RBSE Solutions for Class 11 Biology Chapter 18 Stem: External Morphology

Rajasthan Board RBSE Class 11 Biology Chapter 18 Stem: External Morphology

RBSE Class 11 Biology Chapter 18 Multiple Choice Objective Questions

Question 1.
The Branches originate from Stem
(a) Endogenous
(b) Exogenous
(c) Leaves
(d) Root Apex

Question 2.
The thread like structures visible from stem are
(a) Thom
(b) Tendril
(c) cladode
(d) Hook

Question 3.
The fladode are examples of-
(a) Opuntia
(b) Asparagus racemosus
(c) Lemon
(d) Beetelnut

Question 4.
The Nasturtium (Jalkumbhi) is an example of-
(a) Stem
(b) Leaf
(c) Root
(d) Thom

Answers:
1. b, 2. b, 3. c, 4. a

RBSE Class 11 Biology Chapter 18 Very Short Answer Questions

Question 1.
How many type of trees are there?
Answer:
Habit of Plants:

• Herbs: They are small plants with soft non-woody stem. They usually grow to a height of less than 2 m. The herbs may be annual (e.g., Ground nut. Wheat, Mustard. Rice, Gram, etc.), biemiial (e.g., Henbane. Beet) or perennial (e.g., Canna).
• Shrubs: These are medium sized plants which branch typically at or near the base and do not have a main trunk. They are woody below and herbaceous near the apices. These plants attain a bushy appearance, e.g., Rose, Capparis, Cotton, Jas mine. etc.
• Trees: They are taller than shrubs in height. They have a stout and tall trunk with profuse branching (e.g.. Mango, Neem, Eucalyptus, Ficus bengcilensis, etc.). However, the trunk of some trees may remain unbranched when it is called caudex or columnar (e.g., Palm, Coconut, etc.).

Question 2.
Name two stem which are used as edible part?
Answer:
Ginger turmeric and potato.

Question 3.
Explain different types of stem?
Answer:
Different types of shapes of stem is observed. The name signifies the shape they are acquiring.

1. Cylindrical: Example – Most of the plants
2. Triangular: Example – Cyprus
3. Quadrangular: Example – Ocimum
4. Ribbed: Example – Casuarina
5. Flat: Example – Opuntia (Cactus)
6. Jointed: Example – Sugarcane

RBSE Class 11 Biology Chapter 18 Short Answer Questions

Question 1.
Write the functions of stem?
Answer:
Primary functions:

1.  It bears leaves and holds them in such a position so to provide maximum sunlight.
2. The stem conducts water and mineral nutrients from the roots to the leaves, flowers and fruits.
3. The stem conducts organic food from the leaves to the ’ roots and storage organs.
4. It holds flowers in such a position so it facilitates 3. pollination and fertilization.
5. The stem also bears fruits and seeds.

Secondary (or Accessory) functions:

1. Storage: In many species, the stems act as the organs for storage of reserve food material (e.g., rhizome of Ginger, corm of Col ocas la. tubers of Potato, etc.), Sugars (e.g., Sugarcane) and water (e.g., Phylloclade of Opuntia).
2. Synthesis of food: Some modified stems of xerophytic plants are green, flat and leaf like. They perform photosynthesis and help in the synthesis of food. Examples – Opuntia, Muehlenbeckia, Ruscus, Asparagus, etc.
3. Perennation: Underground modified stems of many plants (e.g., Ginger, Curcuma) tide over the unfavourable conditions and help in perennation.
4. Vegetative propagation: In many plants, stems sen e as means of vegetative propagation (e.g.. tubers of Potato, Suckers of Chrysanthemum, corms of Colocasia. etc.)
5. Protection: Some stems bear thorns (e.g., Durcmtai, prickles (e.g., Rosa), etc. to protect the plants from browsing animals.
6. Support: In weak-stemmed plants, the stem branches get modified into tendrils (e.g., Grape vine, Passiflora), thorns (e.g.. Bougainvillea) and hooks (e.g., Artabotrys) to help such plants to climb the support.

Question 2.
Differentiate between stem and root?
Answer:
Differences between stem and root

Stem	Root
mostly epidermis is covered by distinct cuticle.	Epiblema lacks cuticle.
Stem hairs are either unicellular or multi cellular they are separated from epider­mal cells by walls.	Root hairs are unice­llular.
Stomata may occur in epidermis.	They are not separated from cells of epiblema by w alls.
cortex is generally differentiated into zones like hypodermis, general cortex and endodermis.	Stomata are absent.
Hypodermis is either collenchvmatous or selerenchymatous.	Cortex is an undifferentiated mass.
Endodermis lacks casparian strips; cells of endo­dermis contain starch grains.	Hypodermis is absent
Pericycle is not present in all stems.	Thick walled exoder­mises is present in some roots in its place
Lateral branches are superficial.	Endodermis is distinct with casparian strips, pericycle is present.
They do not develop from pericycle.	Lateral roots arise from pericycle.
Vascular bundles are conjoint, collateral or concentric.	Vascular bundles are radial in arrangement.
Xylem is endarch.	Xylem is exarch.

Question 3.
Differentiate between Phylloclade and Cladode?
Answer:

Phylloclade	Cladode
Both main stem and its branches are modified into flat or circular, fleshy, green, leaf-like phylloclades.	Only the branches of stem are modified into flat leaf – like eladodes. It represents only one or two intemodes.
They have unlimited or indefinite growth.	They have limited or definite growth.
True leaves are usually caduceus.	True leaves are not caduceus. They are either reduced or modified into spines.

RBSE Class 11 Biology Chapter 18  Long Answer Type Question

Question 1.
Explain different aerial modifications of stem?
Answer:
1. Stem tendrils:
Tendrils are long, thin, win and spirally coiled climbing organs of the plants.They coil around the nearby support and help the plant to climb.They may be branched or unbranched. Stem tendrils are of the following four types-

1. Axillary , e.g., Passiflora.
2. Extra-axillary; e.g., Luffa (vern. Ghiatori), – Cucurbita (vem. Kaddoo), Lagenaria (vern. Lauki).
3. Apical bud tendrils, e.g., Grape Vine (litis vinifera). The main stem of the weak-stemmed plant stems shows sympodial branching of scorpoid type. The apical buds of successive branches are pushed to one side and modified into tendrils.
4. Floral bud or inflorescence tendrils, e.g., Antigonon In Antigonon, the flowers occur in bunch which the upper floral buds are modified to tendrils.
   

2. Stem thorns :
The axillary buds of some plants become arrested and get modified into stiff, sharp and pointed structures, called thorns. The are deep seated -structures having vascular comiections with stem. Besides reducing transpiration, they protect the plants from browsing animals. Examples – Citrus, Duranta, Bougainvillea, Pomegranate.

3. Phylloclades:
In some xerophytes, the leaves are highly reduced or modified to spines. The function of leaves (mainly photosynthesis) is taken over by the stems which get modified into flattened (e.g., Opuntia, Muehlenbeckia = cocoloba) or cylindrical (e.g., Casuarina. Euphorbia royleana, E. tirucalli, etc.) structures, called phylloclades. They are green in colour due to presence of photosynthetic pigments, show, unlimited growth and coated by thick cuticle in or wax surface to reduce transpiration.
Some phylloclades store water mucilage and latex. In Opuntia, the stem is modified into jointed, branched, flat, flesh and succulent leaf like structure.

• The leaves are small, fleshy and caducous.
• The leaves of axillary branches are modified into spines to check the loss of water.
• Each segment of phylloclade arises in the axil of a caducous leaf.
• The later falls down leaving behind a scar.
• The segment bears several nodes or areoles. The flowers are borne on the nodes.

In Euphorbia tirucalli. the phylloclade is green, cylindrical, shining and succulent. It is much branched. The leaves are delicate and formed during rainy season but shed in dry season, i.e., they are drought deciduous. The plants possess a milky latex which helps them in retaining moisture and plugging the area of injury

4. Cladodes:
These are the modifications of stem and branches of limited growth. Each cladode is green, flat or cylindrical, leaf-like structure which per-forms photosynthesis. The cladode of Asparagus consists of single intemode. They arise in clusters in the axils of scale leaves. The true leaves are modified to spines. In Riiscus aculeatus, the leaf – like cladodes are borne in the axils of scale leaves. They are the cladodes of two intemodes. The nodal portion between the two internodes is sometimes provided with a floral bud arising in the axil of bract in the middle of cladode.

5. Thalamus:

1. Thorns: They are modified stem structures. Thoms are deep seated and have vascular connection The central vascular cylinder is surrounded by bark of thick walled cells.
2. Spines: They are modified leaf parts. SpinesB have vascular connections but the vascular cylinder is not surrounded by bark of thick walled cells.
3. Prickles:They are superficial pointed outgrowths of stem and leaves. They have no vascular connections.
4. Bristles: They are hard and stiff hair on plant parts. They have thick cell walls due to deposition of silica or Calcium carbonate.

Question 2.
Explain some of the sub-aerial modifications of stem?
Answer:
Creepers: These weak – stemmed plants grow prostrate and develop adventitious roots from their nodes. Creepers are of three kinds – runners, stolons and offsets.

I. Runners:
In certain creepers, the axillary buds from the lowest leaves give rise to special narrow modified branches which creep over the surface of soil. Each modified branch is a basal intemode of the axillary bud which elongates and trails along the soil carrying the bud to a distance from the mother plant. Finally the bud gets fixed to the soil, develops adventitious roots and becomes a new daughter plant.

The process is repeated so that the single plant spreads in all directions and covers a large area. Each runner has generally one or sometimes more nodes. The nodes bear scale leaves and axillary buds. The runners serve as means of vegetative propagation. Examples – Doob grass (Cynodon dactylon), Oxalis, Colocasia, Centella (vern. Brahmi Booti).

II. Stolons: They are special kinds of runners which initially grow upwards like ordinary branches and then arch down to develop new daughter plants on coming in contact with the soil. Examples- Strawberry7 (Fragaria vesica), Mentha, etc.

III. Offsets: The are one internode long condensed runners usually found in rosette plants. Each offset produces at its apex a tuft of leaves above and a cluster of roots below. Examples – Eichhorma (water hyacinth), Pistia, etc.

Question 3.
Explain different types of weak stem?
Answer:
Weak stems: They are thin, soft, delicate and cannot stand erect without some help. They are of two types –

1. Upright weak stems
2.  Prostrate weak stems.

(a) Upright weak stems:
1.Twiners: The stems are long, slender, flexible and very sensitive. They twin or coil around an upright support on coming in its contact due to a special type of growth movement called nutation. They may coil the support to the right (anticlockwise from the top or sinistrorse) e.g., Convolvulus sp., Clitoria tematea, etc. or to the left (clockwise or dextrorse), e.g., Lablab

2. Climbers: The stem is weak and unable to coil around a support.They usually climb up the support with the help of some clasping or clinging structure. They are of four types

• Tendril climbers: Tendrils are specialised climbing organs which are capable of growing spirally around a support and help the shoot to climb up. They may be modified stem (e.g., Vitis) stem branches (e.g., Passiflora), leaves (e.g., Lathyrus aphaca), leaf parts (e.g; Gloriosa superba, Smilax, Clematis, Pea, etc.) or inflorescence stalk (e.g., Antigonon).
• Root climbers: Adventitious roots arise from the nodes and penetrate into the upright support so that the climber climbs up, e.g., Betel vine (Piper betel), Tecoma, Ivy, etc.
  
• Scramblers: These weak stemmed plants slowly grow over other bushes and rest there. They attain this position with the help of curved prickles (e.g., Rose), curved hooks on flowering peduncle (e.g., Artabotrys), prickles on stem (e.g., Lantana), spines (e.g., Climbing Asparagus) or spinous stipules (e.g., Zizyphus).
  
• Lianas: These are woody perennial climbers found in deep forests. At first, they are just like ordinary twiners but once they reach to the top and get sunlight, become woody. Example – Tinospora, Ficus, Bauhinia, Bignonia, etc.

(B) Prostrate weak stems: These stems spread over the ground for proper exposure of the leaves.

(i) Trailers (= Stragglers): These weak – stemmed plants trail along the surface and do not try to climb up any support. They do not root at intervals. They are of three types-

1. Procumbent: They lie flat on the ground, e.g., Tribulus, Bassela, Evolvulus.
2. Decumbent: Branches, after growing horizontally for some length, grow vertically up-wards, e.g., Portulaca, Tridax, Lindenbergia, etc.
3. Diffuse: Branches grow1 profusely in all directions, e.g., Boemaavia.
   

(ii) Creepers: These weak-stemmed plants grow prostrate and develop adventitious roots from their nodes. Creepers are of three kinds – runners, stolons and offsets.
I. Runners: In certain creepers, the axillary buds from the lowest leaves give rise to special narrow modified branches which creep over the surface of soil. Each modified branch is a basal intemode of the axillary bud which elongates and trails along the soil carrying the bud to a distance from the mother plant. Finally the bud gets fixed to the soil, develops adventitious roots and becomes a new daughter plant.  The process is repeated so that the single plant spreads in all directions and covers a large area.

Each runner has generally one or sometimes more nodes. The nodes bear scale leaves and axillary buds. The runners serve as means of vegetative propagation. Examples – Doob grass (Cynodon dactylon), Oxalis, Colocasia, Centella (vern. Brahmi Booti).

II. Stolons: They are special kinds of runners which initially grow upwards like ordinary branches and then arch down to develop new daughter plants on coming in contact with the soil. Examples – Strawberry (Fragaria vesica), Mentha, etc.

III. Offsets: The are one internode long condensed runners usually found in rosette plants. Each offset produces at its apex a tuft of leaves above and a cluster of roots below. Examples – Eichhorma (water hyacinth), Pistia, etc.

Question 4.
Explain different functions and general characteristics of stem?
Answer:
Primary functions:

1. It bears leaves and holds them in such a position so to provide maximum sunlight.
2. The stem conducts water and mineral nutrients from the roots to the leaves, flowers and fruits.
3. The stem conducts organic food from the leaves to the ’ roots and storage organs.
4. It holds flowers in such a position so it facilitates 3. pollination and fertilization.
5. The stem also bears fruits and seeds.

Secondary (or Accessory) functions:

1. Storage: In many species, the stems act as the organs for storage of reserve food material (e.g., rhizome of Ginger, corm of Col ocas la. tubers of Potato, etc.), Sugars (e.g., Sugarcane) and water (e.g., Phylloclade of Opuntia).
2. Synthesis of food: Some modified stems of xerophytic plants are green, flat and leaf like. They perform photosynthesis and help in the synthesis of food. Examples – Opuntia, Muehlenbeckia, Ruscus, Asparagus, etc.
3. Perennation: Underground modified stems of many plants (e.g., Ginger, Curcuma) tide over the unfavourable conditions and help in perennation.
4. Vegetative propagation: In many plants, stems sen e as means of vegetative propagation (e.g.. tubers of Potato, Suckers of Chrysanthemum, corms of Colocasia. etc.)
5. Protection: Some stems bear thorns (e.g., Durcmtai, prickles (e.g., Rosa), etc. to protect the plants from browsing animals.
6. Support: In weak-stemmed plants, the stem branches get modified into tendrils (e.g., Grape vine, Passiflora), thorns (e.g.. Bougainvillea) and hooks (e.g., Artabotrys) to help such plants to climb the support.

general characteristics of stem:

1. Stem develops from the plumule and epicotyl of the embryo.
2. Generally, it is an aerial and ascending part of the plant axis.
3. A terminal bud is present at the apex of the main axis and lateral branches and is responsible for growth in length.
4. A stem is differentiated into nodes and alternating intemodes. A node occurs where leaves are attached to the stem. Intemode is the portion of stem between the two nodes.
5. Leaves emerge from the nodes .of the axis and its branches. The lateral organs of stem (i.e.. leaves and branches) are exogenous in origin.
6. The young stem is green and performs photosynthesis.
7. Multicellular hair may be present on the stem.
8. Branches of the stem and its leaves have exogenous origin.
9. Flowers and fruits develop on the stem of mature plants.
10. It is generally erect and grows away from the soil towards light. Generally, a stem is negatively geotropic and positively phototropic.

Question 5.
Describe different types of modifications of underground stem.
Answer:
modifications of stem:
(A) underground (subterranean) Stern Modifications:
In some plants. the main stem or its branches lie below the surface of soil (underground). store food material and help in perennation(i.c.they tied over the unfavourable season). These modified stems also helps in vegetative propagation During unfavourable conditions, the aerial shoots die but the underground stem survives which gives rise to new aerial shoots on the approach of next favourable season Underground sterns are non-green and resemble roots in their general appearance. However. they can be distinguished b the presence of-

1. nodes and internodes.
2. scale leaves
3. adventitious roots,
4. axillarv and terminal buds,
5. exogenous branches.

1. Sucker.
It is a modified under – ground branch of stem that arises from the axillary7 bud of the underground part of aerial stem. It grows obliquely below the surface of soil to some distance and then emerges out in the fonn of aerial shoot. Each sucker is differentiated into nodes and inter nodes, bears scale leaves and develops adventitious roots at the base of nodes. Examples- Chrysanthemum, Mentha an’ensis (Mint, vem. Podina).

2. Rhizome:
It is a perennial underground main axis of the plant which grows indefinitely below the soil surface producing aerial leaves or shoots above ground and adventitious roots on lower,side in favourable season. They perennate during unfavourable season. Most of the rhizomes are fleshy and store reserv e food materials. They are differentiated into nodes and intemodes. On the basis of their position in the soil, rhizomes may be of two types-

• Rootstock: They are upright or oblique with their tips reaching the soil surface, e.g., Alocasict indicct;
• Straggling: They are horizontal in position and generally branched, e.g.. Nelumbo nucifera (Lotus, vern. Kamal), Ginger (Zingiber officinale), Turmeric (Curcuma domestica), Saccharum, etc.
  

3. Corm:
It is very much condensed, fleshy solid and vertically growing root stock with a large apical bud. It is usually unbranched. The shape of corm is almost spherical or somewhat It bears adventitious roots which arise from its basal portion. Each corm bears a number of circular nodes with scales and one or more axillary buds in the axil of scale leaves.

In favourable growing season, some of these buds develop adventitious roots and grow into new aerial shoots They store food material in the basal portion and form new conns. Examples, Amorphophallus (Elephant foot, ver. Zamin- kand or Suran) , Colocasia (vern. Kachalu), Fressia, Crocus sativus (Saffron, vern. Kesar),Alocasia, Glodiolus, etc. The corm of Amorphophallus campanulatus is a huge condensed single intemode with several axillary buds.

4. Tuber:
The stem tubers are swollen ends of specialized underground stem branches which arise as suckers or stolons. They are oval or spherical in shape which do not bear adventitious roots. They are usually protected by corky skin having lenticels for aeration. Each tuber bears spirally arranged depressions called eyes. These represent nodes. The eyes are more crowded towards the apical end (rose end) and less towards the stolon end (heel end).

The common example of stem tuber is potato (Solanum tuberosum). It stores starch as reserve food material. The potato tubers are used for vegetative propagation. When they are placed in the soil under favourable conditions, the axillary buds grow out in the form of branches at the expense of stored food material. Some of these branches become green aerial shoots.

5. Bulb:
The stem does not store any reserve food material. It bears a large number of fibrous adventitious roots at its base and a several fleshy sheathing leaf bases on its upper side.The leaf bases bear axillary buds which may also become fleshy to form bulblets. These bulblets grow into new plants under favourable conditions. The bulbs are of two’ types-

• tunicated
•  scaly

(a) Tunicated bulbs:
The bulb consists of concentric layers of fleshy scales surrounding the reduced stem having terminal bud. The outermost few scales of the bulb become dry and membranous forming a protective covering called tunic. These bulbs are of two types –

• Simple tunicated bulbs,eg., Allium cepa (Onion), Tulipa (Tulip), Narcissus (Nargis) and
• Compoum tunicated bulbs, e.g., Allium sativun (garlic).

In case of simple tunicated bulbs, the scale leaves are arranged in a concentric manner forming a series of rings. They are surrounded by acommon tunic on outer side. In case of compound tunicated bulbs, the fleshy scales represent bulblets or cloves which occur in two or more concentric rings surrounding the central floral axis.Each bulblet is surrounded by its own tunic. The concentric rings of bulblets are surrounded by common overlapping tunics.

(b) Scaly bulbs:
These bulbs have small, narrow and separated scale leaves which are not concentric. The fleshy scale leaves overlap one another on the margins only. The enveloping tunic or covering sheath is absent, e.g., Lilium bulbifera (Lily).

RBSE Solutions for Class 11 Biology