Important crop plant families
The crop plant families every agriculture student must know: grasses, legumes, Solanaceae, crucifers, cucurbits, Malvaceae and more, with key features.
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Plant taxonomy is the science of identifying, naming and classifying plants. For agriculture, the family is the most useful level. Crops in the same family share flower and fruit structures, often the same pests and diseases, and sometimes the same weeds. That is why rotations alternate families: following tomato with potato, both Solanaceae, carries over bacterial wilt and late blight, while following it with a cereal breaks the cycle.
Each plant has a two-part binomial name, a system established by Linnaeus in 1753. The genus is capitalised, the species epithet is not, both are italicised, and the name is often followed by the author: Oryza sativa L. Names are governed by the International Code of Nomenclature for algae, fungi, and plants (ICN). Family names end in -aceae.
Eight families keep older traditional names that are still allowed: Gramineae (Poaceae), Leguminosae (Fabaceae), Cruciferae (Brassicaceae), Compositae (Asteraceae), Umbelliferae (Apiaceae), Labiatae (Lamiaceae), Palmae (Arecaceae) and Guttiferae (Clusiaceae).
Classification systems
| System | Type | Notes |
|---|---|---|
| Linnaeus (1753) | Artificial | Based on the number of stamens; a "sexual system" |
| Bentham and Hooker (1862–1883) | Natural | Used in Indian herbaria and many textbooks |
| Engler and Prantl (1887–1915) | Phylogenetic (early) | Started from simple flowers |
| Hutchinson (1926, 1934) | Phylogenetic | Split woody and herbaceous lines |
| APG IV (2016) | Molecular phylogenetic | Based on DNA; the current scientific standard |
Bentham and Hooker divided seed plants into dicots, gymnosperms and monocots. The Angiosperm Phylogeny Group (APG) system uses DNA sequences, and it has moved some families. For example, the old Tiliaceae, Bombacaceae and Sterculiaceae are now part of Malvaceae, which is why jute (Corchorus) and cocoa (Theobroma) now sit in the cotton family.
Monocot families
Poaceae (Gramineae): the grass family
Poaceae is the most important family for food. Rice, wheat, maize, barley, sorghum, the millets, oats and sugarcane all belong to it, and they supply more than half the calories people eat. Grasses have:
- hollow, jointed stems (culms) with solid nodes;
- leaves with a sheath, blade and ligule;
- tiny flowers in spikelets, enclosed by glumes, lemma and palea, with lodicules instead of petals;
- usually three stamens and a feathery stigma suited to wind pollination;
- a one-seeded fruit called a caryopsis, in which the seed coat is fused to the fruit wall.
Other monocot families
- Musaceae includes banana and plantain. These are giant herbs whose "trunk" is a pseudostem of rolled leaf sheaths.
- Arecaceae (Palmae) includes coconut, oil palm, date palm and arecanut.
- Liliaceae and Amaryllidaceae include onion and garlic, now placed in Amaryllidaceae.
- Zingiberaceae includes ginger, turmeric and cardamom.
Dicot families
Fabaceae (Leguminosae): the legume family
Fabaceae is the third-largest plant family and the second most important for food. It includes all the pulses, groundnut, soybean, berseem, lucerne and trees such as Prosopis and Acacia. Most members form root nodules with Rhizobium bacteria and fix nitrogen.
Legumes have alternate, often compound leaves, and a fruit called a legume or pod. The old family is divided into three subfamilies:
- Papilionoideae (Faboideae) has butterfly-shaped flowers with a standard, two wings and a keel, and diadelphous stamens (9 + 1). Most crop legumes belong here.
- Caesalpinioideae includes tamarind, Cassia and gulmohar.
- Mimosoideae has small, regular flowers in heads, as in Acacia, Prosopis and Mimosa. APG now treats it as part of Caesalpinioideae.
Solanaceae
Solanaceae includes tomato, potato, brinjal, chilli, capsicum and tobacco. Its flowers have five fused petals, five stamens attached to the petals, and an obliquely placed two-chambered ovary. The fruit is a berry, or a capsule in tobacco. Many members contain alkaloids such as solanine and nicotine.
Brassicaceae (Cruciferae)
Brassicaceae includes mustard, rapeseed, cabbage, cauliflower, radish and turnip. Its flowers have four petals arranged in a cross and are tetradynamous: four long and two short stamens. The fruit is a siliqua, or a silicula if short. The family's pungency comes from glucosinolates.
Cucurbitaceae
Cucurbitaceae includes cucumber, pumpkin, watermelon, muskmelon and the gourds. These are climbers with tendrils and usually separate male and female flowers. The fruit is a pepo, a berry with a hard rind.
Malvaceae
Malvaceae includes cotton, okra, roselle and, under APG, jute and cocoa. Its stamens are monadelphous, fused into a tube around the style. There is often an epicalyx below the calyx, and the fruit is usually a capsule. Cotton lint is a hair on the seed coat.
Asteraceae (Compositae)
Asteraceae is the largest flowering plant family. It includes sunflower, safflower, niger, lettuce, chrysanthemum and marigold, as well as the weed Parthenium. What looks like one flower is a capitulum: a head of many tiny florets, with ray florets at the edge and disc florets in the centre. The anthers are fused (syngenesious), and the fruit is a cypsela. A sunflower "seed" is really a fruit.
Other dicot families
- Rutaceae includes citrus and has gland-dotted leaves; its fruit is a hesperidium.
- Rosaceae includes apple, pear, peach, plum, strawberry and rose. Its fruits vary: the apple is a pome (a false fruit), peach and plum are drupes, and the strawberry is an aggregate.
- Anacardiaceae includes mango and cashew, which are drupes with resin canals.
- Apiaceae (Umbelliferae) includes carrot, coriander, cumin and fennel, with flowers in umbels and a fruit called a cremocarp.
- Euphorbiaceae includes castor, cassava and rubber, which have milky latex.
- Lamiaceae (Labiatae) includes mint, basil (tulsi) and the weeds of the mint family. Its members have square stems and opposite leaves.
Using families in crop rotation
A farmer grows tomato, followed by chilli, then potato. All three are Solanaceae, so they share bacterial wilt (Ralstonia), root-knot nematodes and several viruses, and the pressure builds each season. Rotating with maize (Poaceae), then cowpea (Fabaceae), breaks the disease cycle, and the legume adds nitrogen as well. Families are the first thing to check when planning a rotation.
Key points
- Taxonomy covers identification, nomenclature and classification. Linnaeus introduced binomial names, and family names end in -aceae.
- Bentham and Hooker is the classic natural system; APG IV (2016) is the modern DNA-based system.
- Poaceae has culms, ligules, spikelets and a caryopsis. It supplies the cereals, millets and sugarcane.
- Fabaceae has pods, root nodules and, in Papilionoideae, diadelphous stamens and papilionaceous flowers.
- Solanaceae has an oblique ovary and berries; Brassicaceae has cruciform, tetradynamous flowers and a siliqua.
- Malvaceae has monadelphous stamens; Asteraceae has a capitulum and cypsela; Cucurbitaceae has tendrils and a pepo.
What the exams ask
- Give the family of a named crop, or name crops in a given family.
- State the diagnostic features of Poaceae, Fabaceae, Solanaceae, Brassicaceae and Malvaceae.
- Give the old and new names of the eight conserved families.
- Name the fruit types of major crops (caryopsis, legume, siliqua, pepo, hesperidium, pome, cypsela).
- Compare Bentham and Hooker with the APG system.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why is groundnut in the same family as peas?
Groundnut has the same papilionaceous flower, compound leaves, root nodules and pod as other legumes. Its unusual trait is geocarpy: after fertilisation, a stalk called the peg pushes the young pod into the soil to mature underground.
Why does family matter for pest management?
Related plants share chemistry, so pests and pathogens adapted to one species often attack others in the same family. Diamondback moth feeds across the crucifers, and fruit flies attack most cucurbits. Weed control also depends on this: herbicides that are selective for grass crops often fail against grassy weeds from the same family.
Is a potato a root?
No. Potato is a stem tuber: it has nodes (the "eyes") with buds and scale leaves, and it forms on underground stems called stolons. Sweet potato, from Convolvulaceae, is a true root tuber.
Sources
- Simpson, M. G. Plant Systematics, 3rd edition. Academic Press, 2019.
- Angiosperm Phylogeny Group. "An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG IV." Botanical Journal of the Linnean Society 181, 1–20 (2016).
- Judd, W. S., Campbell, C. S., Kellogg, E. A., Stevens, P. F. and Donoghue, M. J. Plant Systematics: A Phylogenetic Approach, 4th edition. Sinauer, 2016.
- Turland, N. J. et al. (eds) International Code of Nomenclature for algae, fungi, and plants (Shenzhen Code). Koeltz Botanical Books, 2018.
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