RowQ
The Vault
RowQ
The Vault
CBSE Class 11 Biology · 10 questions · 24 marks
Before you can explain how a plant works you have to be able to describe exactly what you are looking at, and that vocabulary is what this chapter delivers. Root, stem, leaf, flower, fruit and seed each come in many modified forms, and a swollen underground potato turns out to be a stem while a swollen carrot is a root. The chapter finishes with the floral formula, a compact shorthand that lets you summarise an entire flower in a single line.
Pneumatophores found in mangrove plants such as Rhizophora are modifications of the:
Answer
Root is correct — these specialised roots grow vertically upward out of the waterlogged soil and bear pores through which the underground tissue obtains oxygen.
The potato tuber is regarded as a modified stem rather than a root because it:
Answer
Bears nodes with buds in the form of 'eyes' is correct — nodes, internodes and axillary buds are stem characters, and storing starch underground without chlorophyll is something true roots do as well, so those features cannot decide the question.
In a flower where the ovary lies above all the other floral parts on the thalamus, the flower is said to be:
Answer
Hypogynous is correct — here the gynoecium occupies the highest position and the sepals, petals and stamens are inserted below it, so the ovary is described as superior, as in mustard and China rose.
Placentation in which ovules are borne on a central axis in a multi-chambered ovary, as in tomato and China rose, is called:
Answer
Axile is correct — the ovary is divided by septa into chambers and the placenta runs along the central axis with ovules attached to it, whereas in free central placentation the septa are absent.
Assertion (A): In a pinnately compound leaf of neem, an axillary bud is found only at the base of the whole leaf and never at the base of an individual leaflet. Reason (R): The leaflets of a compound leaf are outgrowths of a single lamina and do not themselves occupy the position of a leaf on the stem.
Answer
Both A and R are true and R is the correct explanation of A — because an axillary bud develops only in the axil of a true leaf, the presence of a bud is the reliable test for telling a compound leaf from a twig bearing several simple leaves.
Differentiate between racemose and cymose inflorescence.
Answer
In a racemose inflorescence the main axis continues to grow and does not end in a flower; the flowers are borne laterally on it in acropetal succession, so the oldest flowers are at the base and the youngest at the tip, as in radish and mustard. In a cymose inflorescence the main axis terminates in a flower and so its growth is limited; further flowers develop from lateral branches below in basipetal succession, so the oldest flower is at the tip and the younger ones lower down, as in Jasminum and Solanum.
Describe three modifications of the stem for functions other than support, with one example each.
Answer
First, stems are modified for food storage and perennation underground: the potato forms a tuber swollen with starch and bearing buds in its 'eyes', while ginger forms a horizontal rhizome with scale leaves and nodes. Second, stems are modified into tendrils, thin coiled structures that develop from axillary buds and help weak-stemmed plants such as gourds and grapevine to climb. Third, stems are modified into thorns, hard pointed axillary structures that protect the plant from browsing animals, as in Citrus and Bougainvillea. In addition, stems such as the runner of grass and the sucker of mint spread horizontally and give rise to new plants, serving vegetative propagation, and the flattened green stems of Opuntia carry out photosynthesis where leaves are reduced to spines.
Describe the parts of a typical flower and explain how flowers are classified on the basis of symmetry, the presence of sex organs and the position of the ovary.
Answer
A typical flower is a modified shoot in which the internodes are condensed and the floral leaves are arranged in whorls on a swollen tip called the thalamus. The outermost whorl is the calyx, made of sepals which are usually green and protect the flower in the bud stage. Inside it lies the corolla of petals, which are brightly coloured and attract insects for pollination; the sepals and petals may be free or fused, and when calyx and corolla are indistinguishable the units are called tepals forming a perianth. The third whorl is the androecium, made of stamens; each stamen has a stalk called the filament and a terminal anther, typically with two lobes containing pollen sacs where pollen grains are produced. The innermost whorl is the gynoecium, made of one or more carpels; each carpel has a swollen basal ovary containing ovules, a slender style, and a receptive stigma at the top on which pollen lands. If the carpels are free the gynoecium is apocarpous, and if they are fused it is syncarpous. Flowers are classified in several ways. On the basis of symmetry, an actinomorphic flower can be divided into two equal halves by any vertical plane through the centre, as in mustard and Datura, while a zygomorphic flower can be divided into equal halves in only one particular vertical plane, as in pea and Cassia; a flower that cannot be divided into two similar halves by any plane, as in canna, is asymmetric. On the basis of sex organs, a flower carrying both androecium and gynoecium is bisexual, while one carrying only stamens or only carpels is unisexual, staminate or pistillate respectively. On the basis of the position of the ovary with respect to the other whorls, the flower is hypogynous when the ovary is highest and the other parts are inserted below it, so that the ovary is superior; perigynous when the thalamus grows up around the ovary so that the other whorls stand at about the same level and the ovary is half inferior; and epigynous when the thalamus completely encloses and fuses with the ovary so that the other parts arise above it and the ovary is inferior.
What is a floral formula? Explain the symbols used in it and describe the different types of placentation found in flowering plants.
Answer
A floral formula is a compact line of symbols that summarises the structure of a flower so that it can be recorded and compared quickly. Symmetry is shown first, a plus sign standing for an actinomorphic flower and a percentage-like slanting symbol for a zygomorphic one, and the sex of the flower is indicated by the male and female signs or by a symbol for a bisexual flower. The whorls follow in order: K for calyx, C for corolla, P for perianth where sepals and petals are alike, A for androecium and G for gynoecium, each followed by the number of parts in that whorl. Fused parts of a whorl are shown by enclosing the number in brackets, and members of two different whorls that are joined to each other are linked by a line drawn above them. A bar placed above the G indicates a superior ovary and a bar placed below it indicates an inferior ovary. A bract present at the base is noted by Br. Placentation is the arrangement of ovules on the placenta inside the ovary and takes several forms. In marginal placentation the placenta forms a ridge along the ventral suture of a single carpel and the ovules are borne in two rows on it, as in pea. In axile placentation the margins of several fused carpels project inward to meet at the centre, dividing the ovary into chambers, with the ovules attached to the central axis, as in China rose, tomato and lemon. In parietal placentation the ovules develop on the inner wall of the ovary or on peripheral parts; the ovary is one-chambered but may become two-chambered by a false septum, as in mustard and Argemone. In free central placentation the septa are absent and the ovules are borne on a central column that stands free inside the single chamber, as in Dianthus and Primrose. In basal placentation a single ovule is attached at the base of the ovary, as in sunflower and marigold.
A school garden club is preparing labelled displays. They set out four items: a carrot with a swollen conical root, a piece of ginger with scale leaves and visible nodes, a branch of neem with many leaflets on a common stalk, and a cut tomato showing several chambers with seeds attached to a central column. (a) Name the type of tap root modification shown by the carrot and state its function. (b) Which organ is the ginger a modification of, and give the evidence from the description. (c) What kind of leaf does the neem branch show, and what single test distinguishes it from a twig bearing many simple leaves? (d) Identify the placentation seen in the cut tomato and describe it.
Answer
(a) The carrot is a conical storage tap root; the primary root is swollen and broad at the top, tapering gradually to the tip. Its function is to store food, mainly in the form of sugars and starch, which the plant uses when it flowers. (b) Ginger is a modification of the stem, specifically an underground rhizome. The evidence is the presence of nodes with scale leaves and buds, which are stem characters that a root never shows. (c) The neem branch bears a pinnately compound leaf, in which many leaflets are attached along a common axis called the rachis. The distinguishing test is the axillary bud: a bud is present only in the axil of the whole compound leaf where it joins the stem, and never at the base of an individual leaflet. (d) The tomato shows axile placentation. The fused carpels form septa that divide the ovary into several chambers, the placentae meet along a central axis, and the ovules that become the seeds are borne on this axis inside each chamber.
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