What is a plant? Several basic questions are relevant in reviewing crop farming or crop 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).
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, would not fit into this page. But their primary importance is that they are vital to life. They are a basis of the food pyramid for living organisms. They are the source of energy for most heterotrophic organisms. They likewise generate oxygen which is essential to aerobic organisms including humans and which makes the atmospheric conditions favorable for life. Even when some plants are considered weeds in agriculture, they still perform these life-sustaining functions.
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.
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. This is the most important characteristic that describes what is a plant in relation to crop farming and in perpetuating life on Earth (click to read What is Photosynthesis). Starch is the primary storage product of photosynthesis and their food reserve.
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.”
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).
6. Unlike animals, plants have no nervous system and organized muscle fibers.
In addition, plants are classified into different grouping based on various criteria. Some general classifications are provided below.
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)