Plant Anatomy: Cells, Meristems, Organs, Steles, Vascular Tissues and Secondary Growth

This chapter answers Section XL2.2 of XL2 Botany, one of the five optional sections of the Life Sciences (XL) paper: “Anatomy of root, stem and leaves, floral organs, embryo and young seedlings; Primary and secondary meristems; Stellar organization; Vascular system and their ontogeny; Xylem and phloem structure; Secondary growth in plants and wood anatomy; Plant cell structure and differences from animal cells.” It begins with the plant cell, then the meristems that build the plant body, the primary anatomy of root, stem, leaf, flower, embryo and seedling, the types of stele, how the vascular tissues arise and what their cells look like, and finally secondary growth, periderm and wood. Its numericals are counts read from sections and organs: vessel elements and perforation plates, the pollen output of an anther, and growth-ring widths.

1. The plant cell and how it differs from an animal cell

A plant cell is a eukaryotic cell boxed in a wall. The primary wall, laid down while the cell grows, is a network of cellulose microfibrils in a matrix of hemicelluloses and pectins; once growth stops, many cells add a thicker, layered (S1, S2, S3) and often lignified secondary wall inside it. Neighbouring cells are cemented by a pectin-rich middle lamella and joined by plasmodesmata, channels lined by plasma membrane with a strand of appressed ER (the desmotubule) running through, which make the living protoplasts one symplast. A large central vacuole bounded by the tonoplast keeps the cell turgid, stores solutes and pigments, and does much of the lytic work that lysosomes do in animal cells. Plastids (chloroplasts, chromoplasts, amyloplasts and other leucoplasts) develop from proplastids and can interconvert.

Flowering-plant cell versus animal cell
FeatureFlowering-plant cellAnimal cell
Outer supportCellulose wall; middle lamellaNo wall; extracellular matrix rich in collagen
Cytoplasmic channels between cellsPlasmodesmataGap junctions
VacuoleLarge central vacuole, lytic and storageSmall vacuoles; lysosomes digest
CentriolesAbsent; the spindle forms without themPresent at the spindle poles
CytokinesisCell plate built in the phragmoplast, growing outward to the wall at the site marked by the preprophase bandCleavage furrow pinched in by an actomyosin ring
Stored carbohydrate; main sterolsStarch in plastids; phytosterols such as sitosterolGlycogen in the cytosol; cholesterol
⚠️ Scope ‘no centrioles’ to flowering plants
Angiosperm cells lack centrioles, but bryophytes, ferns, cycads and Ginkgo make motile sperm with basal bodies. A statement that ‘plants lack centrioles’ is safe only for flowering plants.

2. Primary and secondary meristems

Meristems are regions of persistently dividing cells. By position they are apical (shoot and root tips; growth in length), intercalary (at the bases of grass internodes and leaves; regrowth after grazing or mowing) and lateral (vascular and cork cambium; growth in girth). By origin they are primary, descended directly from embryonic cells: the apical meristems and their immediate products, the protoderm, procambium and ground meristem, which give the epidermis, primary vascular tissue and ground tissue. Secondary meristems arise later from permanent cells that regain the power to divide: the cork cambium, and the interfascicular cambium formed from medullary-ray parenchyma.

Three classic ideas describe apical organisation. Hanstein’s histogen theory (1868) traced three layers, dermatogen, periblem and plerome, to epidermis, cortex and stele; it fits roots better than shoots. Schmidt’s tunica–corpus theory (1924) describes the shoot apex as one or more outer tunica layers that divide only anticlinally, adding surface, over an inner corpus that divides in all planes, adding volume. In the root, Clowes found a quiescent centre, a cup of slowly dividing cells behind the root cap that acts as an organiser and a reserve of initials; grasses also have a separate calyptrogen for the cap.

The vascular cambium has two kinds of initials: long, pointed fusiform initials, which give the axial cells (vessel elements, tracheids, fibres, sieve-tube members, axial parenchyma), and short, nearly isodiametric ray initials, which give the rays. Periclinal divisions add xylem to the inside and phloem to the outside; anticlinal divisions add initials so that the cambial cylinder keeps pace with the growing circumference.

3. Primary structure of root, stem and leaf

In the root, a single-layered epidermis (epiblema) bears root hairs; a wide parenchymatous cortex ends in the endodermis, whose radial and transverse walls carry the suberised and lignified Casparian strip, which forces water and ions into the symplast (thin-walled passage cells face the protoxylem). Inside lies the pericycle, which starts lateral roots (so they are endogenous) and, in dicots, part of the cambium. Xylem and phloem form separate, alternating strands on different radii (radial arrangement), and the xylem is exarch, protoxylem outermost. In the stem the bundles are conjoint, with xylem and phloem on the same radius, and the xylem is endarch.

Dicot and monocot organs compared
OrganDicotMonocot
RootUsually two to six xylem strands (diarch to hexarch); pith small or absent; cambium arises laterPolyarch, often more than six xylem strands; large pith; no secondary growth
StemCollenchymatous hypodermis; conjoint, collateral, open bundles in a ring round a pith (eustele)Sclerenchymatous hypodermis; closed bundles, each in a sclerenchyma sheath, scattered in undifferentiated ground tissue (atactostele); protoxylem cavity in maize; no phloem parenchyma
LeafDorsiventral: palisade on the upper side, spongy mesophyll below, more stomata on the lower surfaceIsobilateral in grasses: undifferentiated mesophyll, stomata on both surfaces, bulliform cells in the upper epidermis that roll the leaf when they lose turgor

Bundle types follow from how xylem and phloem are arranged: radial (roots); conjoint collateral, phloem outside xylem, open with cambium in dicot stems and closed in monocots; bicollateral, phloem on both sides, as in Cucurbitaceae and Solanaceae; and concentric, either amphicribral (phloem surrounding xylem, common in ferns) or amphivasal (xylem surrounding phloem, as in the secondary bundles of Dracaena). C4 leaves show Kranz anatomy: chloroplast-rich bundle-sheath cells ringed by mesophyll.

4. Floral organs, embryo and young seedling

Sepals and petals are built like small leaves (petals often with conical, light-catching epidermal cells), and every floral organ receives vascular traces from the receptacle much as leaves do from the stem, one line of evidence that the flower is a modified shoot. A typical anther is dithecous and tetrasporangiate. Its wall, from outside in, is the epidermis; the endothecium, whose fibrous thickenings drive dehiscence at the stomium; one to three short-lived middle layers; and the tapetum, which nourishes the microspores and supplies callase, sporopollenin precursors and pollenkitt, and is either secretory or amoeboid. The pollen wall has a resistant sporopollenin exine, absent at the germ pores, over a pectocellulosic intine. The ovule is a nucellus wrapped in one or two integuments that leave a micropyle, with the chalaza at its base and a funicle joining it to the placenta; most angiosperms have anatropous (inverted) ovules.

A dicot embryo has an axis with two cotyledons: the epicotyl, ending in the plumule, above them and the hypocotyl, ending in the radicle, below. A grass embryo has one shield-like cotyledon, the scutellum, pressed against the endosperm, a coleoptile sheathing the plumule and a coleorhiza sheathing the radicle. In epigeal germination the hypocotyl elongates and lifts the cotyledons above ground (bean, castor, cotton); in hypogeal germination the epicotyl elongates and the cotyledons stay below (pea, gram, maize). In the young seedling, the transition region in the hypocotyl is where the root’s radial, exarch arrangement turns into the stem’s collateral, endarch bundles.

5. Stelar organisation

The stele (van Tieghem and Douliot, 1886) is the central column of vascular tissue, with the pericycle and any pith, bounded by the endodermis. Its types form an evolutionary series. The protostele, a solid core of xylem without pith, is the most primitive. The siphonostele has a pith and is ectophloic (phloem only outside the xylem) or amphiphloic (phloem on both sides). Where leaf traces leave leaf gaps that do not overlap, a siphonostele is a solenostele; where gaps overlap, the stele is broken into a net of strands (meristeles), a dictyostele. Seed plants have a eustele (a ring of collateral bundles, in dicot and gymnosperm stems) or an atactostele (scattered bundles, in monocot stems). Leaf gaps go with megaphylls; the microphylls of lycophytes leave none.

Types of stele
SteleStructureExample
Haplostele (protostele)Smooth, solid xylem core ringed by phloemRhynia
Actinostele (protostele)Star-shaped xylem core with phloem between the armsPsilotum
Plectostele (protostele)Xylem in separate plates alternating with phloemLycopodium clavatum
Ectophloic siphonostelePith present; phloem only outside the xylemOsmunda
Amphiphloic siphonostelePith present; phloem outside and inside the xylemMarsilea rhizome
DictyosteleSiphonostele dissected into meristeles by overlapping leaf gapsDryopteris
EusteleRing of collateral bundles round a pithDicot and conifer stems
AtactosteleBundles scattered through the ground tissueMonocot stems such as maize

6. Xylem and phloem: ontogeny and structure

Procambium differentiates into primary xylem and primary phloem. The first-formed elements, protoxylem and protophloem, mature while the organ is still elongating, so protoxylem has annular or helical secondary thickenings that let the dead cell be stretched; the later metaxylem matures after elongation and has scalariform, reticulate or pitted walls. The direction of maturation names the xylem: exarch (centripetal, protoxylem outside; roots), endarch (centrifugal, protoxylem inside; seed-plant stems), mesarch (protoxylem in the middle of the strand; many ferns) and centrarch (protoxylem at the centre of the stele). Protophloem always lies outermost.

Cells of xylem and phloem
CellStructure and occurrence
TracheidDead, lignified, imperforate; water passes through pit pairs; in all vascular plants; conifer tracheids have bordered pits with a torus–margo membrane
Vessel elementDead, wide, with perforation plates (simple or scalariform) in the end walls, stacked end to end into vessels; in nearly all angiosperms and in Gnetales; absent from Amborella and the Winteraceae
Xylem fibre and parenchymaFibres (libriform fibres, fibre-tracheids) give strength; parenchyma, the living cells of xylem, stores food and forms rays
Sieve-tube memberLiving but enucleate at maturity; end walls are sieve plates with callose-lined pores; P-protein; angiosperms
Companion cellNucleate, dense cytoplasm, many plasmodesmata to its sieve-tube member, a sister cell from the same mother cell; loads and maintains it
Sieve cellNarrow, with no sieve plates but sieve areas on all walls; in pteridophytes and gymnosperms; in gymnosperms it is served by Strasburger (albuminous) cells that are not its sisters
Phloem fibre and parenchymaBast fibres (jute, flax, hemp); phloem parenchyma is absent in most monocots
⚠️ Vessels, tracheids and the fence-post count
A vessel is a column of many dead cells; a tracheid is one cell. A perforation plate is the perforated wall where two successive elements meet, so a vessel of n elements has n − 1 plates, its two outer ends being closed. Vessels are not unique to angiosperms (Gnetales have them), and not every angiosperm has them.

7. Secondary growth, periderm and wood anatomy

In a dicot stem the intrafascicular cambium inside each bundle joins the interfascicular cambium formed from medullary-ray cells between bundles, making a complete ring. It cuts off far more secondary xylem inward than secondary phloem outward; the primary phloem is crushed, and rays run through both tissues. In a dicot root, cambium arises from conjunctive parenchyma inside each phloem strand and from pericycle opposite each protoxylem point, forming a wavy ring that later becomes circular. Monocots normally lack secondary growth, but in Dracaena a cambium arising outside the primary bundles adds amphivasal secondary bundles inward.

Seasonal climates give annual (growth) rings: early wood (spring wood) with wide, thin-walled cells, then dense late wood (autumn wood) with narrow, thick-walled cells. The sharp boundary between one year’s late wood and the next year’s early wood marks a ring, so ring counts at the base of a trunk estimate age (dendrochronology). Wood whose wide early-wood vessels form a distinct band is ring-porous (oak, ash, elm); wood with vessels of similar size spread evenly is diffuse-porous (maple, birch, poplar). Gymnosperm softwood is non-porous (tracheids only); angiosperm hardwood is porous. Older inner wood becomes heartwood: dead, darker and decay-resistant, because its vessels are plugged by tyloses (parenchyma ballooning in through pits) and its walls are impregnated with tannins, resins and gums. The outer sapwood still conducts. Leaning trunks form reaction wood: compression wood on the lower side in conifers, tension wood with gelatinous fibres on the upper side in angiosperms.

Wood is read in three planes. In transverse section rings appear as concentric arcs and rays as radial lines, showing ray width but not height. In radial longitudinal section rays run across the grain as horizontal ribbons. In tangential longitudinal section rays are cut end-on as lens-shaped groups, which shows their height and width (uniseriate or multiseriate). Outside the cambium, the phellogen (cork cambium), arising in the outer cortex or epidermis, cuts phellem (cork, suberised and dead) outward and phelloderm inward; the three together are the periderm, pierced by lenticels for gas exchange. Bark is everything outside the vascular cambium: living secondary phloem (inner bark) and the dead rhytidome (outer bark).

Key takeaways

  • Flowering-plant cells have a cellulose wall with pectin-rich middle lamella, plasmodesmata with a desmotubule, a large lytic vacuole and plastids, lack centrioles, divide by a phragmoplast cell plate and store starch.
  • Meristems are apical, intercalary or lateral by position and primary or secondary by origin; the shoot apex is tunica over corpus, the root has a quiescent centre, and the cambium’s fusiform initials give axial cells while ray initials give rays.
  • Roots have radial, exarch bundles inside an endodermis with Casparian strips (dicot two to six xylem strands, monocot polyarch with large pith); stems have conjoint, endarch bundles, open in a ring in dicots and closed and scattered in monocots; dicot leaves are dorsiventral, grass leaves isobilateral with bulliform cells.
  • The anther wall is epidermis, endothecium, middle layers and tapetum; grass embryos have scutellum, coleoptile and coleorhiza; epigeal germination lifts the cotyledons on the hypocotyl, hypogeal leaves them below.
  • Steles run from protostele (no pith) to siphonostele, solenostele and dictyostele, with eusteles in dicot stems and atactosteles in monocots; protoxylem is annular or helical, metaxylem pitted; vessels have perforation plates, sieve tubes have sieve plates and sister companion cells.
  • Secondary growth adds rings of early and late wood; heartwood is dead and tylosed, sapwood conducts; ring-porous and diffuse-porous woods differ in vessel distribution; the tangential section shows ray height and width; periderm is phellem, phellogen and phelloderm.

Practice questions (18)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

  1. Which feature of a dividing cell in an angiosperm root tip distinguishes it from a dividing animal cell?

    1. A spindle organised from a pair of centrioles sitting at each pole of the cell
    2. A cleavage furrow that pinches the cytoplasm in two from outside
    3. A cell plate formed in a phragmoplast and growing out to the parent walls
    4. Breakdown of the nuclear envelope at the onset of prometaphase
    Show answer

    Answer: C — A cell plate formed in a phragmoplast and growing out to the parent walls

    Plant cytokinesis builds a new wall from the inside: Golgi vesicles fuse in the phragmoplast into a cell plate that grows outward to join the parent wall at the site marked by the preprophase band. Centrioles are absent from angiosperm cells, and a cleavage furrow is the animal mechanism. Nuclear-envelope breakdown is the tempting choice because it is a real mitotic event, but it happens in both plant and animal cells and so distinguishes nothing.

  2. Which of the following statements about plant cell structure are correct?

    1. The middle lamella that cements adjacent cells is rich in pectins
    2. Lignin is deposited mainly in the primary wall while the cell is still elongating
    3. A plasmodesma carries a strand of appressed endoplasmic reticulum, the desmotubule
    4. Glycogen granules in the cytosol are the main carbohydrate store of leaf cells
    Show answer

    Answer: A — The middle lamella that cements adjacent cells is rich in pectins; C — A plasmodesma carries a strand of appressed endoplasmic reticulum, the desmotubule

    The middle lamella is pectin-rich, and each plasmodesma has an ER-derived desmotubule running through it. Lignin is laid down mainly in secondary walls after elongation has stopped (a lignified wall cannot extend), and plants store starch in plastids, not glycogen.

  3. The interfascicular cambium of a dicot stem is classed as a secondary meristem because it

    1. arises from medullary-ray parenchyma cells that resume division after differentiating
    2. produces secondary xylem and secondary phloem rather than primary xylem and primary phloem
    3. appears only in the second growing season, after the primary body is complete
    4. lies between the vascular bundles instead of inside them as the procambium does
    Show answer

    Answer: A — arises from medullary-ray parenchyma cells that resume division after differentiating

    A meristem is secondary by origin: it arises from permanent cells that dedifferentiate and divide again, which is how interfascicular cambium forms from ray parenchyma. Producing secondary tissues is the tempting reason, but the intrafascicular cambium does the same and is a residue of procambium, primary in origin. Neither season nor position defines the class.

  4. In the tunica–corpus interpretation of the shoot apex, the cells of the tunica layers

    1. divide in all planes and so add bulk to the interior of the apex
    2. divide mainly anticlinally and so add to the surface of the apex
    3. rarely divide and act as a reserve of initials, like a quiescent centre
    4. divide periclinally to give the procambial strands of the leaf traces
    Show answer

    Answer: B — divide mainly anticlinally and so add to the surface of the apex

    Tunica layers divide anticlinally (new walls perpendicular to the surface), so they stay as discrete surface layers while increasing in area; the corpus beneath divides in all planes and adds volume, which is the tempting confusion. A slowly dividing reserve of initials is the root’s quiescent centre.

  5. A transverse section shows an epidermis with hairs, a wide cortex, an endodermis with Casparian strips, about fourteen xylem strands alternating with as many phloem strands on separate radii with protoxylem towards the outside, and a large pith. The section is most likely of

    1. a dicot root, because radial bundles with exarch xylem are found only in dicots
    2. a monocot stem, because many bundles and a large central pith are present
    3. a dicot stem, because the xylem and phloem form a ring around the pith
    4. a monocot root, because the radial bundles are polyarch and the pith is large
    Show answer

    Answer: D — a monocot root, because the radial bundles are polyarch and the pith is large

    Radial, exarch bundles mean a root; many xylem strands (polyarch) with a large pith mean a monocot root. A dicot root is the tempting choice because it shares radial exarch bundles, but it usually has two to six xylem strands and little or no pith. Stems have conjoint, endarch bundles, and a monocot stem has no distinct pith because its bundles are scattered through undifferentiated ground tissue.

  6. A monocot stem in transverse section shows scattered closed bundles in the centre, while towards the outside a meristematic zone is adding new bundles, each with xylem surrounding a central strand of phloem, embedded in lignified parenchyma. This pattern is expected in

    1. Zea mays, whose protoxylem cavities enlarge to form the new outer bundles
    2. Dracaena, whose cambium outside the primary bundles adds amphivasal bundles
    3. Cucurbita, whose bicollateral bundles divide to produce the extra outer strands
    4. Helianthus, whose interfascicular cambium completes a ring between the bundles
    Show answer

    Answer: B — *Dracaena*, whose cambium outside the primary bundles adds amphivasal bundles

    Dracaena forms a secondary meristem outside the primary bundles that cuts off secondary bundles, amphivasal (xylem around phloem), and parenchyma inward. Maize has no secondary growth, and Cucurbita and Helianthus are dicots whose cambium forms a continuous ring producing secondary xylem and phloem, not new discrete bundles.

  7. The bulliform cells of a grass leaf are

    1. large, thin-walled cells of the upper epidermis whose loss of turgor rolls the leaf in drought
    2. chloroplast-rich bundle-sheath cells that concentrate CO₂ around Rubisco in C4 grasses
    3. silica-filled short cells of the epidermis that harden the leaf against grazing animals and insects
    4. dumbbell-shaped guard cells whose swelling opens the stomatal pore in the morning
    Show answer

    Answer: A — large, thin-walled cells of the upper epidermis whose loss of turgor rolls the leaf in drought

    Bulliform (motor) cells are large, vacuolate cells in the adaxial epidermis; when they lose water the leaf rolls or folds, cutting transpiration. The Kranz bundle sheath of C4 grasses is the tempting distractor because it is another grass-leaf feature, but it lies around the veins, not in the epidermis. Silica cells and dumbbell guard cells are real grass cells with other jobs.

  8. Which of the following statements about stelar organisation are correct?

    1. The atactostele, with scattered bundles, is characteristic of dicotyledon stems
    2. A protostele has a solid central core of xylem and no pith
    3. The microphylls of lycophytes leave large leaf gaps in the stele where their traces depart
    4. A dictyostele arises when overlapping leaf gaps dissect a siphonostele into separate strands
    Show answer

    Answer: B — A protostele has a solid central core of xylem and no pith; D — A dictyostele arises when overlapping leaf gaps dissect a siphonostele into separate strands

    A protostele is a solid xylem core without pith, and overlapping leaf gaps break a siphonostele into the meristeles of a dictyostele. Scattered bundles (atactostele) belong to monocot stems; dicot stems have a eustele. Microphylls are served by a single trace that leaves no gap; leaf gaps accompany megaphylls.

  9. In the young root of a dicot, the first-formed xylem elements lie next to the pericycle and later elements mature towards the centre. This xylem is

    1. endarch, with centrifugal maturation from the pith outwards
    2. mesarch, with maturation proceeding both ways from the middle
    3. exarch, with centripetal maturation from the periphery inwards
    4. centrarch, with the protoxylem placed at the centre of the stele
    Show answer

    Answer: C — exarch, with centripetal maturation from the periphery inwards

    Protoxylem at the outside with metaxylem maturing inward is exarch xylem with centripetal development, the root pattern. Endarch, the tempting answer because it is the familiar stem pattern, is the reverse: protoxylem toward the centre and centrifugal maturation. Mesarch and centrarch place the protoxylem in the middle of the strand or at the centre of the stele.

  10. Why do protoxylem elements usually have annular or helical secondary thickenings rather than pitted walls?

    1. They carry water at a higher pressure than metaxylem and need extra rings to resist bursting
    2. They are formed by the vascular cambium, which cannot lay down a continuous secondary wall
    3. They remain alive at maturity, and continuous lignified walls would cut off their exchange
    4. They mature while the organ is still elongating, and rings or helices can be stretched apart
    Show answer

    Answer: D — They mature while the organ is still elongating, and rings or helices can be stretched apart

    Protoxylem differentiates in the zone of elongation; discontinuous rings or helices let the dead cell be stretched as its neighbours keep growing, whereas a continuous pitted wall could not extend. The bursting option is tempting because thickenings do brace the wall, but xylem water is under tension, not higher pressure, and that would not explain why only the earliest elements have this pattern. Protoxylem comes from procambium, not cambium, and it is dead at maturity.

  11. A conducting cell from the phloem of a pine is narrow and elongated, has no sieve plates but sieve areas scattered over all its walls, and is supported by an adjacent cell that is not its sister. This cell is a

    1. sieve-tube member, supported by a companion cell
    2. sieve cell, supported by a Strasburger (albuminous) cell
    3. tracheid, supported by a ray parenchyma cell lying beside it
    4. companion cell, supporting a sieve-tube member
    Show answer

    Answer: B — sieve cell, supported by a Strasburger (albuminous) cell

    Gymnosperm phloem conducts through sieve cells, with sieve areas on all walls and no sieve plates, served by Strasburger cells that do not share a mother cell with them. A sieve-tube member is the tempting choice, but it is the angiosperm cell, with sieve plates on its end walls and a sister companion cell. Tracheids are dead xylem cells, and a companion cell does not conduct.

  12. Which of the following statements about xylem and phloem cells are correct?

    1. A mature sieve-tube member keeps a large nucleus that directs its metabolism
    2. Vessels occur in the Gnetales even though most gymnosperms lack them
    3. A companion cell and its sieve-tube member arise from two unrelated mother cells
    4. Amborella trichopoda is an angiosperm whose xylem lacks vessels
    Show answer

    Answer: B — Vessels occur in the Gnetales even though most gymnosperms lack them; D — *Amborella trichopoda* is an angiosperm whose xylem lacks vessels

    Gnetales (Gnetum, Ephedra, Welwitschia) have vessels although other gymnosperms do not, and Amborella conducts only through tracheids. A mature sieve-tube member is enucleate and depends on its companion cell, and the two are sister cells cut from one mother cell by an unequal division.

  13. A disc cut from the base of a trunk shows 72 annual rings in a radius of secondary xylem of 18 cm, measured from the edge of the pith to the cambium. What is the mean width of one annual ring, in millimetres, to one decimal place?

    Numerical answer — type the value.

    Show answer

    Answer: 2.5

    Each annual ring adds one band to the radius, so mean ring width = radius ÷ number of rings = 180 mm ÷ 72 = 2.5 mm. Forgetting to convert 18 cm to 180 mm gives 0.25, and dividing the diameter instead of the radius doubles the answer.

  14. A vessel 6.0 mm long is made of vessel elements, each 0.25 mm long, joined end to end in a single file. A perforation plate is the perforated wall where two successive elements meet, and the two end walls at the extreme ends of the vessel are imperforate. How many perforation plates does the vessel contain?

    Numerical answer — type the value.

    Show answer

    Answer: 23

    Number of elements = 6.0 ÷ 0.25 = 24. Perforation plates sit only at the junctions between successive elements, and 24 elements in a single file have 23 junctions, so the vessel has 23 plates. Answering 24, or 48 by counting two end walls per element, is the fence-post slip: the two ends of the vessel are closed.

  15. A typical dithecous, tetrasporangiate anther has 18 microspore mother cells in each microsporangium. If every mother cell completes meiosis and every microspore develops into a pollen grain, how many pollen grains does one anther release?

    Numerical answer — type the value.

    Show answer

    Answer: 288

    Four microsporangia × 18 mother cells = 72 mother cells; each meiosis gives a tetrad of 4 microspores, so 72 × 4 = 288 pollen grains. Counting only two sporangia (one per theca) gives 144, the commonest slip.

  16. To measure the height of the wood rays and see whether they are uniseriate or multiseriate, the most useful section of secondary xylem is the

    1. transverse section, in which rays show as radial lines crossing the rings
    2. radial longitudinal section, in which rays run across the grain as ribbons
    3. tangential longitudinal section, in which rays are cut end-on as lens shapes
    4. transverse section of the bark, in which rays flare out as phloem dilatations
    Show answer

    Answer: C — tangential longitudinal section, in which rays are cut end-on as lens shapes

    A tangential section cuts rays across, so each appears as a spindle or lens of cells whose height (cells tall) and width (seriation) can be counted. The radial section is the tempting choice because rays are prominent in it, but it shows them side-on, with their radial length and cell types, not their width. A transverse section shows ray width but not height.

  17. Heartwood resists decay better than sapwood mainly because

    1. its living parenchyma cells produce defence proteins whenever fungi attack the inner wood
    2. it consists only of late wood, whose thick walls are too dense for fungal hyphae
    3. it is the outermost wood and so is the first to be covered by a protective periderm
    4. its vessels are blocked by tyloses and its walls impregnated with tannins and resins
    Show answer

    Answer: D — its vessels are blocked by tyloses and its walls impregnated with tannins and resins

    As wood ages into heartwood its parenchyma dies after depositing extractives (tannins, resins, gums, phenolics) and pushing tyloses into the vessels, which makes it darker, non-conducting and resistant to decay. Active living defence is the tempting idea, but it fits sapwood, whose parenchyma is still alive; heartwood is dead. Heartwood contains both early and late wood and lies inside, not outside, the sapwood.

  18. In the germination of a maize grain, the sheath that protects the plumule as it pushes up through the soil is the

    1. coleorhiza
    2. scutellum
    3. coleoptile
    4. epiblast
    Show answer

    Answer: C — coleoptile

    The coleoptile is a closed sheath over the plumule that pierces the soil; the first leaf breaks through its tip once it reaches light. The coleorhiza is the tempting choice, but it sheathes the radicle, not the plumule. The scutellum is the cotyledon that absorbs food from the endosperm, and the epiblast is a small outgrowth opposite it in some grasses.