Plant science

Categories: Disciplines; history of plant science

Taxonomy

One of the basic subdisciplines of plant science and life science in general, taxonomy (also known as systematics) is the study of relationships and organization of plant species. The great diversity within the plant kingdom requires a system by which plant species are named and classified. The modern system is a modification of the system first established in the eighteenth century by the Swedish botanistCarolus Linnaeus. Each species is placed into a hierarchy of groups that indicate its similarity and dissimilarity to other species. These categories (taxa) are from the most to the least inclusive: domain, kingdom, phylum, class, order, family, genus, species.

The species is the most natural and fundamental unit. Similar species are grouped into a genus, similar genera into a family, families into orders, orders into classes, classes into phyla, and phyla into the kingdoms typically studied in plant science courses: true plants, or Plantae, Fungi, and Protista (which include many unicellular organisms and algae). Each species is given a scientific name which includes the genus name followed by the species name. An example is the scientific name of the dwarf crested iris: Iris cristata.

Morphology

Morphology includes the study of the general structure of plants. Morphologists study the parts of a plant and how they are arranged and function. For example, when a seed of an angiosperm (flowering plant) germinates, the radicle of the seed embryo develops downward to form a root system. Growth in the length of the root occurs within the meristem (region of cell division). Branch roots form due to the activity of pericycle cells within the root. Some epidermal cells develop root hairs as extensions of the cells. The shoot system, which includes the stem and leaves, develops from the epicotyl of the seed embryo. Stems are often branched, allowing for the attachment of leaves in such a manner as to permit their maximum exposure to sunlight.

Also included in the study of morphology are the reproductive parts of plants. The pollination of flowers causes the ovary of the flower to mature into a fruit. At the same time, the one or more ovules inside the ovary become seeds.

Anatomy

Anatomy is the study of plant tissues. New plant cells are formed within meristems. There, the cells begin the process of becoming specialized (differentiated) for a particular function. As a result, three categories of plant tissues are formed: dermal, vascular, and ground.

Dermal tissues, which form protective coverings, include the epidermis, which covers all parts of a young plant, and others that develop as a plant matures. The periderm commonly replaces the epidermis and includes tissues found in bark.

Vascular tissues, derived from cambium cells within the meristem, conduct water and dissolved compounds within a plant. They include xylem and phloem.

Ground tissues are the less specialized tissues. Among their functions are storage, support, and photosynthesis. A common type is parynchema.

Cytology

Cytology is the study life at the cellular level; another name for this discipline is cell biology. Plant cells share many features with animal cells. Both forms of life are composed of eukaryotic cells, which have a distinct nucleus surrounded by a nuclear envelope which separates it from the cytoplasm. (By contrast, bacteria have nucleus-free cells, called prokaryotic cells.) Inside the nucleus is chromatin, which becomes organized into chromosomes as a cell divides. Chromosomes are composed of functional units called genes, which serve as the control center of the cell. Genes are composed of nucleoprotein. The nucleus also contains a nucleolus.

The cytoplasm is differentiated into numerous organelles, each specializing in a particular activity. Among those which plant cells share with animal cells are mitochondria (which conduct cellular respiration), ribosomes (which conduct protein synthesis), endoplasmic reticulum (for strengthening), Golgi apparatus (for packaging), and a plasma membrane (which functions as the cell’s outer boundary). Not found in animal cells are chloroplasts (which conduct photosynthesis). Surrounding each plant cell are several layers of compounds (especially cellulose) that form the cell wall.

Physiology

Physiology is the study of the various functions performed in and by living organisms. Physiological processes of plants include the flow of energy, movement of solutes, and control by hormones. Chemical reactions are mediated (their rate is controlled) by enzymes.

For example, those studying plant physiology are concerned with the way that plants trap light energy as light is absorbed by chlorophyll. As a result of a series of chemical reactions, glucose, a six-carbon carbohydrate, is formed (preceding glucose are molecules of PGAL, a three-carbon sugar, which pair to form glucose). The glucose may be oxidized within the same cell or within another cell of the same plant (or utilized by an animal). This oxidation process produces adenosine triphosphate (ATP). As this compound is converted to adenosine diphosphate (ADP), energy is released, allowing organisms to perform other essential energy-requiring life activities.

Plant physiologist would also be concerned with the transport of nutrients and water throughout a plant. By means of their roots, plants absorb water and dissolved materials from the soil, after which they are conducted upward, by means of xylem tissue, to all parts of the plant. This upward movement is called transpiration. The glucose formed in leaves is dissolved in water and transported, by means of phloem tissue, to all parts of the plant. This movement, called translocation, is commonly downward, but also may be upward. By these processes, water, minerals, and sugars are transported to all parts of a plant.

Another area of concern for plant physiologists is the function of plant hormones (phytohormones); in fact, those specializing in this area have their own discipline, endocrinology. Phytohormones are compounds produced within a plant. They are transported to other parts of plant, where they regulate growth and development. Early in the twentieth century, auxin was the first phytohormone to be discovered. It promotes growth by causing cells to elongate but was found also to inhibit growth of lateral buds. Gibberellins, a second group of hormones, also stimulate growth by causing cell elongation. Among their activities is the promotion of seed germination. Cytokinins are abundant in dividing tissues, where they stimulate cell division. Abscisic acid is a growth-inhibiting hormone that maintains dormancy in buds and fruits and also is associated with the falling of leaves in autumn. Ethylene causes fruits to ripen. Several hormones are used in agriculture for increasing growth rates of crops.

Genetics

Genetics, the study of heredity and the mechanisms that control it, is an outgrowth of the studies of Gregor Mendel. In the 1860’s, he performed experiments with garden peas which resulted in a new way of explaining how traits are passed from generation to generation. Mendel’s ideas were revived in 1900 as other European investigators confirmed his basic tenets. Heredity is due to discrete hereditary particles which soon came to be called genes, which are located on located on paired chromosomes. The application of the principles of genetics, begun in the first few decades of the twentieth century, has resulted in the development of greatly improved varieties of crop plants.

Molecular Biology

Genetics today is in many ways the concern of another discipline of plant science, molecular biology. The basic chemical nature of genes and how they express themselves remained in question until the 1950’s, when James Watson and Francis Crick developed the double-helix model of deoxyribonucleic acid (DNA), explaining how genes occur in great variety, replicate (duplicate) themselves, and produce phenotypes (observable traits). Today, genetics is largely concerned with studying the chemical reactions that control DNA replication. Many researchers are also at work mapping the genomes (identifying the genes responsible for expressed characteristics) of various organisms. In 2001, researchers completed a map of the genome of the model plant Arabidopsis thaliana as well as the more complex genome of the rice plant, Oryza sativa.

Molecular biology has many practical applications. A knowledge of the genetic control of cells has already resulted in new crop plants. The term “genetic engineering” indicates that plants can be designed for specific purposes.

Ecology

The early Greek scientist Theophrastus, in the third century b.c.e., recognized environmental effects on plants. Much later, naturalists documented the geographical distribution of plants as determined by various climatic factors. Such studies were the roots of the scientific discipline ecology, which emerged in the late nineteenth century. Plant ecologists of the early twentieth century were concerned largely with describing the nature and distribution of world plant communities and developing a “successional theory” as a means of understanding the dynamics of changing plant communities. Now, ecologists study plants as integral parts of ecosystems that also include animals and microorganisms. They are concerned with countering the threat of loss of species as a result of human activities such as pollution and habitat destruction.

Paleobotany

Fossils have long been recognized as remnants of plants and animals that lived and died many millennia ago. The animal fossil record was an important factor in the development of Charles Darwin’s theory of evolution in the 1800’s. Paleobotany as a subdiscipline can be traced to the efforts of Albert Seward of Cambridge University of England in the late nineteenth and early twentieth centuries. Studies of plant fossils have resulted in a clearer understanding of plant evolution.

Economic Botany

People have always relied on plants to provide basic necessities of life: food, shelter, and clothing. Economic botany developed as a specialty within botany to acquaint botanists with plant uses. Topics considered in economic botany include plant domestication, food and beverage plants, essential oils, oils and waxes, latexes and resins, medicines, fibers, tannins and dyes, wood products, and ornamental plants.

Related Disciplines

Courses in botany and plant science often address organisms that are not, in the strict sense, plants but that nevertheless are appropriately studied in the same context. Hence, although bacteria differ from plants primarily because of their cells, which are prokaryotic (lacking a nucleus and most cytoplasmic organelles), they are often studied in botany courses. Bacteria are early and evolutionarily significant organisms. Some are closely related to the protists known as algae and therefore important in the study of photosynthesis.

Fungi, too, are often studied in the context of plant science. These are mostly multicellular filamentous eukaryotic organisms lacking chlorophyll. Because they do not make their own food but live in or on the food provided by plant and animal tissues, fungi are heterotrophs (rather than autotrophs, like plants, which make their own food through photosynthesis). In some ways, therefore, fungi are more similar to animals than they are to plants. Nevertheless, they are traditionally studied in the context of plant courses because they were once considered to be plants, given their lack of movement and other gross similarities. Their world significance parallels that of bacteria: They, too, are important as decomposers, returning nutrients and other elements to the environment. The study of fungi is mycology.

Protists are unicellular eukaryotes, forming one of the four kingdoms of Eukarya, the others being fungi, plants, and animals. Included among the protists are slime molds and protozoans which, lacking chlorophyll, are said to be heterotrophic protists, obtaining their food from other sources, generally other organisms. Also included are the algae, which are autotrophic eukaryotes—many using photosynthesis, like plants, to generate their own food. For this reason, protists are often studied in the context of plant science, and algae are almost always included in such studies. The study of algae is phycology.

Viruses, the study of which is virology, are noncellular entities that can reproduce only inside specific host cells. Not generally considered to be living, they are nevertheless important because of the infections they cause. To the extent that they cause infection in plants, they are important in the study of plant science, particularly plant pathology.

Bibliography

Madigan, Michael T., et al. Brock Biology of Microorganisms. 9th ed. Upper Saddle River, N.J.: Prentice Hall, 2000. This microbiology textbook serves as a reference for other microorganisms.

Magner, Lois N. A History of the Life Sciences. 2d ed. New York: Marcel Dekker, 1994. A concise treatment of the history of biology.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants. 6th ed. New York: W. H. Freeman, 1999. An excellent textbook that gives an overview of plants and related groups and provides rationales for the inclusion of various subdisciplines under the umbrella of plant science.