Plant evolution

Question 1

The greening of Earth

Answer 1

• For more than 3 billion years of Earth’s history, the terrestrial surface was lifeless
• Cyanobacteria likely existed on land ~1.2 billion years ago
• 475-500 million years ago, small plants, fungi, and animals emerged on land
• The first plants were types of green algae and were aquatic
• This group included freshwater green algae, called charophytes (Around 480 million years ago unicellular charophytes were evolving features that enabled
  them to survive out of water)
• From the fossil record we see that some ancient charophytes grew in wide flat mats in shallow water or on mud flats (Tested by periods of drying and increased sunlight,
  provided a ground for natural selection to act upon)
• The first land plants were non-vascular and needed to grow directly on or near water
  Benefits:
• Unfiltered sunlight
• More carbon dioxide
• Few/no herbivores and
  pathogens
• Mineral substrates
  Challenges:
• Scarcity of water
• Solar radiation
• Lack of structural support
  against gravity

Question 2

Water challenges

Answer 2

• To avoid desiccation, plants evolved a cuticle; a waxy coating that is present on
  most exposed surfaces (waterproofing)
• This limited the ability for the plant to conduct gas exchange = stoma and guard cells
• This also would limit the ability of the plant absorb water, and thus plants begin evolving roots that would be protected from the sun and could draw moisture from soil
• A vascular system is then required to move water and minerals from areas where they are absorbed
• Reproduction out of water is also a major barriers, for aquatic algae gametes can float
  to where they need to go – but out of water they are at risk of desiccation too = evolved sporopollenin, a durable polymer that covers exposed zygotes of charophytes and the walls of plant spores
• plants eventually evolving a wide range of
  structures to safeguard offspring: pollen grains, seeds, cones, fruits

Question 3

Gravity challenges

Answer 3

• Growing larger required increased structural integrity to offset gravity
  Solved two ways:
  1. Turgor Pressure:
• Also called hydrostatic pressure, is the ability to regulate the pressure of fluid within cells
• Changes in osmolarity allow the flow of water into cells filling the vacuole
• Due to their tough cell wall the cell can become swollen and rigid
  2. Increased structural support
• Collenchyma and sclerenchyma are supporting tissues in land plants
• Collenchyma has thick cellulose cells walls
• Sclerenchyma has a thick cell wall of cellulose and a lignified secondary cell wall in mature cells (which do not contain cytoplasm and mostly filled with lignin) - woody plants

Question 4

Direction (up or down?)

Answer 4

• Auxins are group of plant hormones that control the growth of plants by modulating cell division
• Phototropism causes an unequal concentration of auxins to cluster in the stem away from the light, causing that side to grow faster and thus bend toward the light
• Geotropism causes an unequal
  concentration of auxins to cluster on the underside of organs, in roots it slows growth (causing them to grow down), while in shoots they grow more (causing them to grow up)

Question 5

Non-Vascular plants

Answer 5

• Early colonizers of land
• Modern examples are mosses, liverworts and hornworts
• Evolved the:
• Cuticle (to avoid desiccation),
• Stoma (to breath)
• Rhizoids (to hold to the
  substrate, and began the
  fungal partnership)
• Sporopollenin (to protect the
  gametes)
• Were the dominant (well only) land plant for 40 million years during the Ordovician period

Question 6

Vascular seedless plants

Answer 6

• The beginning of being able to exploit the soil and enter drier habitats
• Modern examples are ferns and club mosses
• Evolved the:
• Roots and shoots
• Proper leaves
• Vascular system
• Became the dominant land plant for the next 150-200 million years … becoming a major dense forest forming plant. With various species of tree fern covering the landscape

Question 7

Vascular seeded plants

Answer 7

• Evolved to live across a range of habitats and niches
• Divided into gymnosperms and angiosperms
• Evolved the:
• Seeds and means of seed
  dispersal
• Collenchyma and
  sclerenchyma tissue

Question 8

Gymnosperms

Answer 8

• Evolved in the Triassic and dominated for 150 million years
• All are perennial woody plants, include
  conifers, cycads, and ginkgo
• Name means ‘naked seeds’, based on
  the unenclosed condition of their seeds
• Which contrasts with the seeds and ovules
  of angiosperms, which are enclosed within
  an ovary
• Gymnosperm seeds develop either on the
  surface of scales or leaves, which are often
  modified to form cones

Question 9

Angiosperms

Answer 9

• Evolved in the Cretaceous and have
  dominated since (65 million years)
• Range from annual to perennial,
  and herbaceous to woody
• They include all forbs (flowering plants
  without a woody stem), grasses and
  grass-like plants, the vast majority of
  broad-leaved trees, shrubs, vines,
  cacti, succulents, and most aquatic
  plants (i.e., non algae)
• Angiosperms are distinguished by:
• Flowers, the reproductive organs of
  flowering plants
• Endosperm within their seeds that forms after fertilization, but before the zygote divides, which provides food for the developing embryo (the cotyledon)
• Fruits, a closed carpel, that completely envelop the ovules (the seed)

Question 10

Plants paved the way for us

Answer 10

• Modulate global CO2 and O2 levels
• Including the expansion of an oxygen-rich
  atmosphere and stabilizing a cooler climate
• Vascular plants were able to colonise land
  away from water
• Significantly increased the bio-available
  chemical energy (the world runs on sugar)
• Produced organically enriched soils that allowed for more plants, and the formation of todays ecosystems