Corn Growth and Development Randy Wayne Shoals Marine Laboratory Summer, 2009

Caryopsis: Embryo within is Bipolar The plant will be growing into two physical phases: The plumule will be growing into a bright, dry phase. The radical will be growing into a dark, moist, nutrient rich phase.

Bipolar Corn Plant

Plumule Inside Caryopsis

Radical Inside Caryopsis

Mesocotyl Inside Caryopsis

Germination

Germination

Stem Apical Meristem The stem and root apical meristems give rise to all the primary tissues in the stem and root, respectively. The stem apical meristem gives rise to the leaves. The apical meristems carry embryonic stem cells with them. A stem cell is a cell that divides to form two daughter cells, one of which differentiates and one of which remains a stem cell.

Intercalary Meristems

Stem: The Organ that Supports the Leaves Primary growth results in an increase in height not girth. Corn and other monocots stems have scattered vascular bundles composed of xylem and phloem tissue. Each bundle is surrounded by a ring of cells called a bundle sheath. Corn and other monocots do not exhibit secondary growth of concentric annual rings produced by a vascular cambium.

Vascular Bundle: One Structure—Many Functions

Phloem and Xylem in Vascular Bundle in Stem

The Location of the Vascular Bundles Provides Mechanical Strength Corn Sunflower

Surface View of Corn Leaf Showing Chloroplast-Containing Mesophyll Cells

Corn Leaf (xs)

Corn Leaves: The Photosynthetic Organs Flat. The high surface-to-volume ratio maximizes the amount of light intercepted for photosynthesis (also maximizes water loss). Covered with cuticle to prevent water loss. Has stomates that allow in the influx of CO2 while controlling water loss.

Stomate on Epidermis of Corn Leaf

The Opening of the Stoma is Regulated by Guard Cells In the morning, light activates pumps in the plasma membrane of guard cells so that KCl accumulates. The increased osmotic pressure in the guard cells causes water to move into the guard cells and they swell. This causes the stoma to open.

The Closing of the Stoma is Regulated by Guard Cells If the plant loses too much water, the roots produce a hormone called abscisic acid, which travels to the leaves and causes ion channels to open and the KCl leaves the guard cells. The decreased osmotic pressure causes water to move water to move out of the guard cells and the they shrink. This causes the stoma to close.

Water is Necessary for Photosynthesis: Corn Leaf Showing a Cross Link between Two Vascular Bundles

Ammophila Leaf with the Large Bulliform Cells in the Epidermis that Allows the Leaf to Roll up as a Way to Minimize Water Loss

Corn Leaf with the Large Bulliform Cells in the Epidermis that Allows the Leaf to Roll up as a Way to Minimize Water Loss

Rolled Up Corn Leaves

Corn Leaf Vascular Bundle with Kranz Anatomy Important for C4 Photosynthesis

Corn Root (ls): The Organ for Absorption and Anchoring Root cap Meristem Developing vylem

Corn Root (xs) The vascular bundles are not necessary for support and perhaps are closer to the center to protect them from predators.

Water Movement through the Plant: Osmosis The solute concentration and thus the osmotic pressure is greater in the cells of the root than in the soil. Water enters the root by osmosis (diffusion). The root hairs increase the surface to volume ratio of the root to increase the rate in which water and dissolved minerals can be taken up. The water moves through the root to the endodermis.

Water Movement through the Plant: Root Pressure The walls between the endodermal cells have a Casparian strip, which is a hydrophobic wax-like substance called suberin that prevents water movement between the endodermal cells and ensures that all water moves across the plasma membrane of the endodermal cells. The endodermal cells allow the whole root to act as an osmometer.

Water Movement through the Plant: Root Pressure The endodermal cells pump ions into the vascular region of the root. The increased concentration of ions in the xylem draws water into the xylem by osmosis. Water moves out of the root and into the shoot through the xylem as a result of root pressure. Root pressure can be inferred by looking at the elimination of water by leaves at night, known as guttation. In more mature roots, the inner wall of the endodermal cells become suberized, which prevents water loss from the xylem.

Guttation Only Occurs When the Humidity is High, Resulting in a Water Surplus

Water Movement through the Plant: Transpiration The water pressure exerted by the root is not great enough to push water against gravitational pressure to the top of a large corn plant or a tree. Shoots separated from the roots still have the ability to transport water against gravity to the leaves, indicating that there is another mechanism responsible for long distance water movement in plants.

Water Movement through the Plant: Transpiration (Cohesion Tension Theory)

Structure of Tracheary Elements (Protoxylem and Metaxylem)

Structure of Tracheary Elements Allows Axial and Lateral Water Movement but Prevents Cell Collapse

Food Movement through the Plant: Translocation (Pressure Flow Theory) Leaves are sources of sugar. Sugar is produced by photosynthesis. It either diffuses into or is pumped into the sieve tube elements of the phloem. The sugar into the phloem pulls in water from the xylem by osmosis, creating a high hydrostatic pressure in the phloem near the sugar source.

Food Movement through the Plant: Translocation (Pressure Flow Theory) In corn, roots and caryopses are sinks for sugar Sugar diffuses or is pumped out of the phloem and is converted in the sink cells to starch, a non osmotic form. The water leaves the phloem by osmosis, causing a drop in the hydrostatic pressure in the phloem near the sugar sink.

Food Movement through the Plant: Translocation (Pressure Flow Theory) The sugar solution within the sieve tube moves through the sieve tube in response to the hydrostatic pressure gradient, thus carrying the sugar from source to sink.

Food Movement through the Plant: Translocation (Pressure Flow Theory) Sieve tube elements, unlike the vessel elements of the xylem, are enucleate living cells. Sieve tube elements get their proteins from the companion cells. Sieve tube elements are separated from each other by sieve plates that become occluded in response to a pressure surge.

Flowering: Switch from Vegetative Growth to Reproductive Growth

Male Flowers

Female Flowers

Each Silk is Attached to its Own Kernel Silk is the stigma that receives the pollen Kernel is the carpel in which the seed grows. The longer the silk, the greater is the competition (selection pressure) between pollen tubes.

Meiosis: Formation of Female Gametophyte in Lily

Meiosis: Formation of Female Gametophyte in Lily

Meiosis: Formation of Female Gametophyte in Lily

Meiosis: Formation of Female Gametophyte in Lily

Meiosis: Formation of Female Gametophyte in Lily

4-Celled Stage of Female Gametophyte

8-Celled Stage of Female Gametophyte

The Egg

Meiosis: Formation of Pollen in Anthers of Stamens

Pollination

Pollination: Pollen Caught by Stigma

Double Fertilization

Seed Formation

Germination

Corn Plants are Very Efficient in Converting Radiant Energy into Chemical Energy because they have a Special Kind of Photosynthesis Involving two Types of Chloroplasts