What is a plant? Several basic questions are relevant in reviewing crop farming or plant agriculture as a science and practice. These include what is: agriculture, crop, weed, agronomy and horticulture, as well as the various factors and processes involved in plant growth and development. But equally necessary is to have a wide understanding of plants and their characteristics. This is so because crop production generally means plant production.
A plant is a multicellular, autotrophic living organism capable of photosynthesis, primarily adapted to living on land, and belongs to the kingdom Plantae. Planta or plantae is also the Latin word for plant, sprout, shoot, slip, young plant, seedling, sole and foot (Kidd 1957; Traupman 1995). This is one short answer to the question What is a plant? However, this will require a long elaboration. And this is not even an exacting answer because there are plenty more about what is a plant and what are those which are not plants.
Although there are aquatic plants, the main adaptation of plants is for living on land. In other words, they are primarily terrestrial dwellers. Living in water is only a secondary adaptation (Mader 1993).
In addition to Plantae, there are at least four other kingdoms (regnum) of living organisms (Moore et al. 2003). The other kingdoms with common acceptance are: Animalia (animals), Monera (bacteria, procaryotic), Fungi (fungi), and Protoctista (algae, protozoans and slime molds). Other organisms placed under kingdom Protoctista are those which do not fit into the other kingdoms.
The uses of plants are without limit and, if comprehensively enumerated, will not fit into this page. Nevertheless, their main use to man and other animals is as food. It can also be said that this is their ultimate function on Earth That is, to sustain or perpetuate life by providing energy and structural elements for use of other organisms.
With a few possible exception, present humans survive on diets that include plants or plant products. Cereals, root and tubers, grain legumes, and vegetables are common inclusions in staple foods. Ruminant animals forage on both grasses and non-grass plants; birds feed on fruits and grains; and bees thrive on nectar from flowering plants.
As to carnivorous animals and other heterotrophs, they derive their energy need and nutrition from plant-eating animals or from the animals that eat them. Even the most minute microorganisms derive their nutrition from food that originates from plants.
Without plants as the basis of food pyramid, all these secondary harvesters of solar energy will perish. Plants, therefore, are vital to life on Earth, particularly on land dwellers.
This is so because plants are able to photosynthesize. The process of photosynthesis is responsible for the generation of chemical energy needed by heterotrophic organisms, those which are incapable of utilizing directly the energy from the sun. In addition, photosynthesis produces oxygen which is essential to aerobic organisms including humans. Even when some plants are considered weeds in agriculture, they still perform these life-sustaining functions.
Aside from food, plants have plenty more uses. To mention a few, plants are used as or sources of materials for clothing, shelter, weapons, medicines, fibers, tools, fuel, furniture, ornaments, and animal feed. Many industrial products are derived from plants such as starch, sweeteners, vegetable oils, beverages and drinks, papers, and biofuels. The paper made into checks and money also originate from living plants.
Even dead plants find many uses. They have been converted into lumber, furniture and decors, or processed into charcoal, or used as posts, firewood and tools. Fossilized sources of energy such as coal and petrochemicals originate mainly from plants that lived in the past.
In addition, there are products derived from animals such as hides, feathers, horns, and bones. There can be none of these without the energy which these animals harvest from plants directly or indirectly or both.
The growing of plants is likewise exploited as a strategy to mitigate climate change. Click here to read relevant article.
There are many more ways of describing what is a plant. Here are some of their distinct characteristics:
1. Plants are multicellular organisms made up of eucaryotic cells. These cells also occur in other organisms except bacteria and cyanobacteria. Eucaryotic cells have membrane-bound organelles and a nucleus. They have chloroplasts that contain chlorophylls a and b and carotenoids; and cell walls which are rich in cellulose.
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2. Plants are generally autotrophic, oxygen-evolving photosynthesizers. They produce their own food (source of energy and carbon skeletons) through photosynthesis in which process oxygen is given off. Starch is the primary storage product of photosynthesis and their food reserve. Like algae, they are considered primary users of the the energy from the sun, the ultimate source of energy.
3. In contrast to other organisms, plants are non-motile. However, plants are not exactly immobile. They also exhibit some form of limited movement like the unidirectional movement of plant parts in response to light, or phototropism, and the folding of the leaves of Mimosa in response to touch.
4. The life cycle of plants consists of a distinct alternation of generations. This means that plants have both the haploid gametophyte which produces gametes and the diploid sporophyte. Depending on the species, the generations can occur simultaneously or successively. Except in the bryophytes, the dominant generation is the sporophyte.
5. Along with the sporophyte, plants have evolved the embryo, an immature diploid sporophyte that is attached to or surrounded by the gametophyte (Simpson 2010). Thus the term Embryophyta or embryophytes, also called land plants as distinguished from the former green plants which include the “green algae.”
6. Plants, like animals, possess an internal protective mechanism by which drying of the zygote is prevented. This is accomplished by containing the zygote in an internal organ during development (click to read also Functions of Fruits). All plants possess adaptations which protect the embryo from drying out, a characteristic that is absent and which differentiates them from the green alga Ulva (Mader 1993).
7. Unlike animals, plants have no nervous system and organized muscle fibers.
1. Bryophyta (mosses, e.g. Polytrichum spp.)
2. Hepatophyta (liverworts, e.g. Marchantia spp.)
3. Anthocerotophyta (hornworts, e.g. Anthoceros spp.)
4. Psilotophyta (whisk ferns, e.g. Psilotum spp.)
5. Lycopodophyta (club mosses, e.g. Lycopodium spp. and spike mosses, e.g. Selaginella spp.)
6. Equisetophyta (horsetails, e.g. Equisetum spp.)
7. Pteridophyta (ferns, e.g. bracken fern or Pteridium aquilinum)
8. Ginkgophyta (maidenhair tree, i.e., Ginkgo biloba)
9. Cycadophyta (cycads, e.g. Cycas spp.)
10. Pinophyta (conifers, e.g. Pinus spp.)
11. Gnetophyta* (gnetophytes, ex. Gnetum gnemon)
12. Anthophyta or Magnoliophyta (angiosperms or flowering
*According to Simpson (2010), the exact placement of the gnetophytes (Gnetophyta, Gnetales or Gnetopsida), that is, whether separate or under Coniferae (or Pinophyta) is still contested.
Nonvascular plants are those without vascular tissues responsible for the internal transport systems that have evolved in later plants. They consist of the bryophytes under the divisions Bryophyta, Hepatophyta and Anthocerotophyta. All other plants are vascular plants.
Nonvascular plants have no true roots but instead have root-like rhizoids. They absorb and transport water, minerals and organic nutrients by diffusion. A moist environment is therefore essential to their growth and development. (Practical application: The use of moss as a groundcover in bonsai culture and presentation serves dual purposes: besides the aesthetic enhancement, moss is also a good indicator of insufficient soil moisture. They easily turn brownish when soil water is limiting.)
In contrast, the vascular plants have evolved vascular tissues consisting of the xylem and phloem. The xylem transports water and minerals and the phloem transports organic nutrients throughout the plant body.
Plants either possess the natural ability to produce seeds or not. The seed is a plant organ and has been defined as an embryo surrounded by nutritive tissue and enveloped by a seed coat (Simpson 2010).
The seedless plants consist of both the nonvascular plants and vascular plants. Their mode of reproduction is by spore. The nonvascular seedless plants are the bryophytes (divisions Bryophyta, Hepatophyta and Anthocerophyta) while the vascular seedless plants consist of the ferns and allies (divisions Psilotophyta, Lycopodophyta, Equisetophyta and Pteridophyta).
On the other hand, the seed plants or spermatophytes have evolved the ability to produce seeds and consist of the plants informally called gymnosperms and angiosperms.
KIDD DA. 1957. Collins Gem Latin Dictionary. 1989 reprint. London and Glasgow: William Collins Sons & Co. Ltd. p. 250.
MADER SS. 1993. Part 3: Biology of Evolution and Diversity. In: Biology. 4th ed. Dubuque, Iowa: Wm. C. Brown Publishers. p. 297-472.
MOORE R, CLARK WD, VODOPICH DS. 2003. Botany. 2nd ed. New York, NY: McGraw-Hill Companies, Inc. 919 p.
SIMPSON MG. 2010. Plant Systematics. 2nd ed. Burlington, MA: Elsevier Inc. 740 p.
TRAUPMAN JC. 1995. The Bantam New College Latin & English Dictionary. New York, NY: Bantam Books. p. 318,594.
(Ben G. Bareja Oct. 2012)