botany lecture - chapter 7

8/12/2019 Botany Lecture - Chapter 7

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Root system

1. Anchoring the plant firmly to a substrate

(soil)

2. Absorbing water and minerals

3. production of hormones


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Three types of root system

1. Tap root

from radicle

Anchorage

In dicot andgymnosperm

Secondary

growth


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2. Fibrous

After the death ofradicle

Delicate and hair-like

Absorption

No prominentenlarged primary root

In monocot
http://www.cactus-art.biz/note-book/Dictionary/aaa_Dictionary_pictures/Fibrous_root_system.jpg


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a. brace root-arise

from main trunk stem

b. prop rootarise

from the lateral branches ofthe main stem.

Brace roots of corn

which arise from themain trunk or stem of

Zea mays

3. Adventitious do not arise from pre-existing roots


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Germinating seed showing the emergence of primary

root from the radicle.


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1. Taproot single prominent

root

2. Lateral or branch roots-

smaller and arise from the

taproot

3. Root hairs

arise fromepidermis, single celled

extension which increase

absorptive area, no line of

demarcation in the epidermal

cells.

4. Root tip- the growing portion

protected by root cap.

structure of roots


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L-section of root shows 3 zones

Zone of maturation

Zone of elongation

Zone of division/apical

meristem


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peripheral

columella

Rootcap

ROOT TIP IS PROTECTED BY A ROOT CAP

procambium

Ground meristem

protoderm


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Root cap

Slime or mucigelsecreted by the

dictyosomes of the root cap

which helps to lubricate the

passage of root through the soil.

Quiescent centeris the mitotically

inactive region of the root apical

meristem, act as reserve of

healthy cells, resistant to toxic

substances and radiations.The three zones of root apical

meristem:

1. protoderm

2. ground meristem

3. procambium


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Cross section of

Ranunculus root (Buttercup)

Internal anatomy of young dicot root

shows three general sections:

1. Dermal region epidermis with

root hairs

2. Cortex outer collenchyma

middle parenchyma

inner endodermis

3. Stele or vascular cyclinder

consists of the following:

1. pericycle outermost layer

2. primary xylem

3. primary phloem

4. vascular cambium


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B

C

D

E

b.Pericycle

c.Primary phloem

d. Vascular cambium

e. Primary xylem

Cross section of young dicot root

Beside is the enlarged view of

Vascular cylinder or stele region.


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Variations in the number of xylem strands in roots

Triarch tetrarch pentarch polyarch

Three xylem poles four xylem poles five xylem poles many xylem poles

eudicot eudicot eudicot monocot

Protostele protostele protostele siphonostele


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Endodermis- controls the

passage of minerals

across the vascular

tissues

Consist of thick-walled cells

(encrusted with

suberin and lignin)

and passage cells

(thin-walled)

Casparian stripsare bandsof lignin and suberin on the radial

walls (top, bottom and side walls)

causing the cell walls to be waterproof.


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Diffusion paths in rootsApoplastic

Water and mineral

diffuses only through

walls in intercellular

spaces

Symplastic

The material has passed

through a plasma membrane

and enters the protoplasm


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Internal anatomy of monocot root Internalanatomy of monocot rootshows 3 regions:

1. Dermal region epidermis

2. Cortex which shows:

a. outer zone of collenchyma

b. middle zone of parenchyma

c. innermost endodermid

3. Vascular Cylinder

a. pericycle

b. xylem

c. phloem

d. pith parenchyma

Siphonostele

a band of vascular

tissues surrounding a

parenchymatous pith.


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Monocot root

Epidermis

Outer collenchyma

Middle parenchyma

Endodermis

PithPericycle

Primary xylem

Primary phloem


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Vascular cylinder of monocot root

siphonostele strands of xylem and phloem

surrounding a parenchymatous pith

pithpith

O i i d D l t f L t l t


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Origin and Development of Lateral roots

Lateral root

Vascular tissues

Expaded epidermis

And cortex of parental

root

Lateral rootis iniatiated by

cell divisions in the

pericycle. As it pushes

outward, it destroys the

cells of the cortex and

epidermis that lie in its

path, ultimately destroying

endodermis.


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Old dicot root

Old dicot root shows two distinct

regions:

1. Region of secondar vascular

tissues which include the

a. secondary phloem

b. vascular cambium

c. secondary xylem

2. Region of periderm layers:

a. phellem or cork

b. phellogen or cork cambium

c. phelloderm or cork parenchyma


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Brace roots in corn plantfor additional supportand absorption


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Water retention 3. velamen in aerial roots of

orchids

photosynthesis 4. chlorophyllous roots


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Movement - Contractile roots of bulbs like onions,

gladiolus, garlic
http://www.cactus-art.biz/schede/HAWORTHIA/Haworthia_emelyae/Haworthia_emelyae_comptoniana/Haworthia_emeliae_v_comptoniana_contractile_roots_540.jpg


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Parasitic (haustorial roots) for

absorption


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Buttresses- expanded roots for

great support


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Pneumatophores- breathing roots

of mangove for aeration


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enlarged fleshy tap root- carrots, ube,

tugue, radish, turnips,


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Mycorrhizae- association between

a soil fungus and roots


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Other types and root modifications

8. Protection 8. spiny roots of tugue


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Other types and root modifications

5. root nodules associations between

nitrogen fixing bacteria

and roots of legumes

such as makahiya.

6. Parasitism 6. haustorial roots of

parasitic flowering

plants like mistle toe

botany lecture - chapter 7

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