Plants to land

- What is a plant?- What are the challenges of land?- How were these challenges met?

Refer to chapter 23 in text.

Short form: a truly multicellular photoautotroph.photoautotroph: self energy and carbon capture

(it photosynthesizes) truly multicellular: showing tissue specialization

Contrast with unicellular photoautotrophs, algae,(which, being unicellular eukaryotes, are protists)Kelp are (very) large communal algae,

without actual tissue specialization↓

- What is a plant?

http://www.smbaykeeper.org/images/site_images/Giant_kelp_adult.jpgwildflowers.jdcc.edu/Common%20Wood%20Violet.html

Prior to 500 million years ago the land was mostly barren:- bare rock and sand,

with maybe prokaryotes scattered about.There had been colonial algae in the seas for about 2 billion years,

and many complex animals were present, all marine.

http://www.cartage.org.lb/en/themes/sciences/Paleontology/Paleozoology/EarlyPaleozoic/cambdiorama.gif

← Artist’s rendition of theCambrian explosion(505 mya)based on Burgess Shale fossilsdiscovered in western Canadain 1909 by Charles Walcott.

What are the challenges of land?

1. DehydrationWhile living in water there is still the problem ofwater balance, but not like being surrounded by air.

2. SupportAquatic algae are supported by the water.

3. Nutrient acquisition/gas exchange These algae are surrounded by the nutrients they need, suspended in the water itself.

4. Nutrient dispersal Algae are generally small, or thin, and don’t have far

to move materials internally.5. Dispersal of reproductive structures

Again, surrounded by water, reproductive cells are hydrated and washed away, but on land…

These challenges were met in a step-wise progression,reflected in modern examples.

Charophyceans are the algae most closely related to plants:- similar cellulose-making apparatus (for cell walls)- same peroxisome enzymes

(take care of photosynthesis byproduct)- sperm same as those of sperm-producing plants- similarity in cell plate formation in mitosis- apical growth, lengthening filaments, like stems- gaps through cell walls (plasmodesmata) for cell-cell communication

http://www.life.umd.edu/labs/delwiche/Strp/Chlorophyta/charophyceae/Coleo-irregularis.jpg

But these were still all living in the water…

Bryophyte (e.g. mosses↓ liverworts and hornworts→)the earliest land plants

VascularRootsLeavesSeedsFlowers

No (very thin/flat)No (rhizoids, to anchor only)No No (spores)No They do have a water-retainingouter surface.

http://www.davidlnelson.md/Cazadero/CazImages/Moss_sporangium2.jpg

These were the dominant plants for 100 million years.Most of their life they are haploid (more later).Sperm have to swim in moisture

←haploid gametophyte with diploid sporophyte sticking out

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Lycophytes (e.g. “club mosses”..NOT actual mosses)

VascularRootsLeavesSeeds

Flowers

Yes (strong, hydrophobic lignin in cell walls)Yes, off of rhizomes (underground stems)Sort of (single vein microphylls) No (windblown spores and

flagellated sperm)No

- Leaf forms evolved independently(not direct ancestor of modern branched-vein leaves)

- Forests of lycophytes contributed to coal deposits from Carboniferous era; most genera are extinct.- Spores are flammable…

http://www.palaeos.com/Plants/Lycophytes/Images/Lycopodium.jpg

Lycopodium venustulum ↓ Selaginella plana →(note sporangia-bearing strobili… ‘clubs’)

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Pteridophyte/ Filicophyta (e.g. ferns)

VascularRootsLeavesSeedsFlowers

YesYes (rhizome and root)Yes (megaphylls, or “fronds”) No (spores)No

http://www.davidlnelson.md/Cazadero/CazImages/Fern_structure.jpg

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Two other groups,horsetails and whisk ferns,seem more like lycophytes – They have lost traits throughdevolution.

Gymnosperms (e.g. conifers and cycads) “naked seeds”

VascularRootsLeavesSeedsFlowers

YesYesYes (often needles)Yes (not in fruit) No

- Taxonomically not one clade.- Wind-borne pollen

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Redwoods are conifers →Sagos are cycads ↓

Angiosperms (i.e. all flowering plants)

VascularRootsLeavesSeedsFlowers

YesYesYesYes (in fruit)Yes

- Evolution of flowers and fruit has provided for a wide range ofreproductive strategies (e.g. co-evolution of animal pollinators)

- Two broad divisions: ←monocots (e.g. ginger) and eudicots (e.g. dogwood)↑

(More on this later.)

http://www.sky-bolt.com/images/curcumaflower.jpg

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Alternation of generations

Along with the trend to gain the traits noted,there has been a trend to spend less time in a haploid state.

(recall: diploid (2n) is 2 sets of chromosomes,usual for most eukaryotes,

while haploid (n) is one set, as is gametes in sexual reproduction.)

The next series of slides goes over the life cycles showing this: There is a lot of stuff. Don’t panic.

We will have more details on angiosperm fertilization later

Bryophytes

note-The gametophyte (n) is the conspicuous stage.-Sporophyte is the only diploid (2n) stage, depends on the gametophyte.-Sperm (upper right) have to swim in moisture to egg.

Pteridophytes note - Dominant frond is diploid (2n).- Haploid gametophyte still vegetative.- Sperm still need to swim

Gymnosperms

note- Megaspore, microspore, and food reserves are the haploid structures- Microspore (pollen) is wind blown.

Angiosperms note - As with gymnosperms, little is haploid.- A triploid (3n) food storage structure (endosperm)

has been added.- Pollen may be carried by wind or pollinators.

Alternation of generation, in summary:

Describe how each of the 5 major plant groupsrepresents adaptations to life on land.

What do you think are the advantages of haploidy?Of diploidy?

Charophyceans have a lot in common with land plants:Why aren’t they aquatic plants, then?

photoautotroph. conifer

Charophycean cycad

cellulose Angiosperm

Bryophyte alternation of generations

rhizoid diploid

lignin haploid

Lycophyterhizome gametophyte

Pteridophyte sporophyte

Filicophyta pollen

Gymnosperm endosperm