Bio 360, Fall 2001 September 28, 2001
Early development and germination
I. Overall life cycle
A. Overview
embryo development
seed germination
development of specialized cells and tissues
roots
shoots
flowers and reproduction
B. Today - start with the embryo sac
antipodal cells
endosperm
synergids
zygote
double fertilization has just occurred
Why is there double fertilization in angiosperms? What is the function of the endosperm?
II. Embryogenesis
A. Early stages - similar in monocots and dicots
1. Zygote divides
first transverse division forms the suspensor and the chalazal region
a) chalazal region produces the main growth of the embryo
b) suspensor anchors the embryo and may be a conduit for resources
2. After several divisions, a spherical embryo is formed - the globular stage
3. Early differentiation
a) Three primary meristems are formed - meristems are relatively undifferentiated cells that are capable of division followed by specialization
1) protoderm
2) procambium
3) ground meristem
b) development of cotyledons begins during or after initiation of the primary meristems
moncots vs dicots
c) torpedo stage - cotyledons, axis and primary meristems elongate
d) embryo gets larger and the suspensor cells are crushed
B. Initiation of new cells gradually becomes restricted to the meristems of roots and shoots
III. Structure of the seed
A. General features
seed coat
stored food
embryo
embryo structure
B. Cotyledons
1. Dicots
a. Sometimes the endosperm is absorbed and the cotyledons become the food storage organ - e.g., beans
b. Sometimes the endosperm is still present in the mature seeds and the cotyledons remain thin and act as absorptive structures
2. Monocots - usually the cotyledon acts in absorption of food by the embryo from the endosperm
C. Compare embryo structures
D. Stored food in seeds
Seeds may store
1. Protein
2. Lipid
3. Carbohydrate
What are the costs and benefits of each kind of storage?
IV. Germination
A. Cues for germination
1. Water
2. Temperature
3. Daylength
4. Light
What might these cues signal? Why might a particular kind of germination response be beneficial to a plant?
B. Dormancy -
1. seeds are in a physical or physiological state in which immediate germination is not possible
a. Embryo is immature - needs afterripening
b. Seed coat is impermeable to water - requires physical or chemical scarification
c. Seed contains chemical inhibitors
2. Why be dormant?
a. Prevents germination under conditions that are unsuitable for establishment
b. Allows annual plants multiple opportunities to establish offspring
3. How common is seed dormancy
|
Soil type |
Seeds/m2 |
|
agricultural |
1000 - 40,000 |
|
Grassland |
6400 - 54,000 |
|
Forest |
3 - 53 |
b. Not all kinds of seeds are equally represented
annual, colonizing species are most frequent
forest species, especially tropical, are less frequent
4. Ecological and evolutionary implications of dormancy
a. Species in seed pool may differ from present vegetation
b. Seeds in seed bank may provide a "genetic memory" of previous conditions
c. Species with no dormant seeds may not regenerate easily
5. Kinds of dormancy
a. Innate - seeds are initially dormant
b. Enforced dormancy - immediate conditions produce dormancy that ends as soon as the conditions change
c. Induced dormancy - seed is originally able to germinate but initially unfavorable conditions produce dormancy
6. Length of dormancy
a. May be possible for 100's of years
b. Buried seed studies
C. Successful germination - safe sites
1. Seed takes up water
2. Enzymes become active
3. Cells enlarge and begin to divide
4. Respiration begins
D. Emergence from seed coat
radicle emerges first
E. Emergence above soil
1. Epigeous germination - cotyledons emerge above ground
2. Hypogeous germination - cotyledons stay below ground
F. Establishment
1. Seedling is established when it is no longer dependent on the cotyledons and is photosynthesizing independently
2. Crucial phase in plant life histories