Seed Structure and Development Quiz

Seed Structure and Development

• Seeds are mature fertilized ovules containing an embryo
• Plant reproduction involves pollination, fertilization, and seed dispersal
• Ovules transform into seeds, and the ovary develops into fruit enclosing the seeds
• Seeds disperse to various locations, germinating into new plants under suitable conditions

Ovule Structure

• Integuments encase and protect the nucellus and ovum
• Micropyle is an opening for pollen tube entrance
• Nucellus is parenchymatous tissue surrounding the ovum
• Chalaza is the basal part where integuments originate
• The ovule is attached to the placenta by the funicle
• Two poles exist; chalazal and micropylar ends

Embryo Sac (Megaspore)

• Contains 8 nuclei (3+2+3)
• Three nuclei at the micropylar end, including one central and egg cell, develop into zygote, and adjacent synergids adjust pollen tube orientation.
• Two central nuclei fuse with the male gamete, forming endosperm (nutritive tissue)
• 2–3 antipodal nuclei degenerate at the chalazal end

Double Fertilization

• One sperm unites with the egg cell, forming the zygote (2n)
• The other sperm unites with two central nuclei to form the endosperm (3n)

Ovule to Seed Development

• The zygote develops into an embryo consisting of radicle and cotyledons
• Triploid cells proliferate to produce the endosperm
• Nucellus might develop into perisperm
• Integuments form the seed coat (testa) after lignification/suberization

Seed Development

• The zygote develops into an embryo (one or two cotyledons, radicle, Plumule)
• Endosperm forms the nutritive tissue
• Perisperm might develop
• Integuments form the seed coat (testa)

Parts of a Typical Seed

• Testa (seed coat): protecting layer
• Endosperm: nutritive tissue
• Cotyledons: food storage parts in the kernel
• Embryo: radicle and plumule
• Kernel: all tissues within the testa
• Perisperm: tissue outside the embryo sac

Types of Seeds

• Albuminous: endosperm and perisperm are present
• Exalbuminous: lack endosperm or perisperm

Types of Ovules

• Atropous (orthotropous): straight ovule; funicle, chalaza, and micropyle are in a straight line
• Anatropous: inverted ovule; micropyle is next to hilum; chalaza is at the opposite end
• Amphitropous: ovule is bent such that micropyle and chalaza are at right angles to hilum
• Campylotropous: ovule is curved upon itself with micropyle and chalaza positioned close to each other

Types of Outgrowths

• Arillus: covers whole seed, forms from funicle or hilum (e.g., cardamom)
• Arillode: forms from micropyle, covers parts of seeds (e.g., nutmeg)
• Caruncle: fleshy outgrowth from tissue near the micropyle (e.g., ricinus)
• Strophiole: local enlargement over the position of the raphe (e.g., colchicum)
• Wing: membranous extension (from the funicle or integuments)
• Tuft of hairs: sometimes found on the seed (e.g., apocyanaceae or strophanthus)

Testa Structure

• Composed of one or two integuments
• Four layers: epidermis, hypodermis, sclerenchymatous layer, and pigment/nutritive layer
• The testa's surface may be smooth, striated, pitted, papillose or hairy
• Testa contents can vary, from membranous to woody or fleshy

Reserved Food Materials

• Carbohydrates: starch, cellulose, and hemicellulose
• Fixed oils and proteins (aleurone grains)

Aleurone Grain Types

• Simple in structure or consist of amorphous protein mass with inclusions

Microscopic Seed Characters

• Testa and outgrowths (if present)
• Perisperm
• Endosperm
• Embryo
• Cell contents (starch, protein, fixed oils)

Types of Embryos

• Straight (e.g., linseed)
• Accumbent (e.g., fenugreek)
• Incumbent
• Coiled
• Spiral