1. Roots, Stems and Leaves
Part 2

2. Leaves
The structure of the leaf is optimized for absorbing light and carrying out photosynthesis. The major structures of the leaf include:
The blade – thin, flattened sections where most of the photosynthesis takes place
The petiole – a thin stalk that connects the blade to the stem

4. Leaf Types
Simple leaf – a single blade, which can be a variety of shapes
Compound leaf – the blade is divided into many separate leaflets

5. Tissue Layers
Leaves are covered on the top and bottom by a layer of epidermis made of tough, irregularly shaped cells. In many leaves the epidermis is covered by the cuticle. These two layers form a waterproof barrier to protect tissues and limit the loss of water through evaporation

6. Stomata & Guard Cells
The bottom layer of epidermis contain stomata, which are pore-like openings to allow carbon dioxide and oxygen to diffuse into and out of the leaf
Specialized epidermal cells called guard cells control the opening and closing of the stomata by responding to changes in water pressure

7. Veins
The xylem and phloem are gathered together into bundles that run from the stem into the petiole. Once in the blade of the leaf, these vascular bundles are known as veins.

8. Mesophyll
The bulk of the leave consists of a specialized type of ground tissue known as mesophyll (meso means middle, phyllon means leaf), which is specialized for photosynthesis
Palisade mesophyll is found below the epidermis layer, tightly packed cells that absorb light
Spongy mesophyll are the next layer, a loose tissue with air spaces between the cells

10. Photosynthesis
The main purpose of the leaves is as the primary site of photosynthesis. In particular, most of the photosynthesis takes place in the mesophyll cells.
They absorb light from the top of the leaf, and carbon dioxide from the stomata.
These leaves contain many chloroplasts, which are the organelles that carry out photosynthesis.

11. Transpiration
The surfaces of the spongy mesophyll cells are moist to increase the ability for gases to diffuse in and out.
Unfortunately, this also means that water easily evaporates from the surface of these cells.
Transpiration is the term that refers to this type of evaporation from the leaves of a plant.
This lost water is then replaced by water drawn into the leaf through the vascular tissues.

12. Gas Exchange
Leaves take in carbon dioxide and give off oxygen during photosynthesis. Also, when plants use cellular respiration to turn food into quick energy (ATP), they use up oxygen and give off carbon dioxide (just as animals do).

13. Gas Exchange Plant leaves are specialized to allow gas exchange.
The spaces between the spongy mesophyll cells and their moist surfaces allow optimal conditions for gas exchange.
The stomata are openings to the outside of the plant, which is necessary for gas exchange, since the rest of the leaf is essentially waterproof.

14. Stomata & Guard Cells
The opening and closing of the stomata is controlled by the guard cells:
Specialized epidermal cells on the underside of the leaves
Control the stomata to regulate the flow of gases

15. Guard Cells Respond to changes in water pressure
High water pressure  guard cells curve, opening the stomata
Low water pressure  guard cells relax, closing the stomata
Generally stomata are open during the day to allow photosynthesis and gas exchange to occur, and closed at night
Stomata can also be closed during hot, dry conditions when water conservation is critical for survival

16. Adaptations of Leaves
The leaves of certain plants have specific adaptations for dry or low-nutrient conditions

17. Pitcher Plant
The pitcher plant has modified leaves to attract and then digest insects and other small prey. These plants usually live in nutrient-poor soils, so they use the insects to supplement their source of nitrogen.
Venus Fly Trap & Pitcher Plant video
(3:30 min)

18. Cactus
Cactus leaves are non-photosynthetic spines or thorns that help to protect against being eaten by herbivores. The cactus carries out most of its photosynthesis on its stem.

19. Rock Plant
The rock plant is adapted for hot, dry conditions. These plants have round leaves with very few stomata, and they have clear tissue that allows light to penetrate into the leaf.

20. Structure Video – Bozeman Science
(14 min – stop at 10:50)

21. Water Transport
Root pressure created by water entering the root tissues is enough to push the water into the vascular system and out of the root, however root pressure alone isn’t enough to lift water throughout the entire plant (especially a tall tree like a giant redwood!)
Plants must take advantage of some of the unique properties of water in order to move water throughout the plant body and to such great heights.

22. Capillary Action
There are 2 major forces that go into making capillary action:
Water molecules are attracted to each other by a force called cohesion (the attraction of molecules of the same substance to each other).
Water molecules are also attracted to other substances through a force called adhesion (attraction between molecules of different substances)

23. Capillary Action
Capillary action occurs when water molecules stick to the sides of a small tube through adhesion, and then cohesion causes other water molecules to be pulled up after them. The thinner the tube, the higher the water will rise in the tube.
Both the tracheids and the vessel elements form hollow connected tubes, and capillary action in these tubes causes water to rise well above the ground.

24. Transpiration
For trees and other tall plants, root pressure and capillary action together don’t provide enough force to lift water to the top branches and leaves. The remaining major force in water transport is transpiration:
Water is lost from the leaves through transpiration
Osmotic pressure moves water from the vascular tissue and into the leaf to replace the lost water

25. Transpiration
A combination of osmotic pressure and cohesion among the water molecules causes water from lower in the plant to move higher, all the way up from the roots.
This is known as transpirational pull

26. Impact of Transpiration
On a hot day, a small tree might lose as much as 100 litres of water to transpiration
Water loss increases with heat, dryness, and windiness of the weather conditions
As water loss increases, the plant must draw up even more water from the roots

27. Controlling Transpiration
The plant must control this rate of transpiration to maintain optimal conditions
When water is abundant, it flow into the leaf, raising the water pressure in the guard cells and keeping the stomata open
When water is scarce, the water pressure in the leaf and guard cells falls, and the stomata closes, preventing further loss of water

28. Wilting
Osmotic pressure also keeps plant stems and leaves rigid and stiff by keeping the central vacuole full of fluid, producing the turgor pressure the plant cells need to keep their shape.
When a plant loses too much water through transpiration, the turgor pressure drops, the cell walls bend inward, and the plant starts to wilt.
Wilting also causes the guard cells to relax, closing the stomata and reducing further water loss. Therefore wilting actually helps a plant to conserve water.

29. Water Movement Video
(3:30)

30. Functions of Phloem
Most nutrients, including sugars, minerals, and complex organic compounds are pushed through the phloem. This process is used to move sugars and other nutrients to where they are needed, including:
Plants push sugars and nutrients into fruits in order to make them more attractive to animals, so they increase their chances of spreading their seeds
In cold climates, plants often pump food down into their roots for winter storage, and then this food must be pumped back into the stems and branches before growth begins again in the spring.

31. From Source to Sink
Sugars are moved through the phloem from a source to a sink.
A source is any cell where sugars are produced by photosynthesis.
A sink is any cell that uses sugars or stores them.
What causes this type of phloem transport?

32. Pressure Flow Hypothesis
One possible explanation is the pressure-flow hypothesis.
Sugars are pumped into the phloem at the source (often a leaf)
As concentrations of sugars increase in the phloem, water moves in by osmosis
This increases the water pressure at that point in the phloem, forcing the fluid to move through the phloem away from the source and toward a sink

33. Pressure-Flow Hypothesis
When a part of a plant (sink) actively absorbs nutrients from the phloem, this reduces the concentration - osmosis causes water to also move away, reducing the water pressure
This reduction in water pressure causes a movement of sugar-rich fluid toward this area

34. Summary
When nutrients are pumped into or out of the phloem, the change in concentration causes a movement of fluid in the same direction – this allows phloem to transport nutrients in any direction to meet the needs of the plant.

35. Summary Video https://www.youtube.com/watch?v=xGCnuXxbZGk (5:30 min)
If time, can also watch this summary video (Bozeman Science) while you start studying
(14 min)