The root: types, regions and modifications
If you pull a small weed from damp soil, you see a mass of fine threads at the bottom. If you pull a carrot, you see a single, thick, orange root. Both are roots, but of different kinds.
The root is the underground part of the plant that comes from the radicle. Roots anchor the plant, absorb water and minerals, and sometimes store food. The main types are the tap root system, the fibrous root system and adventitious roots.
In the figure : Four regions of a root from the tip upward.
Rule 1: Types of root systems — A tap root is the direct elongation of the radicle (mustard, most dicots). In monocots, the primary root is short-lived and is replaced by many fibrous roots (wheat). Roots that arise from parts other than the radicle, such as the stem or leaves, are adventitious (grass, Monstera, banyan).
- Easy : A mustard plant has one main root with branches.
- Medium : Roots of wheat are all of the same thickness and spread out like a mat.
- Exam-type : Q: Give one example of adventitious roots. A: Grass, Monstera or banyan.
Rule 2: Regions of the root and modifications — The root cap protects the tip. The region of meristematic activity makes new cells, the region of elongation increases length, and the region of maturation bears root hairs for absorption. Modified roots store food (carrot, turnip, sweet potato), give support (prop roots of banyan, stilt roots of maize and sugarcane) or help breathing (pneumatophores in Rhizophora, which grows in swamps).
- Easy : A carrot is a swollen tap root that stores food.
- Medium : Prop roots of a banyan hang from branches and hold the heavy tree.
- Exam-type : Q: What are pneumatophores? A: Vertical aerial roots of mangrove plants like Rhizophora that take up oxygen.
Dicots have a tap root system; monocots have fibrous roots. Root hairs are in the maturation zone.
Thinking that root hairs are in the root cap or at the tip. They are in the region of maturation.
Memory clue: C-M-E-M — Cap, Meristematic, Elongation, Maturation, from tip upward.
Spot the difference
| Point | Tap root | Fibrous root | Adventitious root |
|---|---|---|---|
| Origin | Radicle | Replaces radicle | Stem or leaf |
| Plant | Dicots | Monocots | Banyan, grass |
| Example | Mustard, carrot | Wheat | Maize stilt roots |
Practice questions
- 1Name the two main root systems.
- 2Which region of the root has root hairs?
- 3Give one example of a root that stores food.
- 4What are stilt roots? Give an example.
- 5What is the function of the root cap?
- 6What are adventitious roots?
- 7Why do mangrove plants have pneumatophores?
- 8Compare the tap root and the fibrous root.
Show answers (try them yourself first)
- 1Tap root system and fibrous root system.
- 2The region of maturation.
- 3Carrot, turnip or sweet potato.
- 4Supporting roots coming from the lower nodes of the stem, as in maize and sugarcane.
- 5It protects the delicate growing tip as the root pushes through soil.
- 6Roots that arise from parts other than the radicle.
- 7The soil is waterlogged and short of oxygen, so these roots take air.
- 8A tap root has one main root with branches and arises from the radicle; fibrous roots are many similar roots that replace the radicle.
The stem and its modifications
Ginger and potato look like roots, but they are underground stems. You can tell this by their small buds ('eyes') at the nodes, which sprout into new shoots.
The stem is the part of the plant axis that develops from the plumule. It has nodes (where leaves arise) and internodes, and it bears buds, flowers and fruits. It carries water and minerals to the leaves and food from them.
In the figure : A stem has nodes with leaves and internodes between them.
Rule 1: Features of a stem — A stem shows nodes and internodes, it is usually green when young, and it bears buds: terminal and axillary. Axillary buds can grow into branches or flowers. Stems provide support, spread of leaves, flowers and fruits, and transport.
- Easy : A bud in the angle of a leaf is an axillary bud.
- Medium : A potato eye is a node with an axillary bud.
- Exam-type : Q: Where do leaves arise on the stem? A: At the nodes.
Rule 2: Modifications — Underground stems store food: potato, ginger, turmeric and colocasia. Tendrils help in climbing (grapevine, cucumber, pumpkin, watermelon). Thorns protect (Citrus, Bougainvillea). Flattened green stems perform photosynthesis (Opuntia). Sub-aerial stems of grass and strawberry spread along the soil and give rise to new plants.
- Easy : Ginger is a stem, so it is not a root.
- Medium : Citrus thorns are modified axillary buds that protect the plant.
- Exam-type : Q: Which stem modification helps in climbing? A: Tendrils.
Stems have nodes and buds. Roots have none. So potato, ginger and turmeric are stems.
Calling a potato a root because it is underground. It is a stem tuber with buds.
Memory clue: TTT — Tendril climbs, Thorn protects, Tuber stores.
Spot the difference
| Point | Root | Stem |
|---|---|---|
| Develops from | Radicle | Plumule |
| Nodes and buds | Absent | Present |
| Leaves | Absent | Present |
Practice questions
- 1What is a node?
- 2Name two underground stems.
- 3Which plant has a stem modified into a tendril?
- 4What is the function of a thorn?
- 5What is the internode?
- 6Which stem of Opuntia does photosynthesis?
- 7How can you prove that ginger is a stem?
- 8A student claims that a bulb of onion is a root. Correct the statement.
Show answers (try them yourself first)
- 1The point on the stem at which a leaf arises.
- 2Potato and ginger (or turmeric, colocasia).
- 3Grapevine, cucumber, pumpkin or watermelon.
- 4Protection against grazing animals.
- 5The portion of the stem between two nodes.
- 6The flattened green stem.
- 7It has nodes with scale leaves and buds.
- 8An onion bulb is a modified underground stem with fleshy leaves, not a root.
The leaf: venation, types and phyllotaxy
Hold a mango leaf and a blade of grass against the light. The mango leaf shows a net of veins, but the grass shows straight lines. This pattern is called venation.
The leaf is a flattened lateral outgrowth of the stem at a node. It has a leaf base, a petiole and a lamina (blade). Its main work is photosynthesis. Venation is the pattern of veins; phyllotaxy is the arrangement of leaves on the stem.
In the figure : A typical leaf has a leaf base, a petiole and a lamina.
Rule 1: Venation and type of leaf — Reticulate venation is a net-like pattern in dicot leaves; parallel venation is in monocots. A simple leaf has an undivided lamina (mango). A compound leaf has a lamina cut into leaflets: pinnately compound (neem) with leaflets on a common axis (rachis) and palmately compound (silk cotton) with leaflets at a common point.
- Easy : A grass leaf has parallel veins.
- Medium : A neem leaf is pinnately compound, as its leaflets are along an axis.
- Exam-type : Q: A leaf shows net-like veins. Is the plant a monocot or dicot? A: Dicot.
Rule 2: Phyllotaxy and modifications — Alternate (one leaf per node: China rose, mustard, sunflower), opposite (two leaves per node: guava, Calotropis) and whorled (more than two leaves per node: Alstonia). Leaves can be modified into tendrils (pea), spines (cacti), fleshy leaves that store food (onion, garlic), a pitcher (Nepenthes), and phyllodes in Australian acacia.
- Easy : China rose has alternate phyllotaxy.
- Medium : Guava has two leaves arising at each node.
- Exam-type : Q: Give an example of whorled phyllotaxy. A: Alstonia.
Alternate = one leaf per node; opposite = two; whorled = more than two.
Calling a leaflet a leaf. A leaflet has no axillary bud at its base, but a leaf has.
Memory clue: 1-2-many — Alternate 1, Opposite 2, Whorled many.
Spot the difference
| Point | Simple leaf | Compound leaf |
|---|---|---|
| Lamina | Undivided | Divided into leaflets |
| Bud at base | Present in axil | Only at base of the whole leaf |
| Example | Mango | Neem, silk cotton |
Practice questions
- 1Name the three parts of a typical leaf.
- 2Which venation is found in monocots?
- 3Give an example of a pinnately compound leaf.
- 4Define phyllotaxy.
- 5Name a plant with opposite phyllotaxy.
- 6What is a pitcher in Nepenthes?
- 7How does a compound leaf differ from a simple leaf?
- 8A leaf of onion is fleshy. What is its function?
Show answers (try them yourself first)
- 1Leaf base, petiole and lamina.
- 2Parallel venation.
- 3Neem.
- 4The arrangement of leaves on the stem or branch.
- 5Guava or Calotropis.
- 6A leaf modified into a pitcher-shaped structure that traps insects.
- 7In a compound leaf the lamina is divided into leaflets; in a simple leaf it is a single piece.
- 8It is modified for storing food.
Inflorescence: how flowers are arranged
A sunflower looks like one big flower but is actually a cluster of hundreds of tiny flowers. The way flowers sit on a stalk is called inflorescence.
An inflorescence is the arrangement of flowers on the floral axis (peduncle). It is of two main types: racemose, where the main axis keeps growing, and cymose, where the main axis ends in a flower.
In the figure : Two types of inflorescence.
Rule 1: Racemose — The main axis continues to grow, so flowers are borne laterally in an acropetal order: the youngest flower is at the tip and the oldest at the base.
- Easy : A mustard raceme has open flowers at the base and buds at the tip.
- Medium : In radish, new flowers keep forming at the tip.
- Exam-type : Q: In which order do racemose flowers open? A: Acropetal (older at base, younger at tip).
Rule 2: Cymose — The main axis ends in a flower, so growth is limited. Flowers are borne in a basipetal order: the oldest flower is at the top and the youngest at the base. Examples include Jasmine and Cotton.
- Easy : In jasmine, the top flower opens first.
- Medium : A cymose inflorescence stops growing when the first flower is formed at the tip of the axis.
- Exam-type : Q: What is the opening order in cymose? A: Basipetal.
Racemose: unlimited growth, acropetal. Cymose: limited growth, basipetal.
Reversing acropetal and basipetal. Acropetal means developing toward the apex.
Memory clue: R-A-C-B — Racemose = Acropetal, Cymose = Basipetal.
Spot the difference
| Point | Racemose | Cymose |
|---|---|---|
| Main axis | Grows continuously | Ends in a flower |
| Order of opening | Acropetal | Basipetal |
| Oldest flower | At the base | At the apex |
Practice questions
- 1What is an inflorescence?
- 2Name the two types of inflorescence.
- 3Which inflorescence has unlimited growth?
- 4What does acropetal mean?
- 5Give an example of a cymose inflorescence.
- 6What is a peduncle?
- 7Why is the sunflower head not a single flower?
- 8In one inflorescence the youngest flower is at the base. Is it racemose or cymose?
Show answers (try them yourself first)
- 1The arrangement of flowers on the floral axis.
- 2Racemose and cymose.
- 3Racemose.
- 4The younger flowers are at the tip and the older ones at the base.
- 5Cotton or jasmine.
- 6The stalk or axis that bears the flowers.
- 7It is an inflorescence with a large number of small flowers on a common platform.
- 8Cymose, because it shows the basipetal order.
The flower: parts, symmetry and position of the ovary
Pick a hibiscus and gently take it apart: green leaf-like sepals, coloured petals, a tall tube with pollen and a swollen base. Every part has a job in making seeds.
The flower is the reproductive unit of an angiosperm. It has four whorls: calyx (sepals), corolla (petals), androecium (stamens) and gynoecium (carpels, with stigma, style and ovary).
In the figure : The four whorls of a flower from outside to inside.
Rule 1: Symmetry, sexes and completeness — A flower with all four whorls is complete; if one is missing it is incomplete. A flower with both stamens and carpels is bisexual; one with only one of them is unisexual. Actinomorphic flowers are radially symmetrical (mustard, datura, chilli); zygomorphic are bilaterally symmetrical (pea, gulmohur, bean, Cassia); asymmetric flowers cannot be divided into two equal halves (canna).
- Easy : A hibiscus is a bisexual, actinomorphic flower.
- Medium : A pea flower is zygomorphic.
- Exam-type : Q: Which flower is radially symmetrical? A: Mustard, datura or chilli.
Rule 2: Position of ovary and other terms — In a hypogynous flower the ovary is superior, placed above other parts (mustard, brinjal). In a perigynous flower the ovary is half inferior at the same level (rose, plum). In an epigynous flower the ovary is inferior and other parts arise above (guava, cucumber). Aestivation is the arrangement of sepals or petals in a bud: valvate, twisted, imbricate and vexillary (pea). Placentation is the arrangement of ovules within the ovary: marginal, axile, parietal, free central and basal.
- Easy : A brinjal flower has a superior ovary, so it is hypogynous.
- Medium : A guava flower has an inferior ovary, so it is epigynous.
- Exam-type : Q: Which placentation is found in pea? A: Marginal.
Superior ovary = hypogynous; half-inferior = perigynous; inferior = epigynous.
Thinking that a superior ovary means that the flower is above the leaves. It means the ovary is above the other floral whorls.
Memory clue: H-P-E — Hypogynous Higher ovary, Perigynous Parallel, Epigynous Below.
Spot the difference
| Type | Ovary position | Example |
|---|---|---|
| Hypogynous | Superior | Mustard, brinjal |
| Perigynous | Half inferior | Rose, plum |
| Epigynous | Inferior | Guava, cucumber |
Practice questions
- 1Name the four whorls of a flower.
- 2What is a bisexual flower?
- 3Give one example of a zygomorphic flower.
- 4What is the male reproductive whorl called?
- 5Name the three parts of a carpel.
- 6What is aestivation?
- 7Differentiate hypogynous and epigynous flowers.
- 8Why is the pea flower called papilionaceous?
Show answers (try them yourself first)
- 1Calyx, corolla, androecium and gynoecium.
- 2A flower having both stamens and carpels.
- 3Pea, bean, gulmohur or Cassia.
- 4Androecium.
- 5Stigma, style and ovary.
- 6The arrangement of sepals or petals in a floral bud.
- 7In hypogynous flowers the ovary is superior; in epigynous flowers it is inferior.
- 8Its corolla of five petals has a vexillary aestivation with a large standard, two wings and two keel petals, like a butterfly.
The fruit and the seed
A mango has a juicy part that you eat and a hard stone inside. Break the stone, and a seed is waiting to become a new tree. Fruit and seed are the final products of the flower.
The fruit is the mature, ripened ovary developed after fertilisation. Its wall is the pericarp. The seed is the mature ovule, with a seed coat, an embryo and sometimes an endosperm.
In the figure : After fertilisation the ovary becomes the fruit and the ovule becomes the seed.
Rule 1: Fruit — Usually the pericarp is divided into the outer epicarp, the middle mesocarp and the inner endocarp. In a drupe (mango and coconut) the fruit develops from a single-seeded superior ovary, and the endocarp is hard. In a berry (tomato) the pericarp is fleshy. Fruits formed without fertilisation are parthenocarpic (such as the banana).
- Easy : A mango has a hard endocarp, called the stone.
- Medium : In a coconut the fibrous part is the mesocarp and the hard shell is the endocarp.
- Exam-type : Q: Which part of the flower becomes the fruit? A: The ovary.
Rule 2: Seeds: dicot and monocot — A dicot seed (bean, gram) has a seed coat of testa and tegmen, a hilum, a micropyle and an embryo with two cotyledons, a plumule and a radicle. A monocot seed (maize) has a thin seed coat fused with the fruit wall, a bulky endosperm, an aleurone layer, one cotyledon called the scutellum, a plumule covered by a coleoptile and a radicle covered by a coleorhiza.
- Easy : A bean seed splits into two cotyledons.
- Medium : A maize grain has one cotyledon, the scutellum.
- Exam-type : Q: What is the micropyle? A: A small pore in the seed coat through which water enters.
Ovary → fruit; ovule → seed; ovary wall → pericarp; fertilised egg → embryo.
Thinking that all seeds have endosperm. Bean seed is non-albuminous: the endosperm is used up in development.
Memory clue: O-F, O-S — Ovary to Fruit, Ovule to Seed.
Spot the difference
| Point | Dicot seed (bean) | Monocot seed (maize) |
|---|---|---|
| Cotyledons | Two | One (scutellum) |
| Endosperm | Absent in mature seed | Present, bulky |
| Covering of plumule | None | Coleoptile |
Practice questions
- 1Which part of the flower becomes the fruit?
- 2What is the pericarp?
- 3Name the three layers of the pericarp.
- 4What is the scutellum?
- 5Name the structure covering the radicle in maize.
- 6What is a hilum?
- 7Compare the dicot and monocot seeds in terms of the cotyledons and the endosperm.
- 8What is a drupe? Give two examples.
Show answers (try them yourself first)
- 1The ovary.
- 2The wall of the fruit.
- 3Epicarp, mesocarp and endocarp.
- 4The single, shield-shaped cotyledon of a monocot seed such as maize.
- 5Coleorhiza.
- 6The scar on the seed coat where the seed was attached to the fruit.
- 7A dicot seed has two cotyledons and usually no endosperm when mature; a monocot seed has one cotyledon and a bulky endosperm.
- 8A fleshy fruit with a hard endocarp that encloses a single seed, as in mango and coconut.
Describing a plant in botanical terms: floral formula and Solanaceae
When a doctor writes a note, she uses a few symbols to describe a patient. Botanists do the same for a flower, using a floral formula and a floral diagram to give a complete description in one line.
A floral formula is a short way to write the main features of a flower using symbols. Br = bracteate; ⊕ = actinomorphic; % = zygomorphic; ⚥ = bisexual; K = calyx; C = corolla; P = perianth; A = androecium; G = gynoecium. Brackets show fused parts, and a number after a letter shows how many parts there are.
In the figure : Floral formula of Solanaceae: ⊕ ⚥ K(5) C(5) A5 G(2), ovary superior.
Rule 1: Reading a floral formula — Write the symbol for bracts and symmetry first, then the sex of the flower, then the numbers of sepals, petals, stamens and carpels. An underline below G shows a superior ovary and a line above G shows an inferior ovary.
- Easy : K(5) means five sepals joined together.
- Medium : A5 means five free stamens, whereas A(5) would mean five joined stamens.
- Exam-type : Q: What does G(2) mean? A: A gynoecium of two fused carpels.
Rule 2: Family Solanaceae — The family includes potato, tomato, brinjal, chilli, tobacco and Datura. The flowers are bisexual, actinomorphic, pentamerous, with a superior ovary and two fused carpels. The corolla is fused and the stamens are attached to the petals. Floral formula: Br ⊕ ⚥ K(5) C(5) A5 G(2). The fruit is a berry or capsule, and the seeds are often endospermous.
- Easy : Potato and brinjal belong to Solanaceae.
- Medium : The stamens of Solanaceae are epipetalous, attached on the corolla.
- Exam-type : Q: Name three economically important plants of Solanaceae. A: Potato, tomato and brinjal (also chilli and tobacco).
Round brackets show fusion, no brackets show free parts. Pentamerous means five of each whorl.
Writing A(5) when the five stamens are free. Brackets are used only for fused parts.
Memory clue: K-C-A-G — Calyx, Corolla, Androecium, Gynoecium.
Spot the difference
| Symbol | Meaning | Example in Solanaceae |
|---|---|---|
| ⊕ | Actinomorphic | Radially symmetrical |
| ⚥ | Bisexual | Both stamens and carpels |
| K(5) | Fused five sepals | Calyx |
| G(2) | Two fused carpels | Superior ovary |
Practice questions
- 1What is a floral formula?
- 2What does the symbol ⊕ show?
- 3What does K stand for?
- 4Give the floral formula of Solanaceae.
- 5Name two plants of the family Solanaceae.
- 6What does a bracket around a number show?
- 7What does the formula tell about the ovary of Solanaceae?
- 8Write the floral formula for a flower with five free sepals, five free petals, many stamens and a single carpel, bisexual and actinomorphic.
Show answers (try them yourself first)
- 1A representation of the features of a flower using symbols and letters.
- 2An actinomorphic flower.
- 3Calyx.
- 4Br ⊕ ⚥ K(5) C(5) A5 G(2), with a superior ovary.
- 5Potato and tomato (or brinjal, chilli).
- 6The parts of that whorl are fused.
- 7It has two fused carpels and a superior ovary.
- 8⊕ ⚥ K5 C5 A∞ G1 (a free-part formula has no brackets).