Lecture 12 Trees and Seeds

Question 1

What is a tree

Answer 1

Simple: It is a growth form, i.e. plant with single stem & branching canopy that can reach large heights thanks to specialized cells

Strict:
* Excludes early tree-like plants (e.g. Gilboa trees)
* Excludes some modern ‘trees’ (e.g. palms, banana trees)

Question 2

Describe first tree-like plants

Answer 2

• Gilboa trees, 8m eventually up to 20 m tall
• Had lignin but no ‘true wood’ – so how did they get so tall?

Xylem strands, bundled together

Question 3

Describe Carboniferous forests

Answer 3

• Ancient fern forests dominated vast wetlands covering most of Pangea
• Tree ferns, tree club mosses, giant horsetails
• 50 m tall, thin stem supported by lignin
• Shallow roots – fell over easily
• When died sank into swamps & never fully decayed because…

Hypothesis:
* Bacteria and fungi had not evolved ability to break down lignin
* 60 million years between evolution of lignin and evolution of ability to break it down
* Weight of accumulating trunks & branches compressed material -> peat -> coal
* 90% of todays coal comes from these forests

Question 4

Describe the rise of trees

Answer 4

• Key innovation of the seed plants: ability to grow wood and the ability to grow as trees
• BUT ‘trees’ are not a single taxonomic group! Trees are a growth form that occurs in many branches of the seed plants.
• i.e. all trees are seed plants, but not all seed plants are trees!
• From a tree-ish ancestor, many lineages later evolved to be herbaceous

Question 5

Describe the process from ancient trees to modern gymnosperms

Answer 5

Archaeopteris – extinct genus of fern-like trees
* considered the first ‘true tree’
* wood similar to modern gymnosperm wood
* trunks (30 m) with reinforced branch joints
* fern-like leaves
* SMll reproduced via spores, but had 2 types of spores (heterospory)

Question 6

Describe innovation of wood

Answer 6

• Modern trees have ring of dividing cells beneath the bark, called the vascular cambium
• Produces new xylem tissue (lignified) & phloem tissue
• Xylem moves water/nutrients up, Phloem moves sugars up and down
• Live and dead xylem accumulates to form wood= composite of cellulose fibres in lignin matrix
• Means trees build support (girth) as they build height

Question 7

Describe innovation of Heterospory

Answer 7

• Homospory = spores all same size (e.g. bryophytes)
• Heterospory = spores are of two sizes (seed plants)
• Small ones (microspores) go onto become pollen (male)
• Big ones (megaspores) go on to become ovule with egg (female)
• Not exposed to air
• Desiccation resistant
• Male gametophyte has to ‘burrow’ to access female gametophyte

Question 8

Describe evolution of pollen

Answer 8

Question 9

Describe evolution of seeds

Answer 9

Question 10

Describe lifecycle of gymnosperm

Answer 10

• Means ‘naked seed’: unfertilized seeds (ovules) are open to the air to be directly pollinated (no ovary)
• Include all modern seed-bearing plants that don’t have flowers or ovaries (as we’ll see next week, ovaries = fruits!)
• Includes conifers (e.g. pines, redwoods, cypress), cycads, ginkos, gnetophytess

Age of Gymnosperms: Jurassic

• Tallest living organism: red wood named Hyperion (115 m)
• Largest living ‘non-clonal’ organism: giant sequoia (1487 m3) bigger than
• Oldest living ‘non-clonal’ organism: bristle cone pine (5000 yr old)

Question 11

Describe gymnosperms today

Answer 11

Cycads
* Not palms! More closely related to pine trees
* ~100 extant species left of once-large group, tropical
* Slow growing and long lived (up to 1000 yr)
* Example of evolutionary stasis: large part of diet of some herbivorous dinosaurs
* Separate male & female plants: cones either produce pollen (male) or ovules (female)
* Still have motile sperm (like nonvascular plants)

Ginkgo biloba
* Big weirdos! Closest living relative is a cycad!
* “Living fossil” – visibly related to prehistoric fossils
* Only 1 extant species (wild but endangered in China, grown globally)
* All other relatives (same order) are extinct! Been alone for 3+ million years
* Share more ancient features with cycads, like motile sperm
* Male and female plants
* Can live for at least 1400 years!

Conifers (“cone-bearers”)
* Most diverse group of extant gymnosperms: ~630 species
* All conifer species are woody & cone-bearing, most are trees
* Include Pines, Cypress, Yews, larches
* Dominate world’s largest forest biome: the boreal forest (taiga)

Conifers - Pines
* Pines specialize in dry environments (both hot and cold)
* Challenges of winter?
- Cold: Thick bark, cones shelter seeds
- Snow: cone shaped, down-facing & flexible branches
- Dry: thin waxy needles, dense to reduce wind

Question 12

Describe climate change and range of gymnosperms

Answer 12

Albedo = light reflectivity of the surface
Forest has lower albedo than snow-covered tundra
Light not reflected warms the surface!
AcceleraJng artic warming

• Montreal is often 10 °C hotter than surrounding land
• Urban heat island effect
• Dark materials that absorb heat
• ‘canyons’ created by tall buildings trap heat
• Lack of plant cover, especially trees
• Climate change will increase extreme heat in cities
• Trees provide shade (reducing absorption by surfaces), but even more importantly…
• Plants shed heat via evaporative cooling (like people sweating) – trees are constantly losing water from their stomata
• Temperature difference under canopy can create breeze
• Total cooling effect of 3 to 9 °C