The grouping of crops probably evolved with their domestication from wild ancestors, and concurrent selection for various uses to satisfy man’s needs for food and non-food purposes. This may account for their initial classification based on their economic importance in terms of the consumable parts of the plant.
Thereafter, the development of scientific agriculture gave rise to other methods, especially the classification based on nomenclature or binomial system. The use of botanical characteristics is of fundamental scientific significance in crop identification.
However, grouping based on economic (agronomic) role in human survival, ranging from food production, fibre supply especially for clothing, and industrial conversion of crops as raw materials to useful products and by-products, is by far the most popular basis for classifying crops, and also the most emphasized in the literature.
1. Classification Based on Nomenclature i.e. Binomial System
This scheme was formulated by Linnaeus, as a universally acceptable system of naming plants, animals, and minerals. This scheme groups plants into divisions, sub-divisions, classes, orders, families, genera, species, subspecies, and varieties.
Plants are either seed-producing (division Spermatophyta) or non-seed producing (viruses, bacteria, algae, fungi, lichens, mosses and liverworts, ferns, and horsetails). Spermatophytes are the most highly evolved and structurally complex plants and are separated into two classes, namely
i. Class Gymnospermae
These are plants that produce naked seeds, usually in cones that constitute the female organs. They often exhibit structural adaptations to reduce water loss. It comprises the others: Ginkgoales, Coniferales (the most important to horticulture), Cupressaceae, and Taxaceae.
ii. Class Angiospermae
These plants produce seeds that are protected by fruits and have flower structures as a means of sexual reproduction. Many of the families are important to horticulture, both as crop plants and weeds. The sub-classes are:
Sub-class Monocotyledonae- contains some horticultural families such as Liliaceae (tulips, onions), Amaryllidaceae (daffodil family), Iridaceae (Iris family), Graminae (all grass species).
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Sub-class Dicotyledonae- has many more families significant to horticulture, including the Compositae (e.g. chrysanthemum); Cruciferae (e.g. cabbage, Brussels sprouts); Rosaceae (e.g. apples, pear, rose); Leguminosae (Fabaceae, e.g. pea and bean family, pod-producing and contain starchy seeds), Solanaceae (e.g. potatoes, tomatoes), Cucurbitaceae (e.g. cucumbers, marrows), Labiatae (mostly culinary herbs), Cactaceae and Crassulariaceae (widely used as ornamentals); and Aceracea (e.g. common sycamore, ornamental Acers); and Salicaceae (e.g. willows).
Generally, plants are identified through the composed generic and specific names viz. chrysanthemum as Chrysanthemum (genus) morifolium (species).
2. Classification Based on Botanical Characteristics
Crop plants are grouped into monocotyledonous species (species having only one seed-leaf or cotyledon) and dicotyledonous species (species having two cotyledons in their seeds). Typical examples of monocots are cereals (maize, barley). Dicot crops include legumes/pulse crops (cowpea, soybean, alfalfa).
3. Classification Based on Duration of Crop Growth
This scheme groups crops into:
i. Annual Crops: These are crops that complete their life cycle in one year e.g. maize, cowpea, tomato, soybean, amaranthus okra, yam, etc.
ii. Biennials Crops: These are crops that complete their life cycle in two years e.g. cassava, brassicas, etc.
iii. Perennial Crops: These are crops that live for more than two years or year-in- year-out e.g. cacao, rubber, plantain/banana, oil palm, ginger, sugar-cane, etc.
4. Classification Based on Economic/Agronomic Importance
This scheme groups crops variously by the use of the parts consumed directly, their food and non-food nutrients, or indirectly by man for the provision of food, fibre, and other essential purposes. The groups include:
1. Starch Plants – These are crops grown primarily for their high calorific value in human diets as starch, carbohydrates, and sugar.
However, a larger part of the crop is processed into pure starch used in industries for sizing threads and cloth finishing, in pharmaceutical preparations, as a powder in cosmetics and medicines, as drying material, for glues (pastes), filters (as dextrin, sugar, dextrose, maltose, and fructose syrup).
Examples include cereals (maize, rice), pseudocereals (rye, oats), roots (cassava, Irish potato) and tubers (yams, sweet potato), and others (breadfruit, banana).
2. Sugar Plants – These crops are essentially grown for the production of sugar. Sugar is used in sweetening drinks, and baked foods, enhancing flavor in many food preparations, the canning industry, and the production of sweets.
Examples are sugar- cane, sugar beet, and others such as sugar palm, sugar-rich Sorghum, and agaves.
3. OilPlants –These are crops grown essentially for their oil supply. The production of oil plants ranks third in world production in terms of value, after starch plants and fruits, and higher than that for beverages and stimulants, and sugar.
Over 90% of the plant oils are used as an edible oil in human diets and the remainder for technical purposes. As edible oil, they supply vitamin A (e.g. carotenoids), vitamin E (tocopherol) and ergosterol (pro-vitamin D2), phospholipids, and sterols, and are used for drying fat.
Technically, they have varied uses including making soaps and detergents, as suppositories, and for lighting and burning. Plant oils include cooking oils (e.g. oil palm, soya oil), vegetable butter (e.g. cacao butter, shea butter), salads and margarine (e.g. sesame), and technical oils (e.g. Brassica juncea, coconut, linseed).
4. Protein-Providing Plants – These are crops desired for their protein value in human diets. Plant proteins mostly contain insufficient quantities of essential amino acids and therefore, are inferior to animal proteins.
However, they contribute a significant proportion of the world’s protein supply, especially cereals which contribute the largest proportion of the plant protein supply, followed by oilseeds, especially soybean. The “true” protein plants, the pulses crops (cowpea, soybean, pigeon pea, lentils, lupins), contribute only a small portion.
5. Vegetables – This group of plants is found in a variety of crop species, including cereals, seasonings, roots and tubers, legumes, fruits, and oil seeds. Vegetables are of great significance in the tropics and subtropics as food (supplying minerals, vitamins, and proteins) and as commercial products of market gardening.
Leafy vegetables are important sources of protein in the tropics. Examples of vegetables are leaf vegetables such as jute mallow, cabbage, spinach, lettuce, fluted pumpkin), fruit vegetables (tomato, cucumber, melon, gourd, eggplant, calabash, peppers, okra), and root vegetables (carrots, turnips, radish, beetroot, clove, sweet potato).
Other crops are sweet corn, asparagus, cauliflower, cocoyam, sweet potato, Irish potato, and seed legumes such as beans.
6. Fruits –These are crops grown and then, eaten for their refreshing and aromatic taste which gives pleasure to human, and even, animal consumers. They have a very important role in human nutrition and health.
They contain substances that regulate and stimulate food digestion, organic acids which act as mild laxatives or diuretics, and pectins and phenolic compounds, which regulate pH in the intestine, thereby normalizing the intestinal flora.
The compounds also act bacteriostatically, and virostatically and detoxify heavy metals. Examples are many fresh fruits (cashew, mango, grapes, avocado) and dry fruits (dates, figs, muskmelon, watermelon).
7. Nuts – These are fruits with dry shells (hazel nuts), parts of fruits (coconuts, walnuts), or seeds (Brazil nuts), which, due to their pleasant flavor, are eaten raw, roasted, or cooked. Thus, they are widely used in confectionery as aromatic agents and other foods.
They have a high nutritional value in respect of their energy and protein contents. Other examples are peanuts, chickpeas, sunflower seeds, cashew nuts, chestnuts, almonds, macadamia, and pecan nut.
8. Beverages, Masticatories, and Stimulants – These are plants that contain chemical compounds that increase the physical and mental effectiveness, quench thirst and hunger, break down psychic inhibitions, or produce fantastic dreams.
Some stimulants are only for consuming their aroma (caffeine-free, coffee substitutes, and nicotine-free tobacco). The active constituents (caffeine, theobromine, morphine, codeine, cocaine, arecoline, glucoside) of some stimulants are used in other areas of activity. Examples include coffee, tea, cacao, kola nuts, betel-pepper, tobacco, quinine, and opium.
9. Spices – These include all culinary herbs, seasonings, and condiments of vegetable origin. They are used for improving the taste of foods in the food industry, beverages industry, and technical raw materials.
They serve as medicinal plants (e.g. Cinchona spp.) and stimulate the flow of saliva, and the secretion of enzymes (protein and fat digestion) in the stomach-intestinal systems. Other examples are mustard, black pepper, onion, clove, pepper, vanilla, ginger, hop, sweet basil, garden thyme, and nutmeg.
10. Medicinal Plants – To the agriculturalist, the most important medicinal plants are those containing active constituents which cannot be synthesized by the pharmaco-chemical industries (or only at a very high cost), and which cannot be substituted by other compounds e.g. alkaloids (e.g. Cinchona, Datura, Papaver), Digitalis glycosides, flavonoids, and mucilages.
They include many spices and stimulants. However, they are of very little economic importance to the cultivator but great importance to the pharmaco-chemical industries.
11. Essential Oils – These are volatile substances. They, therefore, have distinct odors and thinness for paints (turpentine oil, camphor oil). They are used in the perfume industry and for cosmetic articles such as soap, ointments, and powder; for technical purposes as solvents and floatation agents, for masking smells in plastics, artificial leather, rubber, floor wax, and household sprays.
Also, they are valuable in pharmaceutical preparations because of their specific effects (anise oil, fennel oil, camomile oil), their antiseptic properties and to improve taste (toothpaste). Examples are citrus oils (peel oils from lemon, lime, mandarin, grapefruit, and bergamot), leaf oils (petitgrain oils), flower oils (orange flower oils, neroli oil from bitter orange), and grass oils (citronella oil, lemon grass oil, vetiver oil, palmarosa oil).
12. Fibre Plants – These are plants grown for use in weaving a great variety of household and commercial materials.
They are highly valued in textiles and curtains (flax, sunn hemp, cotton), packaging and paper materials (particle boards and composites), floor coverings (jute, kenaf, agave fibre, door mats from coir), making tear-proof paper (halfa grass), brooms, basket-work and building materials (piassava, sorghum, agave fibres, raffia, and other palm fibres).
13. Elastomers – These are natural and synthetic polymers with rubber-elastic properties. The natural elastomers are polyisoproprenes which are either caoutchouc (with highly elastic properties or gutta-percha, with slight elasticity but having strongly thermoplastic properties (softening at high temperatures, and hardening at room temperature).
Polyisoprene is concentrated in the vacuoles of cells, especially in latex tubes. Elastomers are very useful in making tools (e.g. knife handles), natural rubber (vehicle tyres, packings, break pads, insulators, driving belts, golf balls), and caulking material for boats and containers, and for making toys, chewing gum, and wax.
Like milk, elastomers are used for coffee, manioc diseases, etc., and are of medicinal importance. Examples of plants are rubber (caoutchouc), guayule, and gutta sundeck.
14. Resins and Gums – Resins are complicated mixtures of diterpenes with volatile terpenes (e.g. pinene), coniferyl esters, gums, and aromatic compounds. The resins are not always separable from gums but most burn well.
In the plants, they are exuded in a liquid state into secretory ducts in the bark and the wood, but most often, the resin flow is stimulated by wounding the bark of the wood. Most resins are obtained from wild trees or shrubs. The cultivated plant sources are Shorearobusta, arar tree, copaiba balsam, rattan palm, grass-tree gum, and sal tree.
The resins and gums are used as solvents (turpentine oil), as aromatics in perfumes, fumigation products, cosmetics (skin creams), and chewing gum, and techniques for making colophony, putty, picture and photograph lacquers, in cheap paints, incense, and salves and as a flavoring in lemonade, bakery, soaps, and sweets.
15. Tannin Materials – These are complex phenolic compounds with a large number of oxygen functions. They can exist as hydrolyzable tannins (oak gall, chestnut) or condensed tannins, made up of flavonoids, flavones, and catechins (e.g. mimosa bark).
They are found in the parenchymatic tissues of many plants, in usable concentrations, especially in the bark, wood of the trunk, roots, fruits, and galls. They are used for tanning leather through precipitation of protein which thereafter prevents swelling in water and resists rotting, medicinal benefits in pharmaceutical preparations (e.g. tara rubber), techniques for reducing the viscosity of drilling slurry from deep-drilling projects, and as a colorant (e.g. black colorant in Caesalpinia Spinosatara powder). Other examples are black wattle and a brown mallet.
16. Dyesand Colourings –These include plant pigments found in the plant cells, either in the plastids or in vacuoles. They belong to a variety of chemical groups such as anthocyanins (red pigments), betacyanins (betamin), some carotenoids (bixin from annatto, capsanthin from paprika, zeaxan from tomatoes, carthamin from safflower), yellows from the carotenoids xanthophylls, zeaxan and azafrin; curcumin from Curcuma longa and chlorophyll.
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The uses of dyes and colorings are beautifying the body, clothing (e.g. indigo), and home decorations, as colorant additions in food (rice dishes, manioc, fatty foods, fat-free foodstuffs, gums, starch preservatives), drinks, confectionery, and cosmetic industries, as aromatics for sweets, baking, lemonade, and chewing gum, for producing ink (e.g. C. sappan in India) and for dyeing leather, hair, and fingernails.
Other examples are Bixa orellana, Acacia catechu, Indigofera arrecta, Lawsonia inermis and Escobedia scabrifolia.
17. Pesticides – These are plants that contain natural protective compounds (pyrethrins from pyrethrum, rotenone from derris and cube, nicotine from tobacco, anabasine from Anabasis aphylla; alkaloids such as wilfordine from Thundergod vine and ryanodine from ryania) against insect pests, ectoparasites, etc. Other plants have molluscicidal and nematicidal properties.
18. Waxes – These are fatty substances, characteristically containing esters of long-chain fatty acids with long-chain primary alcohols, bivalent alcohols, hydroxyl fatty acids, paraffin, and resins, as unwanted impurities. Most of the waxes of economic importance are exudates from the epidermis of leaves, stems, and fruits.
The most important sources are carnauba, candellia, jojoba, and Japanese wax. Wax is also extracted as a by-product from the processing of sugar-cane, rice, and sorghum (sugar sorghum, grain sorghum) from the production of raffia fibres and the bark of Douglas fir.
Waxes are used for coating fruits (citrus, apples), in cosmetics (especially in lipsticks), textile, leather, and paper industries, in making candles, matches, painting materials, carbon paper, chewing gum, polishing materials for floors, furniture, cars and shoes, and in pharmaceutical preparations.
19. Forage and Pasture Plants – These are plants produced for feeding various domestic animals, including silkworms, bees, and lac insects. The plants include tropical legumes and grasses, fodder melons (Citrullus lanatus), fodder gourds (Cucurbita pepo), and fodder beets (Beta vulgaris), Japanese fodder radish (Raphanus sativus), fodder kale (Brassica spp.) and fodder sorghum and Pennisetum americanum.
Tropical pasture fodder crops include Southern gamba (Andropogon tectorum), molasses (Melinis minutiflora), pigeon pea (Cajanus cajan), calopo (Calopogonium mucunoides), and Centro (Centrosemapubescens). Other plants are maize, grain sorghum, cassava, cowpea, hyacinth bean, breadfruit, and dates. Bees are fed on pawpaw leaves, and silkworms and lac insects on pigeon pea.
20. AncillaryPlants –These are plants cultivated in agriculture and forestry not for direct benefits to man but for their capacity to encourage the growth and development of other plants. These benefits arise from soil improvement, ground covering, prevention of soil erosion, windbreaks, shade provision, support for climbing plants, and “living fences”.
Examples of the plants are Grevillea robusta, black pepper, belch pepper, vanilla, Centrosema pubescens, Gliricidia, vetiver, acacia, and cashew.
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