B6

Question 1

What is the function and structure of roots?

Answer 1

Roots are organs specialised for anchorage, absorption, storage and conduction. In gymosperms and dicotyledons, there is usually a taproot system. In monocotyledons the root systems is usually fibrous. Not all roots are found in soils.

Question 2

What protects the developing root?

Answer 2

Mucigel sheath and root cap

Question 3

What is the root tip?

Answer 3

Can be divided into regions of cell division, elongation, maturity. The root apical meristem gives rise to 3 primary meristems, protoderm, ground meristem, procambium. The apical meristem also gives rise to a root cap which protects the apical meristem. Mucigel which is produced by the outer root cap, protects cells and aids roots in passage through soil. Epidermal cells in the maturation zone develop root hairs which increase the absorbing surface of the root.

Question 4

What is the anatomy of a root?

Answer 4

The primary structure consist of epidermis, cortex and central stele. The central stele is made up of vascular tissue surrounded by pericycle. The central stele is separated from the cortex by the endodermis, a compact layer of cells. Most of the cells in the endodermis have a casparian strip integrated into their primary wall and middle lamella, which makes them impermeable. Endodermis also contains a few unthickened passage cells.

Question 5

Comparison of monocots and dicot roots?

Answer 5

Large central stele in monocots and lots of xylem vessel points.

Question 6

What originates in the pericycle?

Answer 6

Lateral roots.

Question 7

What do some dicots produce?

Answer 7

Woody roots which stem from secondary growth.

Question 8

What is the secondary structure of roots?

Answer 8

Meristematic activity of vascular cambium between the xylem and phloem. In continuum with this, the pericycle opposite the ridges of primary xylem also produces vascular cambium. These meristems also give rise to 2dary phloem and xylem. A cork cambium originates in the pericycle producing a periderm which cuts off cortex and epidermis from rest of root.

Question 9

What are adventitious roots?

Answer 9

The root system of plants is already in place in the seed and gives rise to the main root system in many plants. Roots can also be formed later from other places during development. These are adventitious roots.

Question 10

What is the shoot organised into?

Answer 10

Stem and leaves

Question 11

What are stems?

Answer 11

An axis with nodes and internodes. Leaves are produced at nodes. Axillary buds are produced in the angles between leaves and stem. Buds are miniature shoots with a dormant apical meristem covered by bud scales. As well as the typical upright, above-ground stem the following are also stems ; rhizomes, corms, bulbs, tubers.

Question 12

What is the anatomy of the primary stem?

Answer 12

Dicotyledon - transverse section, vascular bundles form an outer ring.
Monocotyledon - transverse section, scattered vascular bundles
Phloem is outer
Xylem is inner

Question 13

What is the cortex made of in aquatic plants?

Answer 13

Aerenchyma tissue

Question 14

What do leaves consist of?

Answer 14

Petiole (stalk) and lamina (blade). Leaf size and shape is genotypically and phenotypically variable. May be simple or compound. Usually photosynthetic. Sometimes modified. Made up of primary tissue although petioles may become woody in some species. The vascular tissue in the leaf is connected to the vascular tissue of the stem.

Question 15

What is the difference between palisade mesophyll and spongy mesophyll? What is mesophyll?

Answer 15

Palisade - elongate parallel cells
Spongy - non parallel cells with large intercellular spaces.
Mesophyll = parenchyma cells

Question 16

What are stomata?

Answer 16

Pores regulated by guard cells. Guard cells are associated with subsidiary cells, stomata may be present on one or both surfaces.

Question 17

What do leaves require?

Answer 17

Massive conduction systems

Question 18

What is abscission?

Answer 18

Leaf drop. Abscission zone comprises an abscission layer and a protective layer (to preserve the stem).

Question 19

How is palisade and spongy mesophyll arranged?

Answer 19

P at top, s at bottom.

Question 20

In what does true secondary growth occur and what happens?

Answer 20

In certain dicots but not monocots. In woody dicot species, secondary meristems give rise to secondary tissues ; bark and wood. While primary meristems generate primary tissue that supports extension growth, the secondary meristems give rise to secondary tissues that lead to growth in diameter.

Question 21

What is the vascular cambium?

Answer 21

A secondary meristem which gives rise to secondary xylem and phloem. Made up of two types of cells ; fusiform initials and ray initials

Question 22

What do fusiform intials undergo?

Answer 22

Periclinal (parallel) divisions, giving rise to secondary xylem and phloem. Periclinal divisions of the ray initials generate ray parenchyma cells. Anticlinal (perpendicular) divisions of cambium cells increases the circumference of the vascular cambium cylinder, to keep up with increase in diameter of stem.

Question 23

What is the periderm?

Answer 23

Phelloderm consists of living parenchyma cells but cork consists of cells that have suberised and lignified walls, and eventually die. Cork is impermeable to water and gases. The periderm crushed and gradually replaces the primary epidermis and cortex. Lenticels are eruptions in the cork layer that allow air to diffuse into the stem.

Question 24

What is the structure of wood?

Answer 24

Secondary xylem, heartwood is old secondary xylem which no longer conducts water, sapwood conducts water and dissolved minerals. Each year new secondary growth increases diameter and crushes previous phloem. Bark is everything outside the vascular cambium. Secondary growth adds girth to stems and roots in woody plants.

Question 25

What do annual growth rings show?

Answer 25

Age of tree. A consequence of seasonal activity of vascular cambium, secondary xylem and phloem are produced in spring and summer. Early wood is less dense than late wood. In some trees, early wood has much wider vessels than late wood. In other trees, vessels are similar throughout.