2.1 Secondary Thickening

Question 1

Definition of secondary thickening of the stem

Answer 1

Process by which woody dicotyledons and some herbaceous perennials develop further vascular tissue > grow larger > become perennial.

Question 2

Areas of plant affected by secondary thickening

Answer 2

Stem and root

Must be an increase in mechanical strength to support increased size and environmental stresses.

Question 3

Process of secondary thickening

Part one

Answer 3

1. End of first year - cambium divides sideways, cylinder of cambium is produced inside stem.
2. Cambium then divides to produce a layer of cells on the inside > these cells modify to be some xylem.
3. Plant can then move more water and nutrients and allow every part of the plant greater access to xylem cells.

Question 4

Process of secondary thickening if stems.

Part two.

Answer 4

4. The cambium divides again to produce more xylem.
5. Periodically it divides twice, in quick succession, producing xylem on inside and phloem on outside - secondary phloem.

Question 5

Annual growth rings.

Answer 5

Cambia activity is seasonal in temperate climates.

Autumn - new xylem cells are produced by cambium are smaller with thicker walls as less water is needed by plant.

Spring - new cells are larger with thinner walls to allow extra water movement to leaves for their expansion.

Late summer/autumn cells form a harder, darker ring.

Question 6

Secondary thickening in monocots

Answer 6

Not common
Occurs in some plants eg. yucca
Some packing cells within stem to become meristematic.
Secondary tissue mostly towards centre followed by lignin deposition

Question 7

Medullary rays (radical parenchyma)

Answer 7

Originate from small areas of cylinder of cambium.
Parenchyma cells develop instead of xylem and phloem.
1 - 2 cells wide.
10-12 cells deep.
Used for food storage and transport across stem, from cortex to centre.
Transport O2 and CO2 from respiration, tannins and Suberin (preservatives ) to heartwood and water & food materials to peripheral tissue.

Question 8

Functions of bark

Answer 8

Protection from elements, pests and diseases

Provide some mechanical support.

Question 9

Development of new bark

Answer 9

In young stem epidermis acts as bark.
Cork cambium develops in parenchyma
Cork cells develop outside
Cells outside cork cambium die > cut off from water and food supply.

New bark is a good fit for stem.
As girth increases, bark splits > no longer offers adequate protection.
New bark has to develop to fit increased girth.

Question 10

Old bark

Answer 10

Consists of cork cells blocked with tannin, Suberin, dead cork cambium and dead cortex (parenchyma), dead phloem and sclerenchyma, living young cork cells and living cork cambium.

Question 11

Parts of bark

Answer 11

Periderm - outer part.
Cork - phellem
Cork cambium - phellogen
Secondary cortex - phelloderm

Question 12

Structure of bark

Answer 12

Bark - cortex and secondary phloem

Periderm - cork layer (phellem), cork cambium (phellogen) and secondary cortex - phelloderm.

Question 13

Tracheids - brief description

Answer 13

More primitive of two cell types

Occur in the earliest vascular plants inc. gymnosperms and ferns.

Long tapered cells with points at both ends.

Angled end plates that connect cell to cell.

Dead at functional maturity.
Long and tapering.
Lignified cell walls

Question 14

Xylem parenchyma and xylem fibre cells

Answer 14

Provide protection and support for more important cells.

Collectively know as tracheary elements - vessels and tracheids elements,

Tracheary elements are dead at maturity.

Water through them is passive.

Living phase - cell walls thickened > controlled death> result - reinforced empty cells - like open tubes through which water can move and provide support.

Question 15

Names of three types of xylem cells.

Answer 15

Xylem parenchyma and xylem fibre cells.

Tracheids

Vessel elements

Question 16

Xylem - brief description.

Answer 16

Vascular tissue - specialise tissue.
Primary function - transport of water.
Lesser function - transport of nutrients
Additional functions - storage and mechanical support.
In trees and shrubs xylem is wood

Question 17

Complex tissues - brief description

Answer 17

Comprises of more than one type of cell.

One cell will function in a way to support other types of cell.

Permanent tissues within plant.

Perform specific functions.

Comprise the transport system in the plant (vascular system)