Botany: Plant Anatomy, Photosynthesis, Reproduction, Classification and Plant Physiology
Botany is the branch of biology that studies plants, their structure, growth, reproduction, classification and the way they make and use food. Plants are the primary producers of every ecosystem. They trap the energy of sunlight and convert it into chemical energy, and every other living being depends on that energy either directly or indirectly. Because of this central role, botany forms a large part of the general-science and biology papers of APSC, ADRE, DHS, SSC, Railway and police examinations in Assam, and questions are asked on photosynthesis, plant tissues, the parts of a flower, plant hormones, and the classification of the plant kingdom.
This article gives a complete deep-study revision of botany in an organised way. It begins with the working vocabulary, then explains plant classification, plant tissues and morphology, photosynthesis, reproduction, plant physiology and the economic importance of plants. After that, the examination trends, common mistakes and the correct method of preparation are discussed. The reason behind each fact is explained so that the answer follows naturally from the concept instead of being memorised word for word.
Basic Terms
- Botany: the branch of biology that deals with plants.
- Photosynthesis: the process by which green plants make food using sunlight, carbon dioxide and water.
- Chlorophyll: the green pigment in chloroplasts that absorbs light energy.
- Stomata: tiny pores on leaves through which gases are exchanged.
- Transpiration: the loss of water vapour from the aerial parts of a plant, mainly through stomata.
- Xylem: the tissue that conducts water and minerals upward from the root.
- Phloem: the tissue that conducts food from the leaves to other parts.
- Meristem: a region of actively dividing cells responsible for growth.
- Angiosperm: a flowering plant whose seeds are enclosed in a fruit.
- Gymnosperm: a plant whose seeds are naked and not enclosed in a fruit.
- Pollination: the transfer of pollen from the anther to the stigma.
- Fertilisation: the fusion of the male and female gametes.
- Hormone: a chemical substance produced in small amounts that controls growth and other activities.
- Translocation: the transport of food through the phloem.
Core Concepts and Formulas
- The balanced equation of photosynthesis is 6CO2 + 6H2O -> C6H12O6 + 6O2 (in the presence of light and chlorophyll).
- The site of photosynthesis is the chloroplast, and the green pigment is chlorophyll.
- Plants are classified into Thallophyta, Bryophyta, Pteridophyta, Gymnosperms and Angiosperms.
- Angiosperms are divided into dicotyledons and monocotyledons.
- The main plant hormones are auxin, gibberellin, cytokinin and abscisic acid.
- Water is conducted by the xylem and food by the phloem.
- The sex organs of a flower are the stamen (male) and the pistil or carpel (female).
- Respiration in plants takes place day and night, while photosynthesis takes place only in the presence of light.
Classification of the Plant Kingdom
The plant kingdom is divided into two broad groups, the cryptogams, which do not bear seeds, and the phanerogams, which bear seeds. Thallophyta are the simplest plants, with a body not differentiated into root, stem and leaf; algae belong to this group. Bryophyta are called the amphibians of the plant kingdom because they need water for reproduction, and mosses are examples. Pteridophyta are the first true land plants with conducting tissues, and ferns are the common example. Gymnosperms bear naked seeds and include the pine and the cycas. Angiosperms are flowering plants whose seeds are enclosed inside a fruit, and they form the largest group. Angiosperms are further divided into dicotyledons, with two seed leaves and reticulate venation, and monocotyledons, with one seed leaf and parallel venation.
Plant Tissues
Plant tissues are of two main kinds, meristematic and permanent. Meristematic tissue has actively dividing cells and is found at the tips of roots and shoots, where it brings about growth in length, and in the cambium, which brings about growth in thickness. Permanent tissue has lost the power of division and is of two types, simple and complex. Simple tissues include parenchyma, which stores food, collenchyma, which gives flexibility, and sclerenchyma, which gives strength. Complex tissues are the conducting tissues: xylem conducts water and minerals upward and provides mechanical support, while phloem conducts the food prepared in the leaves to the storage organs. The epidermis forms the outer protective layer, and the cork formed by the cork cambium protects older stems.
Morphology: Root, Stem and Leaf
The root fixes the plant in the soil and absorbs water and minerals. Tap roots and fibrous roots are the two main types, and modified roots may store food, as in the carrot and radish. The stem supports the branches, leaves, flowers and fruits and conducts water and food; modified stems such as the potato tuber and the ginger rhizome store food. The leaf is the chief organ of photosynthesis. A typical leaf has a blade with a midrib and veins, and a petiole that joins it to the stem. The arrangement of veins is called venation, which is reticulate in dicots and parallel in monocots. Leaves may be modified into tendrils for climbing, spines for protection, or scales for storage.
Photosynthesis
Photosynthesis is the process by which green plants prepare their own food. It takes place in the chloroplasts, which contain the green pigment chlorophyll. In the presence of sunlight, chlorophyll uses carbon dioxide and water to make glucose, and oxygen is released as a by-product. The process has two stages. The light reaction takes place in the grana of the chloroplast and needs light; it splits water into hydrogen and oxygen and stores energy as ATP and NADPH. The dark reaction, also called the Calvin cycle, takes place in the stroma and does not need light directly; it uses the ATP and NADPH from the light reaction to fix carbon dioxide into carbohydrate. The rate of photosynthesis is affected by light intensity, carbon dioxide concentration, temperature and water availability.
Reproduction in Plants
Plants reproduce by two methods, asexual and sexual. Asexual reproduction takes place by vegetative propagation, in which a part of the plant such as a stem, root or leaf gives rise to a new plant. Examples are the potato, sugarcane and rose, and artificial methods include cutting, layering and grafting. Sexual reproduction takes place in flowering plants through the flower. The stamen is the male part and produces pollen grains in the anther, and the pistil is the female part and contains the ovules in the ovary. Pollination is the transfer of pollen to the stigma and may be self or cross. After pollination, fertilisation takes place and the ovule becomes the seed and the ovary becomes the fruit. In plants such as mosses and ferns, reproduction also needs water at some stage.
Plant Physiology and Hormones
Plant physiology deals with the life processes of plants. Water and minerals are absorbed by the roots and carried upward by the xylem through a pull created by transpiration, which is called the transpiration pull. Food is transported from the leaves to all parts through the phloem. Transpiration also cools the plant and helps in the upward movement of water. Photosynthesis and respiration work together, but photosynthesis stores energy while respiration releases it; respiration goes on day and night. Plant movements are of two kinds, tropic movements, which are directional and are named according to the stimulus, such as phototropism and geotropism, and nastic movements, which are non-directional. The four important plant hormones are auxin, which promotes cell elongation and is used in rooting; gibberellin, which promotes stem elongation and seed germination; cytokinin, which promotes cell division; and abscisic acid, which promotes dormancy and the closing of stomata.
Economic Importance of Plants
Plants provide food, clothing, medicine, timber, fuel and raw materials for industry. Cereals such as rice, wheat and maize are the main food crops, and pulses supply protein. Tea, coffee and sugarcane are important cash crops, and Assam is famous for tea. Plants also give us cotton and jute for cloth, rubber for industry, and hundreds of medicines. Medicinal plants such as tulsi, neem and cinchona are widely used. Plants hold the soil and prevent erosion, add oxygen to the atmosphere, and maintain the balance of carbon dioxide and oxygen. Because of all this, the study of botany is not only an examination requirement but also a subject of everyday importance.
Examination Trends
In competitive examinations, botany questions usually test the equation and raw materials of photosynthesis, the site of photosynthesis, the function of xylem and phloem, the classification of plants with examples, the parts of a flower, the agents and types of pollination, the names of plant hormones with their functions, and the difference between respiration and photosynthesis. Objective questions frequently ask the green pigment, the site of the light and dark reactions, the amphibians of the plant kingdom, the largest group of the plant kingdom, and the examples of modified roots and stems. A smaller number of questions test transpiration, tropism and vegetative propagation.
Common Mistakes to Avoid
- Do not say that plants respire only at night; respiration takes place day and night.
- Remember that xylem carries water upward and phloem carries food, not the reverse.
- Do not confuse pollination with fertilisation; pollination comes first and fertilisation follows.
- Do not call bryophytes vascular plants; they lack true conducting tissue.
- Do not mix the functions of auxin and gibberellin, or of cytokinin and abscisic acid.
How to Prepare This Topic
Begin with the classification of the plant kingdom and fix one clear example for each group. Draw a labelled diagram of a flower, a leaf and a cross-section of a stem, because diagram-based questions are common. Write the equation of photosynthesis and remember the raw materials and the products with their source. Prepare a table of plant hormones with their functions, and another table that compares photosynthesis with respiration. Revise the modifications of root, stem and leaf with examples, and practise previous-year questions so that the exact wording of the examination becomes familiar.
Frequently Asked Questions
What is photosynthesis and where does it take place?
Photosynthesis is the process by which green plants prepare glucose from carbon dioxide and water using sunlight. It takes place mainly in the leaves, inside the chloroplasts, which contain the green pigment chlorophyll.
What is the difference between xylem and phloem?
Xylem conducts water and dissolved minerals upward from the root to the other parts of the plant and also gives mechanical strength. Phloem conducts the food prepared in the leaves to all other parts of the plant.
Which plants are called the amphibians of the plant kingdom?
Bryophytes, such as mosses, are called the amphibians of the plant kingdom because they live on land but need water for their reproduction, as their male gametes swim in a film of water to reach the female gamete.
What is the difference between monocots and dicots?
Monocotyledons have one seed leaf, parallel venation, fibrous roots and flower parts in threes. Dicotyledons have two seed leaves, reticulate venation, a tap root and flower parts in fours or fives.
Name the main plant hormones and their functions.
Auxin promotes cell elongation and rooting, gibberellin promotes stem elongation and seed germination, cytokinin promotes cell division, and abscisic acid promotes dormancy and the closing of stomata during water stress.