CBSE Class 6 Science Getting to Know Plants Exam Notes

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Getting to Know Plants


The Stem
Characteristic Features
It is a direct prolongation of the plumule. It is negatively geotropic i.e. grows away from the soil. It is normally positively phototropic i.e. grows or bends towards the light. It bears branches, leaves and flowers. It is differentiated into nodes and internodes which may not be distinct in some cases. The space between two successive nodes is called as internode.

The terminal bud is present at the growing in different directions by giving rise to new shoots sideways. If any bud terminal or axillary ends in a floral bud, the growth in the length of the shoot stops. It may however grow sideways by lateral buds.

Forms of Stem
Stems can be of several types depending upon their general form. Broadly speaking they may be strong or weak-stemmed plants. Most of the plants have aerial stems but some have underground stem.

Erect forms
They are tall plants with woody trunk. They are always perennials.Erect herbs and trees are of various types on the basis of the nature of their stems.
1. Excurrent: In this type of the stem grows in a vertical manner and the branches are produced in an acropetal succession as in Pinus.

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2. Deliquescent: The tree grows erect but more by means of lateral buds than by the apical bud. As a result of this the tree has a spreading habit, as in neem, banyan etc.

3. Caudex: In palms etc. the stem is of caudex type. It is very long, cylindrical and stout and bears a cluster of leaves at the top.

4. Scape: In some of the monocotyledons aerial stem is absent. The stem remains underground and produces aerial shoots during favourable conditions.

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5. Culm: Plants like bamboo have jointed type of stem, i.e., they have solid nodes and hollow internodes.

Climbers
They are weak-stemmed plants with thin and long stem and are, therefore, incapable of growing vertically without any support. Such plants, therefore, produce certain structures to enable them to climb up the support and are called climbers. They climb the support with the help of tendrils, roots, etc. They are classified on the basis of the nature of their climbing organ. An account of the climbers is given below :

1. Rootlet Climbers: These climbers produce adventitious roots to catch hold the support. The roots may have adhesive discs at their tips or may secrete a sticky juice for the purpose. Sometimes these roots act as a claw or holdfast. Some of the examples are betel, Pothos, Indian ivy, black pepper, tecoma etc.

2 Scramblers: Some climbers produce thorns and spines which act like hooks to help the plant in getting hold of the support. Spines act as hooks in rose and cane. In Bougainvillea and Uncaria the thorns act as hooks whereas in Artabotrys a hook is produced from the flower-stalk. In cat’s nail (Bignonia venusta) the terminal leaflets are modified into curved hooks for the purpose of clinging to the support.

3. Tendril Climbers: Tendrils are thin, leafless and wiry structures. They coil around the support in a spiral manner. Tendril can be modifications of stem, petiole, leaf, leaflet, etc.

Following are some of the common examples of tendril climbers :
a) Stem-climbers – Passion flower (Passiflora), vine, balloon vine (Cardiospermum), Antigonon, Cucurbita etc.
b) Petiole-tendril– Smilax, Clematis, Nepenthes.
c) Leaf tendril – Wild pea (Lathyrus aphaca).
d) Leaflet tendril – Sweet pea (L.odoratus), Garden pea (Pisum sativum), Naravelia.
 

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4. Leaf Climbers: Some climbers climb with the help of the leaf-apex as in glory lily (Gloriosa).

5. Stem Climbers or Twiners: In this type the plant doesn’t produce any special organ of attachment for the purpose of climbing. The weak-stemmed plant climbs the support by twining around it. In some of these climbers the plant always twines in a particular manner, i.e., either anticlockwise (sinistrorse), or clockwise (dextrorse). In others there is no such rule about the nature of twining.

6. Lianes: In the dense rain forests, lianes are seen twining around the tall trees from base to the to They differ from ordinary twiners in being very thick and woody. They are also perennial in nature. On reaching the top of the tree around which it twines, the twiner produces a canopy of leaves to receive direct sunlight. Hiptage is a very good example of this type of climber.

 Leaves 

The leaf is a specialised organ of photosynthesis in a plant. It develops from a node as a lateral outgrowth of the stem or a branch.

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 Types of Leaves

 The leaves are generally classified into six types :

  1. Cotyledonary leaves : In dicotyledonous plants except castor, the cotyledons come above the ground and develop into the first pair of leave
  1. Scale leaves: These are generally minute colourless leave They occur on underground rhizomes, corms, tubers, stem and branches of leaves, etc. They protect the young buds. In onion, however, the scaly leaves are large, thick and fleshy and are meant for storage of food.
  1. Foliage Leaves: They are the most important parts of a plant. They are green, flat structures and are meant for photosynthesi
  1. Bract Leaves: They are small leaves and occur at the bases of flowers or inflorescence
  1. Prophylls (bracteoles)They occur on the floral axis.
  1. Floral Leaves: The sepals, petals, stamens and carpels are modified leave 

The stamens and carpels are also known as microsporophylls and megasporophylls.

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Venation is the arrangement of the veins and the veinlets in a leaf. If the veinlets form a network it is termed reticulatand if the veins run parallel to each other it is termed parallel 

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Modifications of Leaves

1. Leaf-tendrils : Some of climbers climb with the help of tendrils which are modifications of leaves or leaf structure Some of the examples are as follows: 

(iWhole leaf-Wild pea. (ii) Upper leaflets-Garden pea. (iii) Terminal leaflet-NaraveliaBignonia venusta(iv) Leaf apex-Glory lily (Gloriosa). (v) Petiole-NepenthesClematisSmilax.

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2. Leaf spines : In a number of plants leaves are modified into spines either to escape transpiration or for protection. The first category includes plants like OpuntiaAsparagus, etc.
3. Scale-leaves : In several underground modified stems, xerophytic plants like Casuarina the leaves are colourless, thin, membranous, scaly and non-functional. In onion, however, scale-leaves store food and water and are, therefore, thick and fleshy.
4. Pitcher : The leaf-blade is modified into a pitcher-like structure in the pitcher plant (Nepenthes) for the purpose of capturing insect

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5. Bladder: Bladder are modification of leaf-segments in bladderwort (Utricularia), the leaves of which are highly segmented. The plant is a floating aquatic her The bladder like structure is meant for catching the aquatic insects, etc. fortheir nitrogen content. The bladder has a valve-like opening provided with sensitive hairs to let the insects , et c. t o go into the cavity but prevents them from coming out.
6. Phyllode: In Australian acacia (Acacia auriculiformis) and Parkinsonia aculeata the leaf is bipinnate compound. The leaflets gradually fall down and the rachis becomes flattened and green in colour to carry on photosynthesis

Phyllotaxy

The leaves are arranged on the stem or its branches in one of the three following ways. Different arrangements of leaves are meant to prevent over-shadowing to ensure availability of maximum amount of sunlight.

  1. Alternate: It is also known as spiral arrangement. In this type only one leaf arises at each node. Examples of alternate leaves are to be seen in a very large number of plants including sunflower, shoeflower, tobacco, etc.
  1. Opposite: The leaves occur in pairs at every node. When all the leaf pairs lie in one vertical plane, i.e., there are two vertical rows of leaves, the arrangement is known as opposite superposed, as for example in Rangoon creeper (Quisqualis).
  1. Whorled: In this type more than two leaves arise from each node in the form of whorls, e.g., in NeriumAlstonia etc.

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Stipules 

In most of the dicotyledonous plants the leaf bases produce a pair of lateral outgrowths called stipules. Sometimes a stipule is an appendage of the leaf base itself. Their chief role is to protect the young axillary buds. 

Roots 

The Root System : It is the underground system, usually below the soil and originates from the radicle.

Types of Root Systems

The first structure to emerge out of a seed is the radicle or the root. The roots grow in a downward direction for the purpose of anchorage and absorption of water and nutrients from the soil. They are colourless, leafless, cylindrical structures.

  1. Primary or tap root system : A normal root system always develops from the radicle. It is usually underground and positively geotropic. The primary root and its branches constitute the primary or tap root system. While the main root is called tap root or primary, the branches are termed secondary, tertiary, etc. The primary root grows vertically downward whereas its branches grow in an oblique or horizontal manner.
  2. Adventitious root system : In almost all the monocotyledons including the cereals, the main root system is the adventitious root system, which develops from the lower nodes of the stem.

I. Modified Tap Roots

They are modified for the purpose of storage of food and are, therefore, fleshy and swollen. The modified tap roots are classified according to the shape they assume.

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  1. Fusiform root : It is a spindle-shaped root. It is swollen in the middle and is tapering at both the end
  2. Napiform root : It is very much swollen at the upper end and is abruptly tapering towards the lower end.
  3. Conical root : The conical root is broadest at the top (base) and is gradually tapering towards the lower en The hypocotyl does not contribute to the swollen root. The example of conical root is carrot (Daucus carota,).
  4. Tuberous root : When the swollen root has no fixed shape and is irregular in outline it is known as tuberous. An example of tuberous root is four O’ clock plant (Mirabilis).
  5. Nodulated root : In several members of the family leguminosae the roots are symbiotically associated with the nitrogen-fixing bacterium, Rhizobium leguminosarum.

 II. Modified Branched Tap Roots

Pneumatophores : They are respiratory roots of certain halophytes, which grow in saline marshes. The horizontally growing secondary roots of the halophytic trees produce roots which grow in an upward direction. Examples of plants bearing pneumatophores are RhizophoraHeretiera etc.

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III.Modified Adventitious Roots

There are several instances of plants which produce roots from various parts of the plant other than the radicle for per forming specific functions. Such modified adventitious roots are classified on the basis of the functions they perform.

 A. For Storage of Food

  1. Tuberous roots : When t he adventitious root is a swollen and irregular-shaped it is known as tuberous or tubercular root, e.g., sweet potato (Ipomoea batatas).

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  1. Fas ciculated roots : When many tubercular roots occur in a single cluster or fascicle they are called fasciculated roots, e.g., DahliaRuellia,Manihot (Tapioca), Asparagus, etc.
  1. Nodulose roots: When the tips of adventitious roots are swollen like beads they are called nodulose root s , e. g. , mango-ginger (Curcuma amada) turmeric (C. domestica), Costus speciosus, etc.

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B. For Mechanical Supports 

  1. Prop roots: They are downward growing roots which develop from horizontal aerial branches of certain trees, e.g., banyan (Ficus bengalensis). They enter into the soil and grow stouter. They support the long branches of the trees as pillar
  1. Stilt or Brace roots: Several cereals need support for their plants in the form of roots which arise only from their lower node Being monocots they lack tap roots for their anchorage. 
  1. Buttress roots: Huge trees like silk cotton tree (Salmalia malabarica), TerminaliaFicus sp, etc. produce horizontally spreading plank-like buttress roots all around their bases for giving mechanical support to the plant.

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  1. Climbing roots : Climbers like betel (Piper betel) produce roots usually from their nodes for attachment to a supporting object aroundit.

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FLOWER

 The flower is the most characteristic structure of the angiosperms. It is a complex unit consisting typically of those parts which directly or indirectly contribute to the process of reproduction.

 Morphologically, flower represents a short and compact branch which has ceased growth in length and bears lateral appendages. Thus flower may be defined as a modified shoot meant essentially for the reproduction of the plant.

 The flower usually develops as a branch from a bud, growing in the axil of a small leaf - like structure known as bract. A stalk, called pedicel, supports the flower in the axil of the bract. The upper swollen end of the pedicel, called receptacle or thalamus, bears all the floral parts.

 A typical angiospermic flower consists of four different sets of parts – the calyx, the corolla, the androecium and the gynoecium. Of these, calyx and corolla together are known as accessory or helping whorls. The androecium and gynoecium together are called reproductive or essential whorls.

 The internodes are highly condensed. It has four whorls of modified leaves namely calyx, corolla, androecium and gynoecium.

 CALYX

The outermost axillary whorl of the flower is the calyx. It is represented by the sepals. If the sepals are free from one another the calyx is termed polysepalous. In other flowers the sepals are wholly or partially united with one another, and are termed gamosepalous. Sepals may be sepaloid (green) or petaloid (brightly colored as in Delphinum). On the basis of duration sepals may be caducous, deciduous or persistant.

COROLLA

It is the second whorl of flower present inner to calyx and is composed of petals. The bright color of the petals combined with the scent of essential oils present in some flowers make the flower highly attractive to insect s which act as a gent s for pollination. C ylindrical t ube like str uctu re e.g. Disc floret of sun flower.

Aestivation 

It is the arrangement of sepals/petals in relation to one another in a floral bud. It is of five types

i) Valvate: Sepals/petals meet by their edges but don’t overlap.

ii) Twisted: One margin of sepals/petals is overlapped but the other margin is overlapping.

iii) Imbricate : One sepal/petal is completely external, one is completely internal and rest are of the twisted type.

iv) Quincuincial: In this case 2 sepals/ 2 petals are external, two are internal and rest are of the twisted type.

v) Vexillary : Butterfly shaped corolla consists of 5 petal Odd posterior petal is called standard. It is largest and is outermost. It overlaps a pair of smaller lateral petals called wings or alae. Alae in turn overlap a boat shaped two fused petals called keel or carina.

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Length of Stamens

There are two types of arrangements of stamens according to the length of stamens :

 a) Didynamous : In the family Lamiaceae there are four stamens arranged in two whorl Two short stamens are in the inner whorl and two longer stamens are in the outer whorl.

 b) Tetradynamous : In the family Brassicaceae there are six stamens arranged in two whorl There are four longer stamens in the inner whorl and two shorter stamens in the outer whorl. 

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Gynoecium

 The manner of arrangement of placentae inside the cavity of the ovary is called placentation. The ovule or ovules are attached to the placenta. A brief account of various kinds of placentation is given below: 

Marginal: Monocarpellary, one chambered ovary has placentae on the ventral suture, as in Leguminosae.

Parietal:Polycarpellary, syncarpous one chambered ovary having placentae on the wall. The number of placentae correspond to the number of carpels fused as in Brassicaceae and Papaveraceae.

AxileP olyca r p ella r y, s ynca r p ou s , s ever a l chambered ovary having placentae on the central axis, as in Malvaceae, Solanaceae, Liliaceae, etc.

CentralIt is similar to axile placentation except that it becomes one chambered due to breakdown of the partition walls, as in Caryophyllaceae.

Free-CentralThe ovary is one-chambered. The placenta arises from the base of the ovary in the form of a swollen central axis. The ovules are borne a ll over t he s u r fa ce of t he p la cent a , a s in Primulaceae.

Basal: The ovary is one chambered. The placenta is very small and arises directly on the thalamus. Only one ovule is borne at the base of the ovary, as in Compositate (Asteraceae).

Superficial : The polycarpellary, syncarpous gynoecium has a multilocular ovary in which the ovules are borne on partition walls, as in Nymphaeaceae.

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Types of Ovules

Orthotropous: It is also known as straight or atropous ovule. The funicle, chalaza and micr op yle lie in one s t r a ight line. e. g. Polygonum, Piper.

Anatropous: It is also known as inverted ovule. The micropyle and the chalaza are in one straight line. It is the most common type of ovule found in angiosperms.

Campylotropous: It is also known as curved ovule. The ovule is bent in such a way that chalaza and the micropyle do not lie in a straight line. e.g. gram, bean, mustard etc. 

Hemianatropous: In this type, the body of ovule is situated at right angle to the funicle. e.g. Ranunculus

Amphitropous: The curvature of the ovule affects the shape of the nucellus in such a way that the nucellus is also curved like a horse-shoe e.g., Papaver.

Circinotropous: In this type of ovule funicle completely surround the body of ovule. e.g. Opuntia

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The Inflorescence

An inflorescence is a reproductive shoot bearing a number of flowers or sometimes only a single flower.

 

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