Lecture 11 Vascular Plants

Question 1

What are Tracheophytes?

Answer 1

Plants that have vessels.

Question 2

Ally’s meaning in biology

Answer 2

Close relatives.

Question 3

Describe Silurian Landscape

Answer 3

Early: Giant fungi, small bryophyte-like things
Mid: Branching

Question 4

New challenges for plants to colonize land (into seedless plants)

Answer 4

• Competition (for space, light, water, nutrients)
• Colonize drier environments
• Get taller
• Go deeper

Question 5

Describe plants’ innovation of branching

Answer 5

• Earliest plants could not branch (or grow very tall)
• Earliest branching plants = green stems with no leaves
• Branching enabled different stems to specialize:
  -Some grow along ground to acquire nutrients (-> roots)
  -Some grow up to outcompete neighbours for light (stems)

Modularity = important feature of plants

Question 6

Describe plants’ innovation of Stomata

Answer 6

• Cuticle reduces water loss BUT also keeps CO2 out of plant
• Stoma = opening (pore) surrounded by specialized guard cells
• guard cells let plant control stomata opening
• stomata allows gas exchange (open) & prevents water loss (closed)
• Cuticle and stomata evolved together
• Stomata enabled evolution of full cuticle

Bottom side of leaf has more stomata.

Question 7

Describe plants’ innovation of Vascular tissue

Answer 7

• Problem with being tall -> Transport (get things up there) and support (don’t fall over)
• Vascular tissue: Specialized reinforced conductive tissue
• 2 functions: transport (water, sugars, nutrients) & support
• Nonvascular plants today have conducting cells but no secondary thickening – some conductance but no support (but mostly glorified sponges)
• Earliest vascular plants:
  -thickened conducting cells
  -some branches but no leaves (used photosynthetic stems) or proper roots
  -increasingly effective vascular tissue, from spongy cork-like tissue to hollow lignified tubes

Question 8

Describe evolution of water-conducting cells (xylem)

Answer 8

Question 9

Describe plants’ innovation of roots

Answer 9

• 2 main functions: nutrient & water acquisition, physical support
• Multiple independent evolutions through Devonian
• Third function = interface with symbionts: Mycorrhizal fungi symbionts present even in simplest root systems (e.g. ferns)

Question 10

Describe devonian (age of forests)

Answer 10

• Early Devonian: tallest plants ~1m (dwarfed by fungi!)
• Mid Devonian: shrublike forests of ferns & horsetails
• Competition for water/nutrients & need for support -> roots
• Competition for light -> flattened stems -> leaves
• Late Devonian: world’s first forests

Question 11

Describe first tree-like plants in late Devonian

Answer 11

• Gilboa trees
• 8 m tall (like modern palm tree) while rest of vegetation was only 2 m
• From ancient & weird fern-like family
• No leaves! Just increasingly fine braches
• Vasculature tissue allowed it to get taller, but no true wood (like todays ferns)

Question 12

Describe reduction of gametophyte is one of strongest trends in land plant evolution

Answer 12

• Nonvascular plants: sporophyte small, short lived, depends on gametophyte for nutrition
  -Gametophyte-dominant life cycle
• Vascular seedless plants: sporophyte is much larger & longer lived than gametophyte
  -Sporophyte-dominant life cycle
• Seed plants: Gametophytes are microscopic
• Advantages of sporophytes
  -Diploid cells can respond to varying environmental conditions more efficiently
  -Especially if the individual is heterozygous (i.e. two different alleles) at many genes (also can mask disadvantageous mutations)

Question 13

Extant vascular seedless plants

Answer 13

• Ferns, clubmosses (not actual mosses!), horsetails,
• Have vascular tissue (specialized tissue to conduct water & nutrients that is structurally reinforced, but not true wood)
• Complex leaves & roots
• Sporophyte-dominated life cycle
• Disperse via spores (not seeds)

Question 14

Describe Sporophyte-dominant Life Cycle

Answer 14

Question 15

Describe fern evolution

Answer 15

• 11 000 extant species (2nd only to angiosperms)
• Some remarkable examples of ‘evolutionary stasis’
• Eg Osmunda claytoniana unchanged (even fossilized nuclei & chromosomes) for at least 180 million yr
• Most eukaryote groups have some hybridization between close relatives (e.g. liger, Galapagos finches)
• Many species of ferns hybridize in nature
• Hybridization thought to have played major role in fern evolution (often happens between very disparate parents!)

Question 16

Describe ferns today

Answer 16

• Roots very similar to seed plants
• Leaves produce spores (sporophyte)
• Widespread, still dominant in some ecosystems, eg New Zealand (tree ferns 20 m)
• Invasive ferns!
• 25-30% of ferns are epiphytes (live on other plants)
• Host huge diversity of invertebrates

Bioremediation of heavy metals
* Soil heavy metal contamination is common after mining
* Ferns can bioaccumulate metals in their leaves (then harvest and remove)
* Slow, but cheap?
* Fern Pteris vittate can tolerate arsenic that would kill any other plant or animal
* Several mechanisms involved: proteins that help bind it, move it across membranes, and induce stress responses
* Arsenic is stored in the vacuole in less-toxic form

Question 17

Describe plants’ environmental variation for not moving

Answer 17