingdoms Bacteria, Archaea, Protista (Protoctista), Plantae, Fungi, and Animalia. Thomas Cavalier-Smith and coworkers have devised schemes comprised of six, seven, and eight kingdoms. This version did not recognize the division of the prokaryotes into Eubacteria and Archaebacteria. He retained a single kingdom for bacteria, the remaining kingdoms were Protozoa, Chromista, Plantae, Fungi, and Animalia. In 2015, Cavalier-Smith accepted the prevailing view that bacteria should be divided into Kingdoms Bacteria and Archaea, and added them to their other five kingdoms giving a total of seven kingdoms. In 1993, Cavalier-Smith added a newly discovered group of Protists to his system and recognized them as a kingdom. This new group was called the Archezoa. They were Protists that were found to lack mitochondria. This produced an eight-kingdom scheme.
A large group above the kingdom called a domain was created by Carl Woese in 1977. Woese recognized three domains: Domain Archaea, Domain Bacteria, and Domain Eukaryota. Domain Archaea and Domain Bacteria both included prokaryotic organisms lacking a membrane-bound nucleus, and Eukaryota designated eukaryotic organisms composed of cells that contained a membrane-bound nucleus and membranous organelles.
Phylogenetic systematics, also known as cladistics was developed in 1950 by Willi Hennig (April 20 1913 – November 5 1976), a German entomologist who specialized in the study of dipterans (flies). He published his work in the book Basic outline of a theory of phylogenetic systematics in German in 1950, and later published a translated and revised version of this book as Phylogenetic Systematics in English in 1966. The term clade was coined by Julian Huxley in 1958. The term cladistic was introduced in 1960 by Arthur James Cain and G. A. Harrison. Cladistics attempts to place organisms into groups known as clades on the basis of common evolutionary ancestry. The members of a clade have shared derived characters. A clade is intended to be monophyletic, which means that it contains the most recent common ancestor of all of the organisms in the group together with all of its descendants.
Carolus Linnaeus developed the binomial system of nomenclature, in which every species is given a name consisting of two-parts. The first part of the name is the genus. The second word is an epithet (an adjective or modifier) that has no meaning by itself. The species name is the entire two-part name. For example, the species name for man is Homo sapiens.
Characteristics of Scientific Names
No two organisms can have the same scientific name.
Scientific names are in Latin or Greek.
It is a universal system.
Using a standardized system eliminates confusion which often arises from the use of common names.
Definition of a Species
A species is a reproductive community of populations (reproductively isolated from others) that occupies a specific niche in nature. (Ernst 1984 cited in Hickman)
The classification of organisms, taxonomy, is based on a hierarchical system – that is, it consists of groups within groups, with each group being ranked at a particular level. In such a system, a particular group is called a taxon (plural, taxa), and the level at which it is ranked is called a category. For example, genus and species are categories, and Homo and Homo sapiens are taxa.
The original ranks used by Linnaeus were as follows: Kingdoms were divided into Classes, Classes into Orders, Orders into Genera, and Genera were divided into Species. Below the rank of species, Linnaeus sometimes recognized taxa of a lower (unnamed) rank. (For plants, these are now called “varieties.”) Modern taxonomy recognizes new ranks that have been added to Linnaeus’s original system including family between order and genus and phylum between kingdom and class.
The major categories are:
In our current system a taxonomy, a new group known as a domain has been added above the Kingdom level. The above ranks can be subdivided, if necessary, into superclasses, subclasses, superorders, infraorders, and superfamilies.
An example of Biological Classification using man is as follows:
Human (Homo sapiens)
|Kingdom||Animal||Multicellular organisms requiring complex organic substances for food|
|Phylum||Chordata||Animals with notochord, dorsal hollow nerve cord, gill pouches in pharynx at some stage of the life cycle|
|Subphylum||Vertebrata||Spinal cord enclosed in a vertebral column, body basically segmented, skull enclosing brain|
|Superclass||Tetrapoda||Land vertebrates, four limbed|
|Class||Mammalia||Young nourished by milk glands, skin with hair or fur, body cavity divided by a muscular diaphragm, red blood cells without nuclei, three ear bones (ossicles), high body temperature|
|Order||Primates||Tree dwellers or their descendants, usually with fingers and flat nails, sense of smell reduced|
|Family||Hominidae||Flat face; eyes forward; color vision; upright, bipedal locomotion|
|Genus||Homo||Large brain, speech, long childhood|
|Species||Homo sapiens||Prominent chin, high forehead, sparse body hair|
Classification Based on Evolutionary Relationships
Taxonomists decide how to group organisms by looking for similarity among organisms in their structure, physiology, genetics, reproduction, and development. Modern classification uses a comparative approach based upon evolutionary relationships. The reconstruction and study of evolutionary relationships is called systematics (Mason et al.). Differences and similarities among organisms are as products of their evolutionary history, or phylogeny. The evolutionary relationships among organisms have often been depicted as phylogenetic trees. Charles Darwin was the first person to depict evolutionary relationships among living organisms as a branching tree, as shown in his book On the Origin of Species. The base of the tree represented the single common ancestor of living organisms. The branches of the tree represented the descent with modification of new species.
Phylogenies Constructed Solely on the basis of Similarity May Contain Inaccuracies Caused by Inconsistencies in Evolution
Early taxonomists may have assumed that the more time that has passed since two species diverged from a common ancestor, the more different they would be. If species evolved at a constant rate and the environmental conditions to which the organisms had to adapt changed in a constant direction, this assumption would be true. However, evolution can proceed very rapidly at some times and very slowly at others. In addition, evolution is not unidirectional. Sometimes species’ traits evolve in one direction as organisms adapt to changing environmental conditions, and then revert to an earlier state as conditions change back to resemble the original ones. Species entering new habitats may have to adapt to different conditions and may change greatly; those staying in the same habitats as their ancestors may change only a little. Two different species may have adapted to similar environmental conditions in different areas and may have come to resemble one another. This is known as convergent evolution. It does not mean that the two groups have a common ancestor. For these reasons, similarity is not always an accurate predictor of how long it has been since two species shared a common ancestor.
Definition of Cladistics
Cladistics is a biological taxonomic system that groups organisms into taxa known as clades on the basis of shared derived characteristics that represent shared evolutionary ancestry.
Characters can include structure, physiology, development, behavior, and DNA and protein sequences.
Characters exist in recognizable character states. For example, consider the character “teeth” in amniote vertebrates (namely, birds, reptiles, and mammals), this character has two states: presence in most mammals and reptiles, and absence in birds and a few other groups such as turtles. (Mason et al.)
Cladistics Requires That Character States Be Identified as Ancestral or Derived
Definition of Ancestral and Derived
An ancestral character is one that was inherited from the common ancestor of a clade and has undergone little change since.
A derived character is a trait that appears within the clade group.
“The presence of hair is a shared derived feature of mammals. In contrast, the presence of lungs in mammals is an ancestral feature, because it is also present in amphibians and reptiles (represented by a salamander and a lizard) and therefore presumably evolved prior to the common ancestor of mammals. The presence of lungs, therefore, does not tell us that mammal species are all more closely related to one another than to reptiles or amphibians, but the shared, derived feature of hair suggests that all mammal species share a common ancestor that existed more recently than the common ancestor of mammals, amphibians, and reptiles.” (Mason et al.)
Determination of Ancestral versus Derived – Outgroup Comparison
To determine whether a character is ancestral or derived, the method of outgroup comparison is used. To use this method, a species or group of species that is closely related to, but not a member of the group under study is designated as an outgroup. Traits present in the group under study are compared to those in an outgroup. We infer that any character state found both within the group being studied and in the outgroup is ancestral for the study group. A outgroup that is likely to possess ancestral character states compared to the ingroup should be selected. Traits that are present only in the study group are regarded as derived.
Construction of a Cladogram
Systematists depict the evolutionary relationships of organisms by constructing a diagram known as a cladogram. Cladistics attempts to classify organisms by separating them into groups that have shared evolutionary ancestry, that is have descended from a common ancestor. Such groups have branched off from ancestral groups during the course of evolution. A group of species that shares a common ancestor is called a clade, which is recognized by the possession of shared derived characters. The derived characters in the clade are unique features that have arisen by evolution and were not present in distant ancestors. A derived character shared by clade members is called a synapomorphy of that clade. A characters present in the members of a clade that has been retained from its ancestors is a plesiomorphy and shared ancestral states are called symplesiomorphies. A cladogram is a nested series of clades arranged in a hierarchy.
Example of a Cladogram
Homoplasty Complicates Cladistic Analysis
A homoplasy is a shared character state that is not present in the common ancestor of the group of organisms. Homoplasy can result from convergent evolution. For example two separate groups can live in different areas that have similar environmental conditions. As a result, they may develop adaptations that appear to be similar. For example, the eye of the squid has the same parts that vertebrate eyes do. It has a cornea, a lens, a retina and so on. However, zoologists do not believed that the vertebrate eye evolved from the eye of the squid. Instead, they explain the reason for the similarity in structure by proposing that the squid eye and the vertebrate eye evolved under similar conditions. Homoplasy can also arise as a result of evolution proceeding in a reverse direction in which the animal reverts from a derived character state back to an ancestral character state. The presence of homoplastic characters can possibly lead systematists to incorrectly make assumptions about evolutionary relationships that do not truly reflect shared common ancestry.
Resolution of Difficulties Resulting from Homoplasy – Principle of Parsimony
In order to avoid mistakes such as those arising from the presence of homoplastic characters, systematists employ the principle of parsimony. This principle is basically the same as Occam’s razor: the principle that the simplest explanation is usually the right one. In phylogeny, the explanation that requires the fewest evolutionary events is considered the best hypothesis of phylogenetic relationships.
“For example, adult frogs do not have a tail. Thus, absence of a tail is a synapomorphy that unites no only gorillas and humans but also frogs. However, frogs have neither an amniotic membrane nor hair, both of which are synapomorphies for clades that contain gorillas and humans.” “In the example just stated, therefore, grouping frogs with salamanders is favored because it requires only one instance of homoplasy (the multiple origins of taillessness, whereas a phylogeny in which frogs were most closely related to humans and gorillas would require two homoplastic evolutionary events (the loss of both amniotic membranes and hair in frogs).” (Mason et al.)
Terminology for Taxa
A monophyletic group includes the most recent common ancestor of all of the organisms in the group together with all of its descendants. A monophyletic group, by definition, can also be called a clade.
A paraphyletic group includes the most common ancestor of all of the organisms in the group, but does not include all of the descendants of the most recent common ancestor.
A polyphyletic group does not include the most recent ancestor of all of its members.
Biologists recognize three domains: Domain Bacteria, Domain Archaea, and Domain Eukarya. Biologists currently recognize six kingdoms: Kingdom Archaebacteria, Kingdom Eubacteria, Kingdom Protista, Kingdom Fungi, Kingdom Plantae, and Kingdom Animalia.
Eubacteria are true bacteria. They are prokaryotic organisms. Mitochondria and chloroplasts have been included in this domain (see the Tree of Life Project). They were originally free-living bacteria that were engulfed by a eukaryotic cell. This is explained by the Endosymbiotic theory.
Domain Archaea consists of prokaryotic organisms. They are distinct from other prokaryotic organisms in the domain Eubacteria (Bacteria), and also from eukaryotic organisms (Domain Eukaryotes). Domain Archaea includes Methanogens, Halophiles, Sulfolobus, and relatives (Tree of Life Project). They were separated from bacteria by Carl Woese and George E. Fox on the basis of differences in the nucleotide sequences in ribosomal RNA (rRNA). Additional evidence for separating Archaea from bacteria was the lack of peptidoglycan in their cell walls, and unusual coenzymes such as methanopterin, cytochrome B, and cytochrome M. On the basis of these differences from bacteria, Woese and coworkers proposed assigning these organisms to a new domain, Archaea. Archaea were first isolated in extreme environments, such as high methane (methanogens), high temperature environments such as hot springs (thermophiles), and high salt environments (halophiles). However, they have since been discovered living in more typical and diverse environments such as plankton in the ocean, wetlands, and soil.
The domain Eukaryotes consists of the organisms that have eukaryotic cells. A eukaryotic cell is one that has a membrane-bound nucleus and membranous organelles. The nucleus contains the primary genome of the cell, which is present in the form of linear DNA enclosed within chromosomes and associated with histone proteins. An endomembrane system is present that includes the mitochondria, endoplasmic reticulum, Golgi bodies, and vacuoles. Chloroplasts are found in photosynthetic eukaryotes. Eukaryotic cells possess a cytoskeletal system, which is a network of protein filaments and tubules that extends throughout the cytoplasm of the cell. The proteins include actin-based microfilaments and tubulin-based microtubules. The cytoskeleton includes motor proteins such as kinesins and dyneins that transport organelles throughout the cell.
Domain Eukarya is divided into the following groups:
In a number of classification schemes that we have reviewed, such as the five-kingdom scheme, Protista was considered as a kingdom. It included most of the single-celled eukaryotes. It included Green Algae, formerly in the Plant Kingdom which have a number of significant features in common with plants, such as the possession of chlorophyll a and b, cellulose cell walls, storage of food as starch, and cell division by cell plate formation. Green algae are likely the ancestors of plants. It included the Fungi which also were formerly included in the Plant Kingdom. Unlike plants, fungi are heterotrophic not photosynthetic. They store food in the form of glycogen not starch. Instead of having cellulose cell walls, their cell walls are composed of chitin. Protista included Protozoa, which were formerly in the Animal Kingdom and are likely contains the ancestor of animals. It also included many single-celled organisms that were difficult to place in the Animal Kingdom or the Plant Kingdom such as Euglena. One of the problems with Protista was that inclusion in the group was determined by not easily being classified as a plant, an animal, or a fungus instead of possessing a set of defining characteristics.
Currently, the emphasis in Taxonomy is to group organisms on the basis of evolutionary relationships. The goal is to construct clades, which contain the most recent common recent ancestor of a group of organisms and all of its descendants. Protista is considered paraphyletic, containing an assemblage of organisms that have not united on the basis of shared evolutionary descent. The current trend is to no longer regard Protista as a Kingdom. However, it is also true that the group Protista includes many organisms that are still difficult to classify. But as research continues, evidence may be produced that will allow more definitive placements. Although we will no longer recognize Protista as a Kingdom, we will retain Protista as a group used for those organisms in which evolutionary relationships remain uncertain or existing evidence is not sufficient for placement in a specific taxon.
Animals are defined as multicellular, ingestive, heterotrophic, eukaryotes. All members of the animal kingdom are multicellular, that is composed of many cells. The cells of animals are eukaryotic. Animal cells lack cell walls and chloroplasts. Animal cells have centrioles. During cell division, astral rays are formed and the cell divides by constriction of cytoplasm. The bodies of most animals (except sponges) are composed of tissues, a group of similar specialized to perform a specific functions. All animals have a heterotrophic form of nutrition that is they must eat plants or other animals in order to survive. Most animals ingest food and digest it within a digestive system. Carbohydrate food is stored in the form of glycogen. Most animals are capable of movement from place to place. Most animals reproduce sexually by means of egg and sperm. Most animals are diploid with gametes the only haploid stage in the life cycle.
Plants are eukaryotic, photosynthetic organisms that contain chlorophyll a and b, have cell walls containing cellulose, and store food as starch within plastids. Plant cells are eukaryotic. Their DNA is contained within a nucleus surrounded by a nuclear membrane. Plants are photosynthetic. They contain chlorophyll a and chlorophyll b, xanthophylls (yellow pigments) and carotenes (orange pigments). The photosynthetic pigments are concentrated in organelles known as chloroplasts. Food is stored in the form of starch within plastids. Plants have cell walls that are composed of cellulose. In addition, vascular plants have lignin in the cell walls that functions in support and conduction, enabling plants to grow tall. Plant cells have large central vacuoles. Cell division is by means of a cell plate that forms across the mitotic spindle.
Fungi are composed of eukaryotic cells. Fungal cells have membrane-bound nuclei containing chromosomes. Fungal cells are surrounded by a cell wall containing glucans and chitin and have vacuoles. Fungi lack chloroplasts. Fungi have heterotrophic nutrition. They are saprophytic, which means that they excrete enzymes to the outside of the body. They enzymes break down food there and the nutrients are absorbed back into the body. In this way fungi act as decomposers and are important in breaking down wastes in ecosystems. Some fungi are single-celled, while others are multicellular. The bodies of the multicellular fungi are known as mycelia and are composed of interwoven thread-like filaments called hyphae. Asexual reproduction takes place by the formation of spores. Sexual reproduction also takes place in fungi. Sexual spores may be formed. In Zygomycota hyphae fuse together to produce zygospores. Ascomycota produce a spore sac known as an ascus. Basidiomycota produce basidiospores.