Tag Archives: Taxonomy

RHS Level 3: Plant taxonomy, structure, and function

  1. Understand the Plant Kingdom and the taxonomic hierarchy.

1.1 Describe the major groups of the Plant Kingdom.

List the main groups within bryophytes, pteridophytes, gymnosperms and angiosperms.

This is quite an archaic way of grouping plants. The kingdom Plantae is usually divided into 10 divisions, listed below, with the groups in the syllabus in bold. Gymnosperms consists of Pinophyta, Cycadophyta and Ginkgophyta. Angiosperms = Magnoliophyta:

  • Anthocerotophyta – hornworts
  • Marchantiophyta – liverworts
  • Bryophyta – mosses
  • Lycopodiophyta – club and spike-mosses
  • Pteridophyta – ferns and horsetails
  • Gnetophyta – 3 extant genera of woody plants
  • Cycadophyta – cycads
  • Ginkgophyta – Ginkgo
  • Pinophyta/Coniferophyta – conifers
  • Magnoliophyta – flowering plants

Plant Characteristics

Describe and compare the structural and reproductive characteristics of: mosses, ferns, conifers and flowering plants in relation to their adaptation to terrestrial life.


I’ve written about these four groups previously, the information about structural and reproductive characteristics is in the first two paragraphs of each blog

  1. Mosses
  2. Ferns
  3. Conifers
  4. Flowering Plants

Brief description of reproductive characteristics:

Bryophytes – have sporophyte and gametophyte stages. Gametophyte is dominant.

Pteridophytes – have sporophyte and gametophyte stages. Sporophyte is typical fern, gametophyte is small and rarely noticed.

Gymnosperm – have male and female cones. Male cones drop pollen which is carried by wind.

Angiosperm – have flowers that may be dioecious, monoecious or hermaphrodite. Usually wind or insect pollinated (but other methods of pollination exist).

1.2 Describe features of plant classification and nomenclature relevant to horticulture.

State the hierarchy of botanical units and explain how and when they are used.

To include: family, genus, species, subspecies, varietas, forma.

To include ONE NAMED plant example for EACH of the above terms showing how it is written.


Have the ending -aceae (many family names were recently changed to conform to this). Plant families are usually named after the biggest or most well known genus in that family. eg Euphorbiaceae, the family that the genus Euphorbia is in.


Genus is a subdivision of family. The genus of a plant is used as the first part of its binomial name, and is always capitalised. It should be written in italics (or underlined). eg Euphorbia.


Species is a subdivision of genus. The species of a plant is used as the second part of its binomial name and is never capitalised. It should be written in italics (or underlined). eg characias  (as in Euphorbia characias.)


Recommended abbreviation is subsp. but ssp. is sometimes used. Subspecies are written in small italics, but the word subsp. is not. A subdivision of species. Plants within different subspecies but within the same species are capable of interbreeding, but don’t due to geographical separation. eg Euphorbia characias subsp. wulfenii.


A subdivision of species, similar to subspecies (and the two terms often overlap) however, different varieties within a species may geographically overlap, unlike subspecies. Recommended abbreviation is var. Varieties are written in italics, but var. is not. eg Malva alcea var. fastigiata.


If a plant shows uncharacteristic appearance of its species (such as habit or colour) then it can be known as a different form. These differences are usually due to environmental reasons and won’t be passed to the next generation. Recommended abbreviation is f. The form is written in italics, but f. is not. eg Vinca minor f. alba.

Explain the meaning and use of the terms: cultivar, Group, trade designation (selling name), Plant Breeders’ Rights, interspecific, intergeneric and graft hybrids, naming authority.

To include ONE NAMED plant example for EACH of the above terms, showing how it is written.

Cultivar: This is short for ‘cultivated variety’ and refers to plants that have been bred for their characteristics. The names are often chosen as a selling point, for example using somebody’s name, making them a good present for people of the same name. eg Clematis ‘Willy’ (note the cultivar name is capitalized, in single quotes and not italicized. Because of the complexity of cross breeding across species, the species of a cultivar is only sometimes used.)

Group: If several cultivars are similar, they can be grouped together to make customer selection easier. eg Lilium Darkest Red Group (note the group is capitalized, not italicized, and not in quotes.)

Trade designation: Cultivar names cannot be legally protected. If a plant breeder wishes to keep sole legal rights to a plant, then he/she uses a trade name. This a commercial synonym that is legally protected. eg Rosa FASCINATION = Rosa ‘Poulmax’. (note: the writing method for ‘Fascination’ changes, sometimes it is in quotes, like a cultivar; other times it is in square brackets. The correct notation is all in capitals, not italicized, not in quotes, often in a different font.)

Plant Breeders’ Rights: Breeders using a Trade designation have Plant Breeders’ Rights which are recognised internationally. If you own the rights to a cultivar, it cannot be bred by anyone else without your permission. If somebody buys one specimen of your cultivar, you still have exclusive rights to all propagation material of that plant: seeds, cuttings etc.

Interspecific, intergeneric and graft hybrids: Unlike with animals, plants can be bred across species and genera. Plants of different genera can, in some cases, be grafted together, occasionally this will lead to a mixing of cells where the scion and the rootstock meet, this is not a true hybrid. It is also known as a graft chimaera.


  • Interspecific hybrid –  Mahonia × media (bred from Mahonia lomariifolia and Mahonia japonica, note the ‘x’ in the middle and new specific epithet.)
  • Intergeneric hybrid× Cupressocyparis leylandii (bred from Cupressus macrocarpa and Chamaecyparis nootkatensis, note the ‘x’ at the beginning and the genus which is a combination of the parents’).
  • Graft hybrid – +Laburnocytisus ‘Adamii’, (a graft hybrid between Laburnum and Cytisus, note the ‘+’ at the start and genus which is a combination of the parents’.) This graft contains flowers of Laburnum and Cytisus (ie both yellow and purple) but also flowers that are a pinky colour, a mix of the two.

Naming authority: The International Cultivation Registration Authority is a naming authority, responsible for seeing that cultivar names are not duplicated.

State the significance of the ICN (The International Code of Nomenclature for algae, fungi and plants) formerly ICBN (International Code of Botanical Nomenclature) and the ICNCP (International Code for Nomenclature for Cultivated Plants) in the naming of plants.

The ICN (The International Code of Nomenclature for algae, fungi and plants) A code that governs plant discoveries in the world – ensuring that plants aren’t given different names by different discoverers, or that already named plants aren’t given new names without reason.

International Code of Nomenclature website Contains complex set of rules to standardise naming and classification eg changing all plant families to end in -aceae, Compositae > Asteraceae.

ICNCP (International Code for Nomenclature for Cultivated Plants) – a code that governs the naming of newly created cultivars.

Cultivated Plant Code

Explain the reasons for name changes: reclassification (scientific research, new discovery), changes in nomenclature (rule of priority), incorrect identification.

To include TWO NAMED plant examples for EACH.

Reclassification (scientific research, new discovery)

  1. With advances in DNA technology, African Acasias were found to not be related to Australian Acasias. Australian Acasias have kept their name, while African have become Vachellia or Senegalia.
  2. Coleus became Solenostemon, but was then found to be part of the Plectranthus genus. Plectranthus scutellarioides used to be Coleus blumei.

Rule of priority

This is where a plant is discovered to have been named previously, and its old name is found on record. When an existing name is discovered, the plant should revert to this name, but occasionally, if the new name is far more familiar it will be kept.

  1. Platanus ×acerifolia was the name of the London Plane, but this name was recorded in 1805 and it was discovered later that an earlier name of Platanus ×hispanica had been recorded in 1770. Therefore Platanus ×hispanica became the official name.
  2. Festuca subgenus Schedonorus was moved to the genus Lolium and its name became Lolium subgenus Schedonorus.

Incorrect identification

Sometimes a name change is due to a simple mistake, when one plant becomes mixed up with another.

  1. Archontophoenix cunninghamiana was for a long time incorrectly sold as Seaforthia elegans.
  2. Syzygium australe was often sold as Syzygium paniculatum

 Explain how plant names can indicate: plant origin, habitat, commemoration, colour, growth habit, leaf form.

To include TWO NAMED plant examples for EACH. 

It is often the plant species that indicates origin, colour etc, but not always (see below). The Latin will only refer to one characteristic (when Latin plant names were first used, botanists tried to include every characteristic, leading to ridiculously long names, then Linnaeus reduced it to two).

Plant origin: Mahonia japonica (Japan), Arum italicum (Italy)

Habitat: Clematis alpina (alpine plants), Pinus sylvestris (wood or forest)

Commemoration: Photinia fraseri (John Fraser1750-1811 nurseryman), Weigela (Christian Weigel 1749-1831 German botanist)

Growth: Briza maxima (large or largest), Vinca minor (smaller)

Habit: Cotoneaster horizontalis (growing horizontally), Phlomis fruticose (shrubby)

Leaf form: Acer palmatum (palmate leaves), Ilex aquifolium (pointed leaves)

Plant Divisions: Flowering Plants

Leaf Variety in Magnoliophyta

Leaf Variety in Magnoliophyta

Plants in the Magnoliophyta Division may also be called Angiosperms or flowering plants, they include grasses, palms, oak trees, orchids and daisies. Magnoliophyta is the only division that contains plants with true flowers and fruits, and all plants in this division use those flowers and fruits to reproduce. It is not known exactly when flowers first appeared, but definitely by 125mya and probably as far back as 160mya.

Flowers have proved to be an extremely successful adaptation, and despite its recent appearance, Magnoliophyta is by far the largest and most diverse plant division with over 250,000 different species and 500 families. (For comparisons to other divisions and their sizes see here)

Leaf Variety in Magnoliophyta

Leaf Variety in Magnoliophyta


In Magnoliophyta, flowers replaced the cones of more primitive plants, as a means of reproduction. Some flowers are brightly coloured, have a scent or produce nectar in order to entice animals to pollinate them, but others use wind or water and, having no need to draw attention, are barely noticeable.

Flower Variety in Magnoliophyta

Flower Variety in Magnoliophyta

Flower Variety in Magnoliophyta

Flower Variety in Magnoliophyta

Fruit and what that really means…

All plants in this Division produce fruits of some kind, even though what they produce may not be easily recognised as fruit. The botanical definition of a fruit is a matured ovary (the ovary is the female part of the flower that contains the ovules which become the seeds once fertilised), this includes peppers, tomatoes, aubergines, nuts, peas, wheat grains, but not apples or rhubarb. There is another meaning for the word fruit, which is culinary and refers to a sweet part of a plant that is eaten, this is the more familiar term and includes rhubarb and apples, but not tomatoes and nuts, etc. ‘Vegetable’ is only a culinary term, referring to parts of a plant used in savoury cooking, it may refer to any part of the plant: leaves (lettuce) flower buds (broccoli), stems (celery) or roots (carrots) and has no botanical equivalent.


Being such a large and interesting division means that the classification of Magnoliophyta has received more attention and undergone more changes than any other division.

How Many Flowering Plants Are There?

It was believed for some time that there were over 400,000 flowering plants, but it turns out that many species of plant (not known as yet how many) have actually been named twice or even three or four times. The binominal naming system (using two Latin names, eg Helianthus annuus) was designed to make plant naming international and straightforward, but with people all over the world discovering and naming plants and no comprehensive way of cross referencing them, we have ended up with a lot of confusion. Now, partly due to the international power of the internet, serious attempts are being made to work out how many actual species there are and to remove duplications. The Plant List is a collaboration between a number of botanical gardens around the world and has an impressive online collection of these names.

DNA Alters The Family Tree – Cronquist to APG III

Before DNA testing was possible (or DNA was known about) plants were collected into families, classes and orders according to detailed studies of how they looked.

Over the past few hundred years there have been many different classification systems, but one of the most commonly used and straightforward was the Cronquist System, devised in 1968. This System grouped plants into families, with the families grouped into orders, orders then grouped into sub classes and sub classes grouped into two classes: monocotyledons and dicotyledons. However, with genetic testing, it has been found that many of these groupings were wrong. A new system, called APG (Angiosperm Phylogeny Group), was introduced in 1998, but has subsequently been updated twice since then and will no doubt change in the future.

Frustratingly, what was once a very neat and straightforward system of classification has become an unwieldy, confused and messy system, because nature is never neat. The new system, called APG III, does not use classes and subclasses, instead it groups orders within clades, nested within other clades, nested within other clades; with some families not fitting into any clade at all.

The following diagrams are an attempt to show the changes in a simple manner, using images of plants to represent different orders and showing how those orders have altered their connection to others. It is clear that some assumptions were completely wrong, for example some dicots are more closely related to monocots than other dicots; the buttercup is not kindred with the water lily; cacti are more connected to Heuchera than originally thought and oak trees are closer to Euphorbia than London planes.

Cronquist system

Cronquist system

APG III System

APG III System

Key to Magnoliophyta plants

Key to Magnoliophyta plants

Note: I was unable to take photos of a tulip tree or Rhododendron in flower, so used photos I got online from here: Rhododendron and tulip tree


It was also fairly tricky to find all the necessary information about where plants appear in the Cronquist system, if anyone spots any faults, please contact me at the email to the right. Most of my information came from Wikipedia, and from here

To enlarge the key click the thumbnail

Anthurium and Ctenanthe - two flowering plants

Anthurium and Ctenanthe – two flowering plants