Angiosperm Systematics

The phanerogam phylogenesis cluster (APG) is an off-the-cuff international cluster of systematic botanists that collaborate to determine an accord on the taxonomy of flowering plants (angiosperms) that may result in new information regarding plant relationships discovered through biological process studies.

General Characteristics

The angiosperms are a well-defined and well-characterized group. There is no existing plant species whose classification as angiosperm or non-angiosperm is uncertain. Even the fossil record does not contain forms that are related to any other group, although there are some fossils of particular plant components that cannot be categorised well

Angiosperms are typically seed plants. This distinguishes them from all other plants save gymnosperms, of which the conifers and cycads are the most familiar representatives

Angiosperm ovules (forerunners of seeds) are typically enclosed in an ovary, in contrast to gymnosperm ovules, which are exposed to the air during pollination and are never enclosed in an ovary. The stigma of angiosperms collects pollen, a specialised structure that is typically elevated above the ovary on a more slender structure called the style. Pollen grains germinate on the stigma, and the pollen tube must extend through the style (if present) and ovarian tissues in order to reach the ovule. Gymnosperm pollen grains, on the other hand, are received through an aperture (the micropyle) atop the ovule

The female gametophyte of angiosperms (referred to as the embryo sac) is minuscule and contains only a few (usually eight) nuclei; the cytoplasm is not partitioned by cell walls and are associated less or more directly with these nuclei. In sexual reproduction, one of the embryo sac’s numerous nuclei serves as the egg, combining with one of the pollen tube’s two sperm nuclei. Two additional embryo sac nuclei merge with the pollen tube’s second sperm nucleus. This triple-fusion nucleus is a precursor of the endosperm, a multicellular food-storage tissue found in seeds

Angiosperm Phylogeny Group

The Angiosperm Phylogeny Group (APG) is an informal multinational group of systematic botanists who collaborate to produce a taxonomic agreement for flowering plants (angiosperms) that reflects new information about plant relationships uncovered through phylogenetic investigations

As of 2016, this collaboration has resulted in four incremental versions of a classification system, which were published in 1998, 2003, 2009, and 2016. The group was motivated by what they perceived as shortcomings in previous angiosperm classifications, which were not based on monophyletic groups (i.e., groups that include all the descendants of a common ancestor)

APG publications are gaining increasing clout, with a number of large herbaria reorganising their collections to conform to the most recent APG classification

Angiosperm Classification and the APG

Prior to the discovery of genetic evidence, angiosperms (also called flowering plants, Angiospermae, Anthophyta, or Magnoliophyta) were classified primarily on their morphology (especially their flower) and biochemistry (the kinds of chemical compounds in the plant).

After the 1980s, precise genetic data analysed using phylogenetic methods became accessible, and while it confirmed or clarified some old classification systems, it fundamentally altered others. This genetic data accelerated the growth of knowledge, resulting in several proposed modifications; stability was “rudely destroyed.” This created difficulties for all classification system users (including encyclopaedists).

The catalyst was a landmark molecular study released in 1993 on 5000 flowering plants and a photosynthesis gene (rbcL). This resulted in a variety of unexpected findings on the relationships between plant groupings; for example, the dicotyledons were not supported as a distinct group. Initially, there was apprehension about developing a new system wholly based on a single gene.

However, additional research has confirmed these findings. These scientific projects necessitated an unparalleled level of collaboration among a huge number of scientists. Rather than recognising all of the individual contributors, the Angiosperm Phylogeny Group categorization, or APG for short, was chosen. The first publication under this moniker occurred in 1998 and garnered widespread media attention. The goal was to establish a more universally accepted and stable standard for angiosperm classification.

As of 2016, three versions have been released, the first in 2003 (APG II), the second in 2009 (APG III), and the third in 2016 (APG IV), each of which supersedes the preceding system. Thirteen researchers are identified as authors and another 43 as contributors to the three papers.

A categorization expresses an opinion at a given point in time, depending on the state of research at the time. Independent researchers, including APG members, continue to publish their own perspectives on various aspects of angiosperm taxonomy. Classifications evolve, regardless of how uncomfortable this is for users. However, the APG publications are increasingly considered as authoritative sources of information, as evidenced by the following examples of the APG system’s influence:

Numerous large herbaria, including Kew, are reorganising their collections in conformity with the APG

The influential World Checklist of Selected Plant Families (again from Kew) is currently undergoing an update to the APG III system

In 2006, the United States of America and Canada conducted a photographic survey of their plants using the APG II method

In the United Kingdom, the APG III system is used in the 2010 version of the standard flora of the British Isles (by Stace). Earlier iterations used the Cronquist system

Principles of the APG System

The APG’s classification concepts were established in the original publication of 1998 and have remained intact in subsequent editions. These are, in brief, the following:

The Linnean order and family structure should be preserved. “The family is fundamental to the systematics of flowering plants.” An ordinal classification of families is offered as a “universally useful reference tool.” Orders are regarded as particularly valuable in the classroom and in the study of family interactions

Monophyletic groups should exist (i.e. consist of all descendants of a common ancestor). The primary reason for rejecting present systems is that they lack this quality; they are not phylogenetic

The boundaries of organisations such as orders and families are defined broadly. Thus, it is argued that a small number of larger orders will be more beneficial. Families and orders having a single genus and family are avoided wherever possible without breaching the overriding requirement of monophyly

The term clades is more loosely employed above or parallel to the level of orders and families. (Some clades were later given formal names in a study related to the APG system’s 2009 revision.) The authors state that while naming all clades in a phylogenetic tree is “impossible and undesirable,” systematists must agree on names for select clades, particularly orders and families, to promote communication and discussion

Conclusion

The phanerogam phylogenesis cluster (APG) is an off-the-cuff international cluster of systematic botanists that collaborate to determine a accord on the taxonomy of flowering plants (angiosperms) that may result in new information regarding plant relationships discovered through biological process studies.