Topic 9 - Plant Science

Dicotyledonous Stem

Dicotyledonous Leaf

Differences between Dicot and Monocot. plants

Monocotyledons Dicotyledons

Leaf veins run parallel to one another

Leaf veins form net-like pattern

Vascular bundles are spread through the stem randomly

Vascular bundles are in a ring near outside of stem

Stamens and other organs in the flower are in multiples of three

Stamens and other floral organs are in multiples of 4 or 5

Unbranched roots grow from stem Roots branch off from other roots

Meristems

• Regions where cells continue to divide and grow (often) throughout the life of a plant. These are apical meristems.

• Found at apex of root and stem.• Growth in apical meristems allows elongation of stems and roots.

• Dicot. plants also develop lateral meristems.• In young stems, this consists of cambium in the vascular bundles,

but as the cells grows older, a complete ring forms.• Growth in lateral meristems results in thicker roots and stems with

extra xylem and phloem tissue.

Leaf it out… Plant tissues

Epidermis – a single outer layer of cells

Upper epidermis with a thick layer of wax of the outside is called the cuticle.

Lower epidermis with thin waxy cuticle and guard cells. Stomata in between.

Mesophyll – the tissue filling most of the leaf inside the epidermis

Palisade mesophyll – tightly packed cells in the upper half of the leaf; many chloroplasts.

Spongy mesophyll – loosely packed cells in the lower half of the leaf.

Vascular tissue – found in the veins of the leaf

Xylem – long, dead, tubular structures containing water and mineral ions.

Phloem – lines of living cells, containing water with sucrose and other biochemicals

Modifications

• Can be adapted to form bulbs, stem tubers, storage roots and tendrils.

• For example, roots usually absorb water and mineral ions, but in some cases, they form as extra props for support.

• Bulbs consist of swollen leaf bases attached to a short stem used for food storage (tubers are similar)

• Tendrils are narrow outgrowths from leaves that rotate through the air until they touch a solid support to which they attach and allow the plant to grow upwards.

Auxin in phototropism

Auxin is used in plant growth to direct the plant towards sunlight. Auxin production increases on the side of the plant in shade. The plant grows faster on this side, “tilting” the plant towards the light.

Root systems and mineral ion absorption

Mineral uptake by roots

Extensive root systems increase the surface area available for absorption.

Root hairs increase the surface area available also.

Branching of roots increases surface area for absorption. Plants absorb potassium, phosphate, nitrates and other mineral ions from the soil. The concentration of these is often much lower in the soil, so active transport is used. Most roots only absorb mineral ions if they have a supply of oxygen because they produce ATP for active transport.

Terrestrial plants support themselves by means of thickened cellulose, cell turgor and lignified xylem.

(Above; diagram of xylem vessel structure)

Transpiration

• Transpiration is the loss of water from parts of plants, especially leaves but also stems, flowers and roots.

• Water is carried via the transpiration stream. This is driven by capillary action (sometimes osmotic root pressure).

• Evaporation causes the surface of the water to recess into the pores of the cell wall. Water forms a concave meniscus. The high surface tension pulls the concavity outwards, generating enough force to lift water as high as a hundred meters from ground level to a tree's highest branches. This is transpiration pull.

• Guard cells regulate transpiration by opening and closing stomata.• Abscisic acid closes the stomata.

Biotic factors affecting transpiration

• The size of the plant• The thickness of the waxy cuticle• How widely spread the stomata are• Whether stomata are open or closed.

• As the plant can control these itself (within reason), they are classed as biotic factors.

Abiotic factors affecting transpiration

• Light causes stomata to open and increases transpiration. They often close in darkness (no need to absorb CO2). Water can be conserved without reducing rate of photsynthesis.

• Humidity. Due to evaporation of water from moist cell walls, the humidity in the spaces inside the leaf is 100%. The lower the humidity outside the leaf, the higher the transpiration rate.

• Wind. In windy conditions, the rate of transpiration becomes maximal for the humidity of the air.

• Temperature. At high temperatures, evaporation rate rises. High temperatures also increase the rate of diffusion between the leaf and the air outside. The concentration gradient increases and water is lost more rapidly.

Xerophytes and transpiration

Xerophytes have adaptions that help reduce transpiration:

1. Very thick waxy cuticle covers the stem

2. Spines reduce surface area for transpiration.

3. Crassulacean Acid Metabolism (CAM). Opens stomata during cool nights instead of intense heat of the day.

4. Vertical stems absorb sunlight early and late in the day. It does not absorb at midday when the sun is most intense.

Phloem

• Active translocation is the process by which phloem transport sugars and amino acids within plants.

• Phloem cells must use energy to make the process occur.• Sugars and amino acids are loaded into the phloem in parts of the

plant called sources and are translocated to sinks where they are unloaded.

• Sources; photosynthetic organs (stems and leaves) and storage organs (tubers, bulbs).

• Sinks; roots, fruits and seeds.

Draw (copy from the Internet) the structure of a

dicotyledonous animal-pollinated flower.

Structure of dicot. seed

Conditions needed for germination

• Water. To rehydrate the tissues of the seed.

• Oxygen. Cell respiration. Some respire anaerobically if oxygen is not available but ethanol is produced and often results in toxic levels.

• Temperature. At extreme temperatures, enzyme activity is too slow. Some seed remain dormant if temperatures are above or below certain levels, so that they only germinate during certain times of the year.

Pollination transfer of pollen from the anther to the stigma of a plant.

Seed dispersal the movement or transport of seeds away from the parent

plant.

Fertilization creation by the physical union of male and female gametes; pollen

and ovule in a plant.

Fin