BOT3015 - Underground: Seeds and Roots

Seed

A tiny sleeping plant

Seed Functions

Propagate the next generation, Maintain genetical diversity

- pollination, Survive harsh environmental conditions

- Dormancy, and Disperse into new environments

Embryo

The new baby plant

Endosperm/Cotyledon

A nutrient source for the growing embyro

Seed Coat

The protective covering of the seed

Embryogenesis

The development of the new embryo

Embryogenesis - Eudicots (Stage I)

1.) The developing seed contains the

endosperm and the zygote

Endosperm and Zygote

The endosperm is triploid while the

zygote is diploid

Embryogenesis - Eudicots (Stage II)

2.)The zygote divides to form the proembryo which is anchored to the micropyle by the

suspensor . The endosperms has begun to

divide.

Embryogenesis - Eudicots (Stage III)

3.) The embryo proper is in the globular

stage, a spherical mass of cells. Endosperm

continues to divide.

Embryogenesis - Eudicots (Stage IV)

The embryo proper is in the heart stage -

the heart shape gives rise to cotyledons. The

endosperm continues to divide.

Embryogenesis - Eudicots (Stage V)

The embryo proper is in the torpedo stage -

the shoot apical meristem is between the

cotyledons, and the root apical meristem is

anchored to the suspensor.

Embryogenesis - Eudicots (Stage VI)

The mature embryo includes the radicle,

the hypocotyl, the epicotyl, and the

ovule has developed into a seed coat.

Embryo Proper 3 Stage - Eudicots

The globular stage, The heart stage, The torpedo stage

Testa

Outer coat

Tegmen

Inner coat

Hilum

A scar where the seed was attached to

the endocarp (inner layer of the fruit).

Micropyle

a scar where the pollen tube entered the ovule

Embryonic Axis

Runs the length of the embryo.

The Plume

A young shoot in the Embryonic Axis containing the shoot apical meristem and the leaf primordia

The Radicle

The embryonic root (not always

apparent)

Cotyledons

Connected to the embryo via vascular tissue. (This is not included in the Embryonic Axis

Hypocotyl

Region between cotyledon

attachment point and radicle

Epicotyl

Region between cotyledon attachment

point and shoot tip

Endosperm

Some eudicots form endosperm and others do not form endosperm

Monocot Seeds

Within the Poacea family (grasses - including corn

and wheat), seeds are highly specialized. Testa and tegmen are fused. Fruits (grain) have only one seed, and pericarp

(fruit wall) is fused with the seed coat

Monocot Seed - Embryo

Root tip is protected by the coleorhiza

(sheath-like structure) Plume is protected by the coleoptile

Scutellum

Name of the single cotyledon - it has vascular connections to the rest of the embryo

Starchy Endosperm - Aleurone

Outer layer of endosperm,

secretes enzymes during germination

Enzymes

Enzymes degrade stored carbohydrates,

proteins and lipids, which are then

absorbed by scutellum

Seed Dispersal

Drifting (Wind), Floating (Water), Ingested/Sticky (Animals), and Bursting/Balistic (Mechanic)

Delay of Germination - Physical Dormancy

Impermeability to water or oxygen

Delay of Germination - Chemical dormancy

Compounds in seed coat inhibit germination

Delay of Germination - Physiological Dormancy

Embryo requires environmental

conditions to complete maturation

Delay of Germination - Endodormancy

Internal biochemical processes must be met

before germination can begin

Delay of Germination - Ecodormancy

External factors are not optimal for germination

Soil Seed Banks - Persistent Seed Banks

A large number of seeds waiting to germinate

Germination - Requiremenrs

Sufficient light, proper temperature , oxygen - essential for cellular respiration, Water - imbibition is important to start vigorous metabolism & water can leach away inhibitors.

Germination - Mechanisms

1. Imbibition - or water uptake

2. Enzyme activation

3. Breakdown of starch in to sugar

Germination - Epigeous

The hypocotyl elongates, and cotyledons extend above ground

Germination - Hypogeous

the epicotyl elongates, and the

cotyledon(s) remain belowground

Germination - Epigeous Eudicots

The hypocotyl is shaped like a hook with the plume pointing downwards (plume hook). The plume is encased by cotyledons to be protected from damage. After emergence, the hypocotyl straightens out, and the epicotyl elongates.

Germination - Hypogeous Eudicots

The epicotyl forms a hook. The cotyledons and hypocotyl remain underground.

Germination - Epigeous Monocots

The single monocot bends into a hook and

emerges before the coleoptile. Once the coleoptile is exposed to sun light, it stops growing and is pierced by the plume.

Germination - Hypogeous Monocots

The cotyledon remains belowground and

the coleoptile emerges first. Once the coleoptile is exposed to sun light, it stops growing and is pierced by the plume.

Roots - Systems

Tap Root & Fibrous Root

Anchorage

Roots keep the plant in place AND help stabilize the soil, preventing erosion.

Support

Provide the foundation for upright

growth (particularly tap roots)

Absorption

Absorb the water and nutrients needed by the plant

Nutrient Storage

can swell and store high- energy compounds like starch (e.g. carrot)

Tap Root Systems

Persists throughout the life of the plant. Has a primary root (tap root), forming a central axis, from which lateral roots branch at irregular intervals

Fibrous Root Systems

Just as tap roots, begins with the radicle.

However, the primary root stops growing

and rots away.

Adventitious Roots

New roots form directly from the stem

tissue

Apical Meristem

The area of cell division, it is protected by the root cap.

Root Hairs

On the area of Maturation the epidermal cells (outer layer) form extensions. These increase the surface area of the root - improving absorption.