Plant genealogy: a closer look at plant families
With such an extensive variety of plants in the kingdom Plantae, it is sometimes hard to imagine that plants that look so different can be related genetically, or even classified into families. If we look a little closer at the distinguishing features of plants such as leaf shape, plant height and flower form, we can piece together information that can tell us about their genealogy, how they have evolved and which of their plant companions they share traits with.
With more than a quarter of a million species of flowering plants (angiosperms) alone, it would be impossible to know every plant, so grouping plants by family helps us to connect the dots between the preferences, habits and uses of plants in our own gardens and around the world. Although botanists initially differentiated plants by their characteristics, advancements in genetics means that more recent classification relies on DNA. This means plants sometimes change family or name, following new discoveries about their genetic relationships. There are hundreds of known plant families, but here are a few notable ones that can help us make some connections.
Asteraceae (daisy family)
This the largest plant family on earth, with more than 1,620 genera and 23,600 species, including daisy, sunflower, lettuce, dandelion, artichoke, as well as many favourite garden ornamentals such as chrysanthemums, cosmos, dahlias, marigolds and zinnias. Plants in the family Asteraceae can usually be identified by their distinct flower shape with a daisy-like formation of petals. The flower heads are actually composed of many small flowers called inflorescences, that resemble individual flowers when looked at closely. Wormwood (Artemisia), a plant in this family, has been used as a herb in traditional medicine, but is also the source of the poisonous oil that gives the liqueur absinthe its distinctive flavour.
Poaceae (grass family)
The Poaceae (grass) family could be considered the most economically important to humans and includes cereal crops such as rice, corn, oats and wheat, as well as bamboos. This family includes more than 10,550 species, within 600 genera of grasses and 115 genera of bamboo, existing on every continent, in almost every part of the planet. Plants in the grass family form around 20 percent of the world's vegetative cover, and are incredibly resilient to cutting and trampling, making them so successful in our gardens as lawns, as well as producing much of the world’s food supply.
Fabaceae (legume family)
The Fabaceae family consists of around 19,500 species and is the third largest family among flowering plants, after Asteraceae (daisy family) and Orchidaceae (orchid family). Fabaceae includes plants such as soybean, lentil, garden pea, peanut and alfalfa, as well as lupin and broom. Plants in this family range from tiny clovers to stately trees such as mimosa, and the fruit from these plants is typically a legume, or pod, which splits open as it dries, releasing the seeds. Many legume plants have nitrogen fixing abilities, often with a fibrous tap root with nodules that form a relationship with nitrogen fixing bacteria (rhizobia) in the soil, and are sometimes used as a cover crop to improve the quality of soils.
Rosaceae (rose family)
The rose family (Rosaceae) covers 3000 species, including our much-loved garden roses, as well as popular cultivated varieties of great economic importance such as apple, pear, cherry, plum, almond, blackberry, raspberry, strawberry, peach and hawthorn. Other plants in this family are used as gardeners' favourites, such as crab apples for their beautiful spring blossoms or rowan trees for their bright red berries. Many species like the beloved rose, have delightfully scented flowers, which in this family are typically symmetrical and cup shaped, sometimes being bred to have double blooms or even with no petals like we see in the plant Sanguisorba.
Asparagus, agave, lily-of-the-valley, English bluebell and the grape hyacinth are all examples of plants in this extensive family. It wasn’t always this way though, until what used to be a minor family was greatly expanded to over 2,250 species, including trees such as Dracaena and Yucca, and others like Hosta and Hyacinthus. Fibres from the species Agave are used by people to make paper and textiles, as well as the liquor tequila (Agave tequilana). Some much loved house plants, such as mother-in-law’s-tongue (Sansevieria), dragon tree (Dracaena) and spider plant (Chlorophytum) are all part of the Asparagaceae family.
Solanaceae (nightshade family)
The Solanaceae or nightshade family, includes some troublesome characters such as the poisonous jimsonweed, tobacco and belladonna, as well as the friendlier favourites such as tomatoes, potatoes, aubergines and some garden delights such as brugmansia and petunia. This family is most abundant in tropical regions of Latin America, and many species of the family are narcotic, so contain alkaloids that depress the central nervous system and are toxic in excess. Some alkaloids from these plants have been used medicinally in small doses to treat sea sickness and vertigo, and liquid taken from the Atropa species was used by Italian women as eye drops to dilate the pupils, hence the name ‘belladonna’ meaning ‘fair-lady’.
Connecting the dots between plant families can be very rewarding and help us to apply similar principles between plants of the same family in the garden. When attempting to identify a plant by its family, gather as much information as possible, making sketches or taking photos where you can. Look at all features of the plant, leaves, stems, shoots and buds, to help you identify, and learning key characteristics of the largest plant families can be helpful.