RowQ
The Vault
RowQ
The Vault
CBSE Class 11 Biology · 10 questions · 24 marks
Slice a stem across, put it under a microscope, and the plant's whole plumbing and construction plan is laid out in front of you. This chapter deals with the tissues behind that view — the meristems that keep dividing, the simple and complex permanent tissues they produce, and the three tissue systems that make up every organ. You will also learn to tell a monocot stem from a dicot stem at a glance, a skill worth easy marks in any practical.
Which tissue provides mechanical support to young stems and petioles while still allowing them to bend?
Answer
Collenchyma is correct — its walls are thickened with cellulose and pectin only at the corners and the cells remain living, so it gives support without the rigidity of lignified sclerenchyma.
Vascular bundles that are scattered throughout the ground tissue and lack cambium are characteristic of a:
Answer
Monocot stem is correct — its bundles are conjoint, collateral and closed and lie scattered rather than in a ring, which is why a monocot stem such as maize cannot undergo normal secondary growth.
Large, empty, thin-walled cells present in the upper epidermis of a monocot leaf that help roll the leaf in dry conditions are called:
Answer
Bulliform cells are correct — when they lose water they become flaccid and the leaf curls inward, cutting down transpiration; guard cells by contrast control the stomatal pore.
Sieve tube elements are able to function in translocation even though at maturity they:
Answer
Lack a nucleus is correct — the enucleate sieve tube is kept alive and functional by the adjacent companion cell, which retains its nucleus and is connected to it through numerous plasmodesmata in pit fields.
Assertion (A): A monocotyledonous stem such as that of maize does not show annual rings. Reason (R): The vascular bundles of a monocot stem are closed and contain no cambium, so no secondary tissue is added year after year.
Answer
Both A and R are true and R is the correct explanation of A — annual rings are formed by the seasonal activity of the vascular cambium producing early and late wood, so a stem with no cambium can produce no rings and cannot be aged by counting them.
Give two anatomical differences between a dorsiventral leaf and an isobilateral leaf.
Answer
In a dorsiventral leaf the mesophyll is clearly differentiated into an upper layer of elongated, closely packed palisade parenchyma and a lower layer of loosely arranged spongy parenchyma with large air spaces, whereas in an isobilateral leaf the mesophyll is uniform throughout and is not divided into palisade and spongy layers. Secondly, a dorsiventral leaf bears far more stomata on the lower surface than on the upper, while an isobilateral leaf has roughly equal numbers of stomata on both surfaces; the isobilateral leaf additionally has bulliform cells in the upper epidermis, which the dorsiventral leaf lacks.
Describe the composition of xylem and state the function of each of its elements.
Answer
Xylem is a complex tissue made of four kinds of elements. Tracheids are elongated cells with tapering ends and thick lignified walls; they are dead at maturity and conduct water while also giving strength. Vessels are long tubes formed by many cells placed end to end whose common walls have dissolved, so water passes through them with far less resistance; they too are dead and lignified and are found in angiosperms but not in most gymnosperms. Xylem fibres are dead sclerenchymatous cells with thick walls and a very narrow lumen that provide purely mechanical support. Xylem parenchyma is the only living component; it stores food such as starch and fat and takes part in the short-distance radial conduction of water.
Compare the internal structure of a typical dicot stem with that of a monocot stem under the following heads: epidermis and hypodermis, ground tissue, vascular bundles, and capacity for secondary growth.
Answer
In a dicot stem the epidermis is a single outer layer covered by a cuticle and bearing multicellular trichomes and a few stomata. Beneath it the hypodermis is made of a few layers of collenchyma that provide flexible mechanical strength to the young stem. In a monocot stem the epidermis is similarly single-layered and cuticularised, but the hypodermis consists of sclerenchyma, giving a harder and more rigid outer region. The ground tissue of a dicot stem is clearly differentiated into zones: a cortex of parenchyma outside, then a distinct endodermis whose cells are rich in starch and are called the starch sheath, then a pericycle often in patches of sclerenchyma, then medullary rays running between the vascular bundles, and finally a central pith. The ground tissue of a monocot stem is not differentiated at all; it is a continuous mass of parenchyma in which the bundles are embedded, and the centre frequently breaks down to form a hollow pith cavity. The vascular bundles of a dicot stem are conjoint, collateral and open, with xylem towards the centre and phloem towards the periphery and a strip of cambium between them, and they are arranged in a definite ring. The vascular bundles of a monocot stem are conjoint, collateral and closed, with no cambium between xylem and phloem; they are numerous, scattered irregularly through the ground tissue, and each is surrounded by a sheath of sclerenchyma. In monocots the smaller bundles lie near the periphery and the larger ones towards the centre, and each bundle typically shows a water-containing lysigenous cavity below the protoxylem. Because the dicot bundle contains cambium, a continuous ring of vascular cambium can form and produce secondary xylem inward and secondary phloem outward, so dicot stems increase in girth, develop wood and annual rings, and also form a periderm through the activity of the cork cambium. Monocot stems, lacking cambium altogether, show no such normal secondary growth and remain nearly the same thickness throughout life.
Explain how secondary growth occurs in a dicotyledonous stem and describe the formation of annual rings and lenticels.
Answer
Secondary growth is the increase in the girth of a stem brought about by two lateral meristems, the vascular cambium and the cork cambium. In the young dicot stem, cambium is present only as strips inside the vascular bundles, called intrafascicular cambium. Cells of the medullary rays lying in line with these strips then become meristematic and form the interfascicular cambium, so that a continuous cambial ring is established. This ring divides actively, cutting off new cells towards the inside that differentiate into secondary xylem, and cells towards the outside that differentiate into secondary phloem. Since much more secondary xylem is produced than secondary phloem, the wood accumulates and pushes the earlier tissues outward, and the primary phloem is gradually crushed. Some cambial cells also produce narrow strips of parenchyma running radially, the secondary medullary rays, which carry water and food sideways. The activity of the cambium is not uniform through the year. In spring, when water is plentiful and growth is rapid, the cambium is very active and produces xylem with wide vessels and thin walls, called spring wood or early wood, which is light in colour and low in density. In autumn the cambium is less active and produces xylem with narrow vessels and thicker walls, called autumn wood or late wood, which is darker and denser. One ring of light spring wood together with the adjoining ring of dark autumn wood makes a single annual ring, representing one year of growth, so the approximate age of a tree can be estimated by counting these rings. Meanwhile, as the stem thickens, the outer layers are stretched and would rupture, so a cork cambium or phellogen arises in the cortical region. It cuts off cork or phellem outward, whose cells become dead and suberised and form a waterproof protective layer, and secondary cortex or phelloderm inward, which is living parenchyma. At certain points the phellogen cuts off loosely arranged parenchymatous cells instead of compact cork, and these push through the surface to form small lens-shaped openings called lenticels, through which gaseous exchange takes place between the internal living tissue and the outside air.
A practical class is given three unlabelled transverse sections to identify. Section P shows vascular bundles arranged in a ring, each with cambium between xylem and phloem, and a clear pith at the centre. Section Q shows many small bundles scattered in undifferentiated ground tissue, each ringed by thick-walled cells, with a hollow space in the middle. Section R shows xylem and phloem lying on alternate radii with no cambium between them, and a broad pith. (a) Identify the organ and plant type for section P and name the feature that makes secondary growth possible in it. (b) Identify section Q and explain why the middle appears hollow. (c) Name the type of vascular bundle seen in section R and identify the organ and plant type. (d) In which of the three sections would you expect to find a pericycle giving rise to lateral roots?
Answer
(a) Section P is a dicotyledonous stem. The bundles are conjoint, collateral and open, and it is the presence of cambium within each bundle that allows a complete cambial ring to form and produce secondary xylem and secondary phloem, so the stem can increase in girth. (b) Section Q is a monocotyledonous stem. Its ground tissue is not differentiated into cortex and pith, and the parenchyma in the central region breaks down as the stem matures, leaving a hollow pith cavity such as the one seen in a maize or grass stem. (c) Section R shows radial vascular bundles, in which xylem and phloem occur on separate, alternating radii rather than on the same radius. The broad pith along with the radial arrangement indicates a monocotyledonous root. (d) The pericycle produces lateral roots in the root sections. Here that is section R, the monocot root, where the pericycle lies just inside the endodermis; in a dicot root the pericycle additionally contributes to the vascular cambium and cork cambium during secondary growth.
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