Plant Anatomy

(b) Subsidiary cells

The specialised epidermal cells that lie around the guard cells are called subsidiary cells. Bulliform cells are large, thin-walled cells on grass leaves that help in rolling and unrolling. Lenticels are pores in the bark for gas exchange, and "complementary cells" are the loose cells in lenticels.

(d) (d), (a), (c), (b)

In an older woody dicot stem the order outwards to inwards is periderm: phellem (cork), then phellogen, then secondary cortex (phelloderm), then secondary phloem, vascular cambium and finally wood (secondary xylem). Thus phellem, secondary cortex, secondary phloem, wood.

(b) Protoxylem

In an old dicot stem and root both have secondary xylem, secondary phloem and cortical cells, so these cannot separate them. The position of protoxylem does: it is endarch (towards the pith) in the stem and exarch (towards the periphery) in the root.

(a) being imperforate

Tracheids are dead cells with lignified walls and no perforations, while vessels have perforations at their end walls. Lack of nucleus and lignification are shared with vessels, and Casparian strips belong to the endodermis.

(a) Medullary rays

In a dicot stem the cambium inside the vascular bundles is the intrafascicular cambium. Cells of the medullary rays between bundles become meristematic and form the interfascicular cambium, completing the cambial ring.

(b) Gaseous exchange

Lenticels are lens-shaped openings in the periderm of woody stems that allow exchange of gases between inner tissue and the atmosphere. Transpiration through them is minor (lenticular), but gaseous exchange is the main function.

(c) Number of annual rings

In temperate trees one ring of secondary xylem forms each year (spring and autumn wood), so the count of annual rings gives the age. Height, girth, biomass and heartwood diameter vary with conditions.

(c) Phellem

Phellem (cork) cells are dead at maturity, with suberised walls and no protoplasts. Xylem parenchyma and collenchyma are living. Phloem is mostly living (sieve tubes, companion cells), though phloem fibres are dead, so it is not the best answer.

(c) Secondary xylem

The vascular cambium cuts off secondary xylem inwards and secondary phloem outwards. Phelloderm and periderm come from the cork cambium (phellogen). Primary phloem comes from the procambium.

(c) It conducts water and minerals efficiently

Heartwood does not conduct water and minerals because its vessels are blocked by tyloses and it consists of dead elements. It is dark, durable and rich in tannins, resins and oils. Sapwood (alburnum) is the part that conducts water.

(a) Dumb-bell shaped

Grasses (monocots) have dumb-bell shaped guard cells with thick inner walls and thin bulbous ends. Kidney or bean shaped guard cells are typical of dicots.

(b) Vascular cambium

The vascular cambium is a lateral meristem that cuts off secondary xylem towards the inside and secondary phloem towards the outside. Phellogen produces cork (periderm), and apical meristems give primary growth.

(d) Endodermis

Casparian strips are bands of suberin deposited on the radial and transverse walls of endodermal cells in roots. They block apoplastic movement of water and solutes, forcing them through the cell membranes.

(a) Grasses

Monocots such as grasses lack a vascular cambium (their bundles are scattered and closed), so they show little or no secondary growth. Conifers, cycads and deciduous angiosperms (dicots) have cambium and do show secondary growth.

(d) Flaccidity of bulliform cells

Bulliform cells are large, thin-walled motor cells on the upper epidermis of grass leaves. In dry weather they lose water and become flaccid, so the leaf curls inwards and cuts down transpiration. Closure of stomata or shrinkage of air spaces does not move the leaf blade, and tyloses block xylem vessels in heartwood.

(b) Both sieve tubes and companion cells

Gymnosperm phloem has sieve cells and albuminous (Strasburger) cells. It has no sieve tubes and no companion cells, which are features of angiosperm phloem. The albuminous cells do the job of companion cells, so option (c) is wrong because they are present.

(b) Annual rings are not prominent in trees of temperate region.

Annual rings are most prominent in temperate trees, because cambial activity varies sharply with the seasons. Light early (spring) wood and dark late (autumn) wood together make one annual ring. The statement is therefore not true. Tropical trees show weaker rings because the climate varies less.

(c) Monocotyledonous stem

Scattered vascular bundles, each with a sclerenchymatous bundle sheath, a large parenchymatous ground tissue and no phloem parenchyma are characteristic of a monocot stem. The bundles are conjoint and closed, since there is no cambium. A dicot stem has bundles in a ring and a monocot root has a radial arrangement.

(a) Sapwood is the innermost secondary xylem and is lighter in colour

This is the incorrect statement. Sapwood is the outer, younger secondary xylem and is lighter; heartwood is the innermost, dark region. Heartwood is dark because of tannins, resins and oils, does not conduct water, and gives mechanical support.

(d) Both A and R are true and R is the correct explanation of A

In winter the cambium is less active and forms fewer xylary elements with narrow vessels, which constitute late (autumn) wood. So the low cambial activity directly explains the nature of late wood.

(c) Statement I is incorrect but Statement II is true

Endarch and exarch describe the position of protoxylem relative to metaxylem in primary xylem, not secondary xylem, so Statement I is wrong. Roots show the exarch condition (protoxylem towards the periphery), so Statement II is true.

(a) A and D only

Lenticels are lens-shaped openings for gas exchange, and bark refers to periderm and secondary phloem. B is wrong (early bark is soft bark), C is wrong (bark means all tissues exterior to the vascular cambium in a loose sense, but the textbook defines it as above), and E is wrong because phellogen is multilayered.