Seed Structure & Function - Cycle 4

Question 1

Fruits Develop from Ovary

Answer 1

• Seeds develop from ovules, and fruits develop from ovaries
• Double fertilization yields a diploid zygote (embryo) and a triploid central nuclei (endosperm)

Question 2

Endosperm Development

Answer 2

• Usually develops first before the embryo
• The central nuclei divide to form a multinucleate “supercell” with a milky consistency
• Nuclear division occurs because there are no cell walls that enclose these nuclei just yet, which explains the milky consistency of the endosperm. As the endosperm increase in size through nuclear division, tense cytoplasm gathers around the nuclei, and walls are constructed, thus forming cells.

Question 3

Endosperm Development Example

Answer 3

• Coconut “milk” and “meat” are examples of liquid and solid endosperm, respectively.
• In the early development of a young coconut fruit, the endosperm nuclei divide and increase in number. At this time, the endosperm of the coconut is in liquid consistency because the nuclei that makes up the endosperm do not still have partitions brought by the cell walls.
• As the coconut ripens, the liquid endosperm begins to form walls around individual nucleus causing it to harden and become solid

Question 4

Monocot Endosperms

Answer 4

• The endosperm of grains such as corn, wheat and rice occupies the bulk of the kernel and is the main energy reserve for the development of the young seedling
• The endosperm stores nutrients that can be used by the seedling after germination
• Stores the bulk of its energy in the endosperm.

Question 5

Dicot Endosperms

Answer 5

• The food reserves of the endosperm are completely exported to the growing embryo as the seed completes its development.
• Most lack endosperms upon maturity
• What provides nourishment to the embryo is the stored food in the cotyledons not endosperm
• Food reserves of the endosperm are completely transferred embryo
• Stores its food in the two cotyledons

Question 6

Seed Structure: Embryo

Answer 6

Young plant

Question 7

Seed Structure: Endosperm

Answer 7

stores food for embryo

Question 8

Seed Structure: Seed Coat

Answer 8

Encase the seed

Question 9

Seed Structure: Radicle

Answer 9

Embryonic root

Question 10

Seed Structure: Epicotyl

Answer 10

Embryonic shoot

Question 11

Seed Structure: Hypocotyl

Answer 11

Junction between roots and shoots

Question 12

Seed Structure: Cotyledon

Answer 12

Seed leaf

Question 13

Embryo Development

Answer 13

• Zygote undergoes mitotic division and give rise to terminal and basal cells
• Basal cell give rise to suspensor which attaches to the parent plant
• Suspensor helps in transferring nutrients to the embryo
• As the suspension elongates, it pushes the embryo deeper into the nutritive and protective tissues
• Terminal Cell gives rise to the pro embryo (early embryo). It divides several times and forms into proembryo
• In here, cotyledons begin to form as bumps. A eudicot embryo with its two cotyledons is heart shaped at this stage
• As the embryo elongates, shoot and root apex appears. Shoot apex appears between cotyledons and root apex appears just before the tip of the suspensor. The abysses of shoots and roots sustain primary growth

Question 14

Seed Dormancy

Answer 14

• At some point in seed development, hormonal signals tell cells in the embryo and endosperm to stop dividing and the seed gradually loses moisture and enters what we call dormancy
• A survival mechanism by which seeds can delay germination until the right environmental conditions for seedling growth and development
• This is a crucial adaptation because it ensures that seeds have time to disperse away from the parent plant before germinating. Moreover, dormancy enables seeds to postpone development if the environment is unfavorable, such as during drought or frost. So favorable conditions trigger embryo growth to resume when young plants are more likely to survive

Question 15

Seed Germination: Rainfall

Answer 15

• Seeds of desert plants germinate after a heavy rainfall. If they were to germinate after a mild drizzle, the soil might soon become too dry to support the seedlings.

Question 16

Seed Germination: Fire

Answer 16

• Some seeds require fire to break dormancy
  
  • Where natural fires are common, many seeds require intense heat or smoke to break dormancy. Seedlings are therefore most abundant after fire has cleared away completing vegetation

Question 17

Seed Germination: Winter

Answer 17

Where winters are harsh, seeds may require extended exposure to cold before they germinate on the next season

Question 18

Seed Dormancy

Answer 18

• Lettuce varieties break dormancy in shallow soil and ample sunlight
• The requirement for specific cues to break seed dormancy increases the chance that germination will occur at a time and place most advantageous to the seedling.
• Most seeds are durable enough to last a year or two until conditions are favorable for germinating, thus the soil has a bank of ungerminated seeds that may have accumulated for several years. This is one reason vegetation reappears so rapidly after an environmental disruption such as fire.

Question 19

Seed Germination

Answer 19

• Resumption of growth after a period of seed dormancy. With appropriate environmental conditions such as adequate moisture, oxygen, and appropriate temperature, seeds can germinate.
• It is initiated by imbibition, which is the uptake of water due to the low water potential of the dry seed.
• Imbibition causes the seed to expand and rupture its coat and triggers changes in the embryo that enable it to resume growth
• Following hydration, enzymes digest stored food in endosperm and/or cotyledons
• Nutrients are transferred to the growing embryo
• As the seedling soon begins to take on its mature form, young roots grow downward in response to gravity, anchoring the plant in the soil and absorbing water and minerals
• The shoot produces leaves as it grows upward toward the light. Initially, the energy source for the seedlings growth is stored food inside the seed.
• By the time the seedling has depleted these reserves, the new green leaves should begin producing food by photosynthesis
• Seed that are buried too deep in the soil will not emerge. In effect, it will starve to death before reaching the light

Question 20

Germination in Dicots

Answer 20

• The first organ to emerge is the radicle, which is the embryonic root. The development of a root system anchors the seedling in the soil and supplies it with water necessary for cell expansion.
• Hook forms in the hypocotyl, and growth pushes the hook above ground
• In response to light, the hypocotyl straightens, the cotyledons separate, and the epicotyl spreads its first true leaves. These leaves further expand, become green, and begin making food by photosynthesis. The cotyledons shrivel and fall away because their food reserves have been exhausted by the germinating embryo

Question 21

Germination in Monocots

Answer 21

• Shoot tip grows through the coleoptile, which is a sheath protecting a young shoot tip, pushes up through the soil and into the air. The shoot tip grows through the tunnel, provided by the coleoptile and breaks through the column tiles tip upon emergence.
• The sheath are discarded when the seedling begins growing and the immature leaves become the first true leaves of the corn plant
• The cotyledon remains belowground

Question 22

Does seed size matter?

Answer 22

• The size of a plant’s seed reflects an evolutionary trade-off. Large, heavy seeds contain ample nutrient reserves to fuel seedling growth but may not travel far.
• Small seeds store limited nutrients but tend to disperse far and wide
• Larger seeds are better able to support themselves initially, while smaller seeds have a better chance for dispersal over a wide area, helping at least some seedlings survive
• Seed size matter but neither is superior nor inferior over the other. It all depends on what environmental condition calls for a large seed or a small seed as far as plant growth is concerned