Powerpoint: Plant Reproduction Trends

• Define Alternation of Generation
• Describe the evolutionary trends of plant reproduction

One of the signature features of plants is that they reproduce via alternation of generations. Alternation of generations happen when in one complete life cycle, both asexual and sexual reproduction occurs. This is also accompanied by both haploid and diploid life stages.

In most plant life cycles (other than unicellular algae), there will be multicellular haploid and diploid stages.

We call the multicellular diploid stage the sporophyte. In ferns, gymnosperms and flowering plants, the sporophyte stages are the obvious stages. For example, the fern and trees are the sporophyte stage.

The purpose of the sporophyte:

• To produce spores via meiosis
  • The spores, unlike the diploid multicellular stage are haploid 
• To help the plant to disperse

The haploid spores will then duplicate to become the haploid, multicellular gametophyte.

The purpose of the gametophyte:

• To produce gametes (eggs and sperm) by mitosis
• These gametes can then be used for sexual reproduction.
• Sexual reproduction creates new combinations of genes that may produce new traits better suited to the environment. They also provide a better chance at least some may survive.

When the haploid gametes (N) fuse, they create a single diploid (2N) cell called a zygote. This zygote will then duplicate and become a multicellular diploid organism: the sporophyte.

Learning Objectives

• Identify the functions of vascular tissue (xylem and phloem)
• Identify the plant groups that have vascular tissue
• Identify some of the structures (stomata and root cortex) that facilitate water uptake
• Describe the three forces that move water up the xylem

Powerpoint: Vascular Tissue

Because land plants no longer live in an aquatic environment where nutrients, water and nutrients are easily accessible from the environment, land plants must develop some structures that help to conduct all these essential ingredients. One of these evolutionary breakthroughs is vascular tissue.

Vascular tissue are specialized tissues in plants that help to conduct water, nutrients and sugars around to the parts of the plant that need it. They exist in ferns, gymnosperms and angiosperms. There are two types of vascular tissue:

Xylem

• Carry water and nutrients
• Carry their material upwards from the ground to the leaves of the plant
• Made up of tracheid cells that have died

Phloem

• Carry sugars
• Carry their material downwards and upwards, though mainly downward, from the leaves to the rest of the plant
• Made up of live cells with interconnected cytoplasm

But how does water move up into the plant though? Remember, water has no obligation to just move into the plant, so what mechanism do plants have to “suck” water into themselves?

There are three mechanisms that help with that:

1. Capillary Action
2. Root Pressure
3. Transpiration

Capillary action: because water molecules are polar, each single water molecule is attracted to other water molecules. This creates cohesive forces between water molecules and other surfaces that cause water to be rise upwards in narrow spaces. However, because capillary action itself is a weak force, it alone cannot account for how water is able to move up plants.

Root Pressure: Roots also help water to move into the xylem by uptake of nutrients. First of all, let’s backtrack and think about osmosis.

Osmosis dictates that water will always move towards areas of higher solute concentration. Remember, solutes are anything, particles, that are dissolved in water. Concentration is not how much particles there are in the water, but more like how tightly the water are packed together. Therefore, in a beaker, with a semipermeable membrane where only water can go through, water will travel towards the area where the particles are more packed together.

To remember this, imagine that these solute particles were people packed in a room. Water will always travel towards the area where people are more packed together. This provides them with more room to move around in.

Roots take advantage of osmosis by moving nutrients into itself. While the cortex cells of roots cannot actively move water into its tissues, it can actively take in nutrients from its surroundings. By collecting nutrients, it creates a higher concentration of nutrients inside itself. Because of osmosis, the water will then naturally flow into the roots and into the xylem.

The rushing of water into the roots creates what is called root pressure. But even this is not enough to move water all the way up into a tall tree. There must be something else that moves water.

Transpiration: When you breathe out on a cold day, you’ll notice that a mist escapes into the air. This is the water particles from your breathe evaporating.

Did you know plants can do this too? Of course they don’t breathe the same way that people do, but they do have pores that expel water. These little pores are called stomata. They are like little holes on the underside of leaves. When they open and close, they take in carbon dioxide, and expel water to evaporation.

When the water evaporates off of leaves through the stomata, water will be pulled up from the soil, like drinking from a cup of water through a straw.

Powerpoint: Plants General

Learning Objectives

• Identify the distinguishing characteristics of plants
• Describe the evolutionary trend of plants from water to land
• Describe the challenges of life on land
• Identify the structures designed for life on land and which plants developed them
• Explain how these structures help to meet the challenges of life on land

Members of Kingdom Plantae share the following characteristics: 

All members of Kingdom plantae are eukaryotic, which means plant cells have membrane bound nuclei and organelles.

Photosynthetic autotrophs 

Mostly multicellular

Cell walls made out of cellulose

Reproduce in a process known as alternation of generations (reproduce sexually and asexually) – Mistake in the powerpoint, plant life cycles include both an asexual and sexual stage of sexual reproduction.

Evolutionary Trend Amongst Plants

Kingdom Plantae is an extremely diverse kingdom, with many phyla and classes. Nevertheless, evolutionary trends exist to unify the groups. One of them is the movement of plants from a water environment to one on land. To explore this evolutionary trend, we will be focusing on five groups of plants.

Chlorophyta (Green algae)
Bryophyta (mosses, hornworts and liverworts)
Pteropsida (ferns)
Gymnosperms (focusing on coniferae)
Angiospermae (flowering plants)

But why would it be beneficial for plants to colonize the land in the first place? One of the reasons may be that the land has more direct sunlight. Perhaps at the time, the land was less competitive and provided new niches for plants.

Nevertheless, regardless of where plants live, they must have access  to the following: sunlight, water, nutreints, gas exchange, and a means for reproducing. Plants on a land or in a water environment will, however, have different means and difficulties of getting these necessary things.

As we can see from the graph above, land plants have greater access to sunlight, but must meet the challenges of life on land, such as access to water, nutrients and new means for reproduction.

To meet these challenges, some land plants have developed structures.

1.Cuticle – a waxy outer layer on leaves that prevent water loss. It is usually found on the upper side of leaves.

2.Vascular Tissue – tube system in plants that help with transporting water, nutrients and the products of photosynthesis (sugars) around the plant.

3.Pollen Grain – The male reproductive structure of some land plants. It contains the sperm of the plant and is carried by wind or animals and allows for reproduction without water.

4.Seeds – seeds are a protective casing with nutrients that protect the embryo of a developing plant. It allows the plant to develop without the danger of drying out, allowing plants to develop in drier environments than ferns, algae or moss.

5.Fruits and Flowers – Flowers hold the reproductive structures of angiosperms (flowering plants). They often offer a nectar reward for animals, who will then go to other flowers and pollinate it. This allows a higher chance the pollen will actually reach its target (the female reproductive structure).

Flowers contain the seeds of angiosperms. Some are very well developed to provide a reward for animals that eat it, who will then travel and disperse the seed elsewhere along with their fecal matter.