RBSE Solutions Class 11 Biology Chapter 18 Stem External Morphology

Question 1. The Branches originate from Stem
(a) Endogenous
(b) Exogenous
(c) Leaves
(d) Root Apex
Answer: (b) Exogenous
In simple words: Branches on a plant stem usually grow from the outside layers of the stem. This type of growth is called exogenous.

🎯 Exam Tip: Remember that branch growth is exogenous, meaning it originates from superficial tissues, unlike roots which exhibit endogenous growth.

Question 2. The thread like structures visible from stem are
(a) Thom
(b) Tendril
(c) cladode
(d) Hook
Answer: (b) Tendril
In simple words: Thin, twisted structures that help a plant climb are called tendrils. They look like threads coming from the stem.

🎯 Exam Tip: Pay attention to the function described ("thread-like structures visible from stem") as it points directly to tendrils, which are designed for climbing.

Question 4. The Nasturtium (Jalkumbhi) is an example of-
(a) Stem
(b) Leaf
(c) Root
(d) Thom
Answer: (a) Stem
In simple words: Nasturtium, also known as Jalkumbhi, is a plant where the stem is the main part we are looking at in this context.

🎯 Exam Tip: Familiarize yourself with common plant examples for different morphological features, as these are often tested.

RBSE Class 11 Biology Chapter 18 Very Short Answer Questions

Question 1. How many type of trees are there?
Answer: Plants can be categorized based on their growth habits into three main types:

• Herbs: These are small plants with soft, green stems that are not woody. They usually grow less than 2 meters tall. Herbs can live for one year (like groundnut, wheat, rice), two years (like henbane, beet), or many years (like canna).
• Shrubs: These are medium-sized plants that branch out typically near the ground and do not have a single main trunk. Their lower parts are woody, while the upper parts are softer. Shrubs look bushy, like roses, cotton, or jasmine.
• Trees: These plants are taller than shrubs and have a thick, tall main trunk with many branches, such as mango, neem, and eucalyptus. Some trees, like palm or coconut, have an unbranched trunk, which is called columnar or caudex.

In simple words: There are three main kinds of plants: herbs (small, soft stems), shrubs (medium, bushy, woody base), and trees (tall, strong woody trunk with branches).

🎯 Exam Tip: When classifying plants by habit, focus on stem characteristics (soft/woody, branched/unbranched) and overall height.

Question 2. Name two stem which are used as edible part?
Answer: While the question asks for edible stem parts, the provided answer describes different stem shapes. These shapes are:
1. Cylindrical: Most plants have this type of stem shape.
2. Triangular: An example is the Cyprus plant.
3. Quadrangular: An example is Ocimum (holy basil).
4. Ribbed: An example is Casuarina.
5. Flat: An example is Opuntia (cactus).
6. Jointed: An example is Sugarcane.
In simple words: Stems can have different shapes like round (most plants), three-sided (Cyprus), four-sided (Ocimum), ridged (Casuarina), flat (Opuntia), or jointed (Sugarcane).

🎯 Exam Tip: Be prepared for questions that might have an answer describing a related, but not exact, concept as presented in some textbooks. Always state the given information clearly.

RBSE Class 11 Biology Chapter 18 Short Answer Questions

Question 1. Write the functions of stem?
Answer: The stem performs several key roles for a plant:
Primary functions:
1. The stem supports leaves and holds them in a way that allows them to get the most sunlight.
2. It moves water and nutrients from the roots up to the leaves, flowers, and fruits.
3. The stem also carries the food made in the leaves down to the roots and storage areas.
4. It holds flowers in a good position to help with pollination and fertilization.
5. The stem supports fruits and seeds as they develop.
Secondary (or Accessory) functions:
1. Storage: Many plants use their stems to store extra food, like the rhizome of ginger, corm of Colocasia, and tubers of potato. They also store sugars (e.g., sugarcane) and water (e.g., Opuntia's phylloclade).
2. Synthesis of food: Some modified stems in desert plants (xerophytes) are green, flat, and look like leaves. These stems carry out photosynthesis and help make food. Examples include Opuntia, Muehlenbeckia, Ruscus, and Asparagus.
3. Perennation: Underground stems of many plants, such as ginger and turmeric, help the plant survive through harsh conditions and restart growth later.
4. Vegetative propagation: In many plants, stems are used to create new plants without seeds, for example, potato tubers, chrysanthemum suckers, and colocasia corms.
In simple words: The stem holds leaves towards sunlight, transports water and food, and supports flowers and fruits. It can also store food, make food (in some plants), help plants survive tough times, and grow new plants.

🎯 Exam Tip: Categorize stem functions into primary and secondary (or accessory) roles to provide a comprehensive answer, giving examples where applicable.

Question 2. Differentiate between stem and root?
Answer: Here are the differences between a stem and a root:

In simple words: Stems and roots are different: stems have an outer protective layer (cuticle) and pores (stomata), while roots do not. Roots have special root hairs and a distinct pericycle that stems don't always have. Stems grow upwards, and roots grow downwards.

🎯 Exam Tip: Create a clear table for comparison, focusing on key structural and developmental differences like presence of cuticle, stomata, type of hairs, and origin of lateral organs.

Question 3. Differentiate between Phylloclade and Cladode?
Answer: Here are the differences between Phylloclade and Cladode:

In simple words: Phylloclades are modified stems and branches that look like leaves and grow a lot, while cladodes are only modified stem branches that look like leaves but have limited growth.

🎯 Exam Tip: Clearly distinguish between the origin (stem vs. stem branches) and growth pattern (limited vs. unlimited) when differentiating these stem modifications.

RBSE Class 11 Biology Chapter 18 Long Answer Type Question

Question 1. Explain different aerial modifications of stem?
Answer: Stems can be modified in several ways to grow above ground and help the plant survive. Here are some aerial modifications:
1. Stem tendrils: These are long, thin, coiled structures that help plants climb by wrapping around nearby supports. They can be branched or unbranched and come in four types:

• Axillary tendrils (e.g., Passiflora).
• Extra-axillary tendrils (e.g., Luffa, Cucurbita, Lagenaria).
• Apical bud tendrils (e.g., Grape Vine), where the main stem's apical buds change into tendrils.

Stem tendrils. A, Vitis vinifera (grape vine); B, Antigonon; C, Cucurbita.

2. Stem thorns: These are stiff, sharp, and pointed structures that develop from axillary buds. Thorns help protect plants from animals eating them and also reduce water loss. Examples include Citrus, Duranta, Bougainvillea, and Pomegranate.

Stem thorns. A, Citrus; B, Duranta; C, Bougainvillea.

3. Phylloclades: In some desert plants (xerophytes), leaves are small or changed into spines. The stems take over the job of photosynthesis and become flat (like Opuntia, Muehlenbeckia) or cylindrical (like Casuarina, Euphorbia tirucalli). These green stems are covered with a thick waxy layer to reduce water loss. Some phylloclades also store water. For example, Opuntia has jointed, branched, flat, and succulent stem segments.

• The leaves are generally small, fleshy, and fall off easily.
• The leaves on side branches are modified into spines to reduce water evaporation.
• Each part of a phylloclade grows from the axil (angle) of a leaf that falls off.
• The older part of the phylloclade eventually falls off, leaving a mark.
• Each segment has several nodes (points where leaves or branches grow) or areoles, and flowers grow from these nodes.

In Euphorbia tirucalli, the phylloclade is green, cylindrical, shiny, and juicy. It has many branches. Its delicate leaves appear during the rainy season but fall off in dry weather. These plants have a milky sap that helps them keep moisture and seal injuries.

Phylloclades, A, Opuntia; B, Euphorbia, tirucalli; C, Muehlenbeckia

4. Cladodes: These are modified stems and branches that grow for a limited time. Each cladode is green, flat, or cylindrical, and performs photosynthesis like a leaf. The cladode of Asparagus has one internode (space between nodes) and grows in clusters from the axils of scale leaves. The true leaves are modified into spines. In Riiscus aculeatus, the leaf-like cladodes have two internodes and grow in the axils of scale leaves. Sometimes, a flower bud grows between the two internodes.

Cladodes- A, Ruscus; B, Asparagus

5. Thalamus:
1. Thorns: These are modified stem structures. Thorns are deeply rooted and have connections to the plant's vascular system. They are covered by bark and thick-walled cells.
In simple words: Stems can change into tendrils for climbing, thorns for protection, phylloclades (leaf-like stems) for photosynthesis, and cladodes (short leaf-like stems) for photosynthesis. The thalamus, a part of the flower, can also have thorns which are modified stem structures.

🎯 Exam Tip: When describing stem modifications, always explain their function (e.g., climbing, protection, photosynthesis) and provide specific plant examples.

Question 2. Explain some of the sub-aerial modifications of stem?
Answer: Sub-aerial stem modifications are found in weak-stemmed plants that grow close to the ground, developing special roots (adventitious roots) from their nodes. These types of creepers are mainly of three kinds:
I. Runners: In some creeping plants, buds from the lowest leaves grow into narrow, modified branches that spread across the soil. Each branch develops a new plant from its tip once it roots into the soil. This process repeats, allowing the plant to spread widely and cover a large area. Each runner typically has one or more nodes with small leaves (scale leaves) and buds. Runners help the plant reproduce vegetatively. Examples include Doob grass (Cynodon dactylon), Oxalis, Colocasia, and Centella (Brahmi Booti).

Runner of Cynodon dactylon (Doob Grass)

II. Stolons: These are specialized runners that first grow upwards like normal branches, then bend downwards to touch the soil, where they develop into new daughter plants. Examples include Strawberry (Fragaria vesica) and Mentha.

Stolon of Fragaria vesica (Strawberry)

III. Offsets: These are short, condensed runners with a single internode. Each offset produces a cluster of leaves at its top and a group of roots at its base. Examples include Eichhornia (water hyacinth) and Pistia.

Offset of Eichhornia (water hyacinth).
In simple words: Sub-aerial stems grow partly above and partly below ground. Runners spread horizontally, forming new plants. Stolons grow up then arch down to root. Offsets are short, thick runners that form new plants.

🎯 Exam Tip: When explaining sub-aerial modifications, clarify how each type helps the plant spread or reproduce vegetatively, and provide a clear example for each.

Question 3. Explain different types of weak stem?
Answer: Weak stems are thin, soft, and delicate stems that cannot stand upright on their own and need support. They are generally of two types:
1. Upright weak stems: These stems need support to grow vertically. They include:

• Twiners: Plants with stems that coil around a support to climb. They can coil clockwise (dextrorse, e.g., Dolichos lablab) or anti-clockwise (sinistrorse, e.g., Convolvulus).

A-B. Twiners A, dextrorse twiner of Dolichos lablab; B, sinistrorse twiner of Convolvulus.

2. Climbers: These weak stems climb using special structures. They have four main types:

• Tendril climbers: Use spirally coiling tendrils (modified stems, branches, leaves, or flower stalks) to climb. Examples include Vitis (stem), Passiflora (branches), Lathyrus aphaca (leaves), and Antigonon (inflorescence stalk).
• Root climbers: Produce adventitious roots from their nodes that penetrate the support, helping the plant climb. Examples include Betel vine (Piper betel), Tecoma, and Ivy.

A-D. Root climbers, A. Tecoma; B, Pothos (money plant); C, Piper betle; D, Hedera helix. (Ivy).

• Scramblers: These weak-stemmed plants grow over bushes and rest there without coiling. They use curved prickles (e.g., Rose), hooks (e.g., Artabotrys), or spines (e.g., Lantana, Zizyphus) to hold on.

Scrambler of Bougainvillea.

• Lianas: These are woody climbers found in deep forests. They start as ordinary twiners, but as they reach sunlight at the top, their stems become thick and woody. Examples include Tinospora, Ficus, Bauhinia, and Bignonia.

(B) Prostrate weak stems: These stems spread over the ground to expose their leaves to sunlight.
(i) Trailers (= Stragglers): These weak-stemmed plants trail along the surface but do not try to climb or root at regular intervals. They are of three types- (Note: The specific types of trailers are not detailed in the provided content).

Trailers (= Stragglers) of Euphorbia prostrata.

(ii) Creepers: These weak-stemmed plants grow flat along the ground and develop roots from their nodes (points where leaves attach). Creepers are of three main types:
I. Runners: In certain creeping plants, buds from the lowest leaves grow into narrow, modified branches that creep over the soil surface. Each modified branch is a basal internode of the axillary bud, which grows along the soil, carrying the bud away from the parent plant. Eventually, the bud roots into the soil, develops adventitious roots, and becomes a new plant. This process repeats, allowing a single plant to spread and cover a large area. Each runner usually has one or more nodes. These nodes bear scale leaves and axillary buds. Runners are a way for the plant to reproduce vegetatively. Examples include Doob grass (Cynodon dactylon), Oxalis, Colocasia, and Centella (Brahmi Booti).

Runner of Cynodon dactylon (Doob Grass)

II. Stolons: These are special runners that initially grow upwards like regular branches, then arch downwards to touch the soil, where they develop new daughter plants. Examples include Strawberry (Fragaria vesica) and Mentha.

Stolon of Fragaria vesica (Strawberry)

III. Offsets: These are short, condensed runners with a single internode. Each offset produces a cluster of leaves at its top and a group of roots at its base. Examples include Eichhornia (water hyacinth) and Pistia.
In simple words: Weak stems can be twiners (coiling around support), climbers (using roots or tendrils to hold on), scramblers (resting on other plants with hooks), or lianas (woody forest climbers). They can also be prostrate (spreading on ground) as trailers (no rooting) or creepers (rooting at nodes), which include runners, stolons, and offsets.

🎯 Exam Tip: When describing weak stem types, always mention the specific structures or mechanisms they use for support or spread, and provide examples.

Question 4. Explain different functions and general characteristics of stem?
Answer: The stem plays several vital roles in a plant's life, both primary and secondary. Here are its main functions and characteristics:

Primary Functions:
1. Stems hold leaves in a position that helps them get the maximum amount of sunlight.
2. Stems transport water and essential minerals from the roots up to the leaves, flowers, and fruits.
3. Stems carry the food produced in the leaves down to the roots and other storage organs.
4. Stems hold flowers in a way that makes pollination and fertilization easier for the plant.
5. Stems also support the development and bearing of fruits and seeds.

Secondary (or Accessory) Functions:
1. Storage: In many plant species, stems act as storage organs for extra food materials (like the rhizome of ginger or tubers of potato), sugars (like in sugarcane), and water (like in the phylloclade of Opuntia).
2. Food Synthesis: Some modified stems, especially in desert plants (xerophytes), are green, flat, and look like leaves. These stems perform photosynthesis to make food. Examples include Opuntia, Muehlenbeckia, and Asparagus.
3. Perennation: Underground modified stems of many plants (such as ginger and turmeric) help them survive unfavorable conditions, allowing them to live through tough seasons.
4. Vegetative Propagation: In many plants, stems serve as a way to grow new plants without seeds (for example, potato tubers, chrysanthemum suckers, and colocasia corms).

General Characteristics of Stem:
1. A stem grows from the plumule and epicotyl part of the plant embryo.
2. Generally, it is the aerial part of the plant, growing upwards.
3. A main bud (terminal bud) is found at the very top of the main stem and its side branches, which is responsible for the stem's growth in length.
4. A stem is organized into nodes and alternating internodes. A node is where leaves attach to the stem, and an internode is the section of stem between two nodes.
5. Leaves and branches grow out from the nodes of the stem and its branches. These side organs of the stem grow from the outside (exogenous origin).
6. A young stem is typically green and carries out photosynthesis.
7. The stem may have many-celled hairs on its surface.
8. Both the branches of the stem and its leaves also originate exogenously.
9. Flowers and fruits develop and grow on the stem of mature plants.
10. Generally, a stem grows straight up and away from the soil towards light. This means it is negatively geotropic (grows against gravity) and positively phototropic (grows towards light).
In simple words: Stems are the main upright part of a plant. They hold leaves towards the sun, carry water and food, and help the plant reproduce. They also support flowers, fruits, and seeds.

🎯 Exam Tip: When explaining functions, categorize them (e.g., primary, secondary) for clarity. For characteristics, list distinct features like origin, growth direction, and structural components (nodes, buds).

Question 5. Describe different types of modifications of underground stem.
Answer: Stems can change their shape and function significantly, especially when they grow underground. These changes help plants store food, survive tough times, and create new plants.

(A) Underground (Subterranean) Stem Modifications:
In some plants, the main stem or its branches grow entirely under the soil. These underground stems store food and help the plant survive harsh seasons (a process called perennation). They also help the plant spread and reproduce (vegetative propagation). During unfavorable conditions, the parts of the plant above ground might die, but the underground stem survives and sprouts new aerial shoots when good weather returns. These underground stems are not green and might look similar to roots. However, you can tell them apart because they have:
1. Nodes and internodes.
2. Adventitious roots.

Here are different types of underground stem modifications:
1. Sucker: These stems grow diagonally under the soil for a certain distance and then turn upwards to emerge as an aerial shoot. Each sucker has distinct nodes and internodes, bears small scale leaves, and develops adventitious roots at the base of its nodes. Examples include Chrysanthemum and Mint.
2. Rhizome: This is a long-lasting, underground main stem that grows horizontally below the soil surface. It produces leaves and shoots above ground and adventitious roots below. Rhizomes help plants survive unfavorable seasons. Most rhizomes are thick and store reserve food materials. They have clear nodes and internodes. Based on how they grow in the soil, rhizomes can be of two types:
• Rootstock: These grow upright or at an angle, with their tips reaching towards the soil surface.
• Straggling: These grow horizontally and are generally branched. Examples include lotus, ginger, turmeric, and sugarcane.
3. Corm: This is a very condensed, thick, solid, and vertically growing underground stem with a large bud at its top. It is usually unbranched. The shape of a corm is almost spherical. It produces adventitious roots from its basal part. Each corm has several circular nodes with scale leaves and one or more axillary buds in the axil of these leaves. During good growing seasons, some of these buds develop adventitious roots and grow into new aerial shoots and corms. An example is Amorphophallus.
4. Tuber: These are swollen ends of specialized underground stem branches that develop from suckers or stolons. Tubers are typically oval or spherical and do not bear adventitious roots. They are usually covered by a corky skin that has small openings (lenticels) for air. Each tuber has spirally arranged depressions called "eyes," which are actually nodes. These "eyes" are more crowded towards the apical (rose) end and less towards the stolon (heel) end. The potato is a common example of a stem tuber; it stores starch as reserve food material and is used for vegetative propagation. When potato tubers are planted in suitable soil, the axillary buds on the "eyes" grow into new branches, using the stored food, and some of these branches become green aerial shoots.
5. Bulb: In bulbs, the stem itself does not store much reserve food material. It has many fibrous adventitious roots at its base and several thick, fleshy, sheathing leaf bases on its upper side. These leaf bases contain axillary buds which can also become fleshy to form bulblets. These bulblets can grow into new plants under favorable conditions. Bulbs are mainly of two types:
• Tunicated bulbs: These bulbs consist of concentric layers of fleshy scales that surround a small, flattened stem with a terminal bud. The outermost few scales are usually dry and thin, forming a protective cover (tunic). In simple tunicated bulbs (like onions), the scale leaves are arranged in concentric rings and covered by a common tunic. In compound tunicated bulbs (like garlic), the fleshy scales are actually small bulblets or "cloves" arranged in two or more concentric rings around a central floral axis. Each bulblet has its own tunic, and these bulblets are collectively enclosed by common overlapping tunics.
• Scaly bulbs: These bulbs have small, narrow, and separated scale leaves that do not form concentric layers. The fleshy scale leaves overlap one another only at their margins. There is no enveloping tunic or protective sheath. The lily is an example of a scaly bulb.
In simple words: Underground stems change their shape to store food, survive bad times, and grow new plants. They can be suckers, rhizomes (like ginger), corms (like taro), tubers (like potatoes), or bulbs (like onions or lilies), each with unique features and ways of growing.

🎯 Exam Tip: When describing stem modifications, always mention their specific function (e.g., storage, perennation, propagation) and give clear examples for each type. Clearly distinguish features like nodes/internodes which differentiate them from roots.