Plant families

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Plant families

When we think of plants we tend to think of garden and houseplants but in fact the term covers a much broader range of living organisms. Plants range from enormous trees to delicate flowers and grow in a huge variety of environments.

The Oxford English Dictionary defines a plant as: “exemplified by trees, shrubs, grasses, ferns and mosses, typically growing in a permanent site, absorbing water and inorganic substances through the roots and synthesizing nutrients in the leaves by photosynthesis using the green pigment chlorophyll.”

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Plant forms

Family and form are important to plants. Plants come in two main forms: vascular plants that grow in a wide variety of environments and non-vascular plants (includes algae, fungi, liverworts and mosses) that can only grow in a specific type environment.

Vascular plants have specialized conducting tissue that allows them to grow in a wide variety of places and to a greater size than non-vascular plants. Vascular plants are classified into families according to the way they bear seeds, structure of their flowers, fruits and other common features.

There are currently 1,064,035 scientific names of plant species in existence (new plants are regularly being discovered – although perhaps at a slower rate than in the 18th century).

There are 642 plant families and 17,020 plant genera that can be split into four categories:

  • Flowering plants (Angiosperms)
  • Conifers, cycads and allies (Gymnosperms)
  • Ferns and fern allies (Pteridophytes)
  • Mosses and liverworts (Bryophytes).

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Flowering plant (Angiosperm), Bladder campion (Silene cucubalus) © Ita McCobb

Flowering plants (Angiosperms)

Flowering plants are the organisms that most of us visualize when someone mentions plants. There are an estimated 352,000 species of flowering plants (Angiosperms).

Angiosperms are seed-bearing vascular plants. Their reproductive structures are flowers in which the ovules (that part of a seed plant that contains the female germ cell which, after fertilization, becomes the seed) are enclosed in an ovary. They are found throughout the world, from forests to mountain tops and grasslands to seashores and deserts.

These plants exhibit a huge variety of life forms, including trees, herbs, submerged aquatic plants, bulbs and epiphytes (a plant that grows non-parasitically on a tree or another plant).

Angiosperms include everyone’s favourite family of flowers – the rose (Rosaceae) – although the largest group of angiosperm plant families are orchids (Orchidaceae), daisies (Compositae) (said to have 23,000 subspecies) and beans (Leguminosae).

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Conifer, Black spruce (Picea marinana ) © Ita McCobb

Conifer, cycads & allies (Gymnosperms)

The word gymnosperm comes from the Greek word “gymnospermos” meaning “naked seeds”. Not surprisingly, Gymnosperms are seed-bearing vascular plants in which the ovules or seeds are not enclosed in an ovary.

There are around 1,000 gymnosperm species.

Gymnosperm seeds are produced on the exposed or uncovered surface of scales, on the leaf-like structures of cones or at the end of short stalks.

Most gymnosperms are trees. Typical examples are:

  • Palms (Cycadacea)
  • The smallest group, the ginkgos (Ginkogoaceae)
  • Yews (Taxus baccata family Taxaceae)
  • The largest group, the conifers (Cupressaceae).

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Hard fern (Blechnum spicant) © Ita McCobb

Ferns & fern allies (Pteridophytes)

Pteridophytes comprise around 13,000 species. They are vascular plants that have leaves (called “fronds”), roots and sometimes true stems. Tree ferns are Pteridophytes that have full trunks.

Many ferns from tropical rainforests are non-parasitic epiphytes; their water comes from the damp air, water-soaked terrain and host plant. There are also some purely aquatic ferns such as the water fern (Salvinia molesta) and mosquito ferns (Azolla family Salviniaceae).

Pteridophytes do not produce seeds or flowers but reproduce via spores.

Fronds of the largest species of ferns can grow up to six metres long!

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Moss sporophytes © Ita McCobb

Mosses & liverworts (Bryophytes)

There are around 20,000 species of Bryophyte.

The word bryophyte comes from the Greek words “bryon” meaning “tree-moss” and “phyton” meaning “plant”.

Bryophytes are small, non-vascular plants, such as mosses, liverworts and hornworts. They play an important role in regulating ecosystems because they provide a vital buffer for other plants that live close by and so benefit from the water and nutrients that bryophytes collect.

Some bryophyte species were among the first to colonize open ground and may well be the ancestors of the first plants on Earth.

Bryophytes are very good indicators of habitat quality as many plant species in this group are sensitive to levels of moisture in the atmosphere, which are lower in disturbed habitats because there is less shade.

Bryophytes do not have seeds or flowers but reproduce via spores.

Typical examples of bryophytes are hornworts (Anthocerotophyta), liverworts (Marchantiophyta) and mosses (Bryophyta), normally 1-10 cm tall, though the Dawsonia, the largest variety of which is found in New Zealand, Australia and New Guinea, can grow to 50 cm.

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References: Royal Botanical Gardens, Kew, London; Missouri Botanical Garden; Latin for gardeners, Lorraine Harrison, Royal Horticultural Society; International Botanical Congress (IBC); Gardeners’ encyclopaedia, Christopher Brickell ed., Dorling Kindersley; The Royal Horticultural Society.