CBSE Class 6 Science Getting to Know Plants Exam Notes

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GETTING TO KNOW PLANTS

THE STEM

Characteristic features. The stem is a direct prolongation of the plumule. It is negatively geotropic, i.e. it grows away from the soil, and it is normally positively phototropic, i.e. it 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 an internode.

The terminal bud makes the shoot grow in length, and the axillary buds give 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, plants may be strong-stemmed or weak-stemmed. Most plants have aerial stems but some have underground stems.

Erect forms. They are tall plants with a woody trunk and are always perennials. Erect herbs and trees are of various types on the basis of the nature of their stems:

1. Excurrent: the stem grows in a vertical manner and the branches are produced in acropetal succession, as in Pinus.
2. Deliquescent: the tree grows erect but more by means of lateral buds than by the apical bud. As a result the tree has a spreading habit, as in neem and banyan.
3. Caudex: in palms the stem is very long, cylindrical and stout and bears a cluster of leaves at the top.
4. Scape: in some monocotyledons the aerial stem is absent. The stem remains underground and produces aerial shoots during favourable conditions.
5. Culm: plants like bamboo have a jointed type of stem, i.e. solid nodes and hollow internodes.

CLIMBERS

Climbers are weak-stemmed plants with a thin and long stem. They are incapable of growing vertically without support, so they produce certain structures to climb up a support. They climb with the help of tendrils, roots, etc., and are classified on the basis of the nature of their climbing organ:

1. Rootlet climbers: they produce adventitious roots to catch hold of the support. The roots may have adhesive discs at their tips or may secrete a sticky juice. Sometimes these roots act as a claw or holdfast. Examples: betel, Pothos, Indian ivy, black pepper, tecoma.
2. Scramblers: some climbers produce thorns and spines which act like hooks. 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 clinging to the support.
3. Tendril climbers: tendrils are thin, leafless and wiry structures that coil around the support in a spiral manner. A tendril can be a modification of the stem, petiole, leaf, leaflet, etc. Common examples:

1. Stem tendrils: passion flower (Passiflora), vine, balloon vine (Cardiospermum), Antigonon, Cucurbita.
2. Petiole tendrils: Smilax, Clematis, Nepenthes.
3. Leaf tendrils: wild pea (Lathyrus aphaca).
4. Leaflet tendrils: sweet pea (L. odoratus), garden pea (Pisum sativum), Naravelia.

1. Leaf climbers: some climbers climb with the help of the leaf apex, as in glory lily (Gloriosa).
2. Stem climbers or twiners: the plant does not produce any special organ of attachment. The weak-stemmed plant climbs the support by twining around it. In some of these climbers the plant always twines in a particular manner, either anticlockwise (sinistrorse) or clockwise (dextrorse). In others there is no such rule.
3. Lianes: in dense rain forests, lianes are seen twining around tall trees from base to top. They differ from ordinary twiners in being very thick and woody, and they are perennial. On reaching the top of the tree, the liane produces a canopy of leaves to receive direct sunlight. Hiptage is a very good example.

(Items 4 to 6 above continue the numbering 1 to 3 of the types of climbers.)

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.

Types of leaves. 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 leaves.
2. Scale leaves: generally minute, colourless leaves. They occur on underground rhizomes, corms, tubers, and on stems and branches, and they protect the young buds. In onion, however, the scale leaves are large, thick and fleshy and are meant for storage of food.
3. Foliage leaves: the most important parts of a plant. They are green, flat structures meant for photosynthesis.
4. Bract leaves: small leaves that occur at the bases of flowers or inflorescences.
5. Prophylls (bracteoles): they occur on the floral axis.
6. Floral leaves: the sepals, petals, stamens and carpels are modified leaves. The stamens and carpels are also known as microsporophylls and megasporophylls.

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

VENATION OF LEAVES

• Reticulate
  • Unicostate (Pinnate) (Mangifera, Ficus, etc.)
  • Multicostate (Pinnate)
    • Convergent (Zizyphus)
    • Divergent (Hibiscus rosa-sinensis, Ricinus, Carica papaya, Cucurbita sps.)
• Parallel
  • Unicostate (Pinnate) (Musa, Curcuma, Canna, etc.)
  • Multicostate (Palmate)
    • Convergent (Bambusa, grasses, etc.)
    • Divergent (Palms etc.)

Differences between a branch and a compound leaf

MODIFICATIONS OF LEAVES

1. Leaf tendrils: some climbers climb with the help of tendrils, which are modifications of leaves or leaf parts: (i) whole leaf – wild pea; (ii) upper leaflets – garden pea; (iii) terminal leaflet – Naravelia, Bignonia venusta; (iv) leaf apex – glory lily (Gloriosa); (v) petiole – Nepenthes, Clematis, Smilax.
2. Leaf spines: in a number of plants the leaves are modified into spines, either to escape transpiration or for protection. The first category includes plants like Opuntia and Asparagus.
3. Scale leaves: in several underground modified stems and in xerophytic plants like Casuarina, the leaves are colourless, thin, membranous, scaly and non-functional. In onion, however, the 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 capturing insects.
5. Bladder: bladders are modifications of leaf segments in bladderwort (Utricularia), whose leaves are highly segmented. The plant is a floating aquatic herb. The bladder-like structure catches aquatic insects etc. for their nitrogen content. The bladder has a valve-like opening provided with sensitive hairs that lets insects 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 off 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 three ways. The different arrangements are meant to prevent over-shadowing and to ensure the availability of the maximum amount of sunlight.

1. Alternate: also known as spiral arrangement. Only one leaf arises at each node. Examples: sunflower, shoeflower, tobacco.
2. 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 in Rangoon creeper (Quisqualis).
3. Whorled: more than two leaves arise from each node in the form of whorls, e.g. Nerium, Alstonia.

STIPULES

In most 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 is the underground system, usually below the soil, and originates from the radicle. The first structure to emerge from a seed is the radicle, or root. Roots grow downwards for anchorage and for absorption of water and nutrients from the soil. They are colourless, leafless, cylindrical structures.

Types of root systems

1. Primary or tap root system: a normal root system develops from the radicle. It is usually underground and positively geotropic. The primary root and its branches constitute the tap root system. The main root is called the tap root or primary root, and its 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 monocotyledons, including the cereals, the main root system is the adventitious root system, which develops from the lower nodes of the stem.

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

1. Fusiform root: a spindle-shaped root, swollen in the middle and tapering at both ends.
2. Napiform root: very much swollen at the upper end and abruptly tapering towards the lower end.
3. Conical root: broadest at the top (base) and gradually tapering towards the lower end. The hypocotyl does not contribute to the swollen root. Example: carrot (Daucus carota).
4. Tuberous root: the swollen root has no fixed shape and is irregular in outline. Example: 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.

Modified branched tap roots – pneumatophores. They are respiratory roots of certain halophytes that grow in saline marshes. The horizontally growing secondary roots of the halophytic trees produce roots which grow in an upward direction. Examples: Rhizophora, Heritiera.

Modified adventitious roots. Several plants produce roots from parts other than the radicle to perform specific functions. They are classified on the basis of the functions they perform.

A. For storage of food

1. Tuberous roots: the adventitious root is swollen and irregular in shape. Example: sweet potato (Ipomoea batatas).
2. Fasciculated roots: many tuberous roots occur in a single cluster or fascicle. Examples: dahlia, Ruellia, Manihot (tapioca), Asparagus.
3. Nodulose roots: the tips of the adventitious roots are swollen like beads. Examples: mango-ginger (Curcuma amada), turmeric (C. domestica), Costus speciosus.

B. For mechanical support

1. Prop roots: downward-growing roots which develop from horizontal aerial branches of certain trees, e.g. banyan (Ficus bengalensis). They enter the soil, grow stouter and support the long branches like pillars.
2. Stilt or brace roots: several cereals need support in the form of roots which arise only from their lower nodes. Being monocots, they lack tap roots for anchorage.
3. Buttress roots: huge trees like silk cotton (Salmalia malabarica), Terminalia and Ficus species produce horizontally spreading, plank-like buttress roots all around their bases for mechanical support.
4. Climbing roots: climbers like betel (Piper betel) produce roots, usually from their nodes, for attachment to a supporting object.

FLOWER

The flower is the most characteristic structure of the angiosperms. It is a complex unit consisting of those parts which directly or indirectly contribute to reproduction. Morphologically, a flower is a short, compact branch which has ceased growth in length and bears lateral appendages. A flower may thus be defined as a modified shoot meant essentially for reproduction.

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

A typical angiospermic flower consists of four sets of parts: the calyx, the corolla, the androecium and the gynoecium. The calyx and corolla together are known as the accessory or helping whorls. The androecium and gynoecium together are called the reproductive or essential whorls. The internodes are highly condensed. The four whorls are modified leaves.

Calyx. The outermost whorl of the flower is the calyx, 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 and the calyx is termed gamosepalous. Sepals may be sepaloid (green) or petaloid (brightly coloured, as in Delphinium). On the basis of duration, sepals may be caducous, deciduous or persistent.

Corolla. It is the second whorl of the flower, inner to the calyx, and is composed of petals. The bright colour of the petals, combined with the scent of the essential oils present in some flowers, makes the flower highly attractive to insects, which act as agents of pollination. A cylindrical, tube-like corolla is seen, e.g., in the disc floret of sunflower.

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

1. Valvate: sepals or petals meet by their edges but do not overlap.
2. Twisted: one margin of a sepal or petal overlaps the next one, while its other margin is overlapped by the previous one.
3. Imbricate: one sepal or petal is completely external, one is completely internal, and the rest are of the twisted type.
4. Quincuncial: two sepals or petals are external, two are internal, and the rest are of the twisted type.
5. Vexillary: the butterfly-shaped corolla consists of 5 petals. The odd posterior petal is called the standard (vexillum); it is the largest and is outermost. It overlaps a pair of smaller lateral petals called wings or alae. The wings in turn overlap a boat-shaped structure of two fused petals called the keel or carina.

Length of stamens. There are two types of arrangement of stamens according to their length:

1. Didynamous: in the family Lamiaceae there are four stamens arranged in two whorls. Two short stamens are in the inner whorl and two longer stamens are in the outer whorl.
2. Tetradynamous: in the family Brassicaceae there are six stamens arranged in two whorls. There are four longer stamens in the inner whorl and two shorter stamens in the outer whorl.

Gynoecium – placentation. The manner of arrangement of the placentae inside the cavity of the ovary is called placentation. The ovule or ovules are attached to the placenta. The various kinds of placentation are:

1. Marginal: monocarpellary, one-chambered ovary with placentae on the ventral suture, as in Leguminosae.
2. Parietal: polycarpellary, syncarpous, one-chambered ovary with placentae on the wall. The number of placentae corresponds to the number of carpels fused, as in Brassicaceae and Papaveraceae.
3. Axile: polycarpellary, syncarpous, several-chambered ovary with placentae on the central axis, as in Malvaceae, Solanaceae, Liliaceae.
4. Central: similar to axile placentation, except that the ovary becomes one-chambered due to breakdown of the partition walls, as in Caryophyllaceae.
5. Free-central: the ovary is one-chambered. The placenta arises from the base of the ovary as a swollen central axis, and the ovules are borne all over its surface, as in Primulaceae.
6. 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 Compositae (Asteraceae).
7. Superficial: the polycarpellary, syncarpous gynoecium has a multilocular ovary in which the ovules are borne on the partition walls, as in Nymphaeaceae.

Types of ovules

1. Orthotropous: also known as straight or atropous ovule. The funicle, chalaza and micropyle lie in one straight line. Examples: Polygonum, Piper.
2. Anatropous: also known as inverted ovule. The micropyle and the chalaza are in one straight line. It is the most common type of ovule in angiosperms.
3. Campylotropous: also known as curved ovule. The ovule is bent in such a way that the chalaza and micropyle do not lie in a straight line. Examples: gram, bean, mustard.
4. Hemianatropous: the body of the ovule is at right angles to the funicle. Example: Ranunculus.
5. Amphitropous: the curvature of the ovule affects the shape of the nucellus so that the nucellus is also curved like a horse-shoe. Example: Papaver.
6. Circinotropous: the funicle completely surrounds the body of the ovule. Example: Opuntia.

THE INFLORESCENCE

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

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).

2. 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.

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

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.

Axile: P 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.

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

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