The origins of plant life on land

Question 1

Charophytes -> Land plants

Answer 1

Charophytes are the group of green algae whose ancestral lineage gave rise to land plants. It is thought that the chemical xyloglucan played a role in this.

“While cellulose gives the cell walls strength, it’s thought that xyloglucan is a glue that helps organise the cellulose.”

However many algae have been found to possess the ability to produce xyloglucan. “Most of these algae are unicellular or have very simple structure made up of few cells… which leads to the conclusion that it probably didn’t evolve as a molecule associated with the strength needed to the upright growth of land plants.”

“They had found that plants were releasing it from their roots, and it was helping the roots aggregate soil particles by sticking them together…The evolution of xyloglucan is possibly linked to the adaptation of these algae to survive on land, in direct contact with the substrates, making them able to aggregate soil particles around the cells creating a more favorable microenvironment.”

Xyloglucan may play a role in the formation of biological soil crusts

Quotes from : https://botany.one/2018/09/the-first-footholds-for-land-plants-may-have-been-much-smaller-than-imagined/ (Salt, 2018)

Question 2

Plants first appeared on land around 500 Mya.

Answer 2

They needed structural support and methods to disperse gametes/progeny. Larger plants also needed a way to transport water to all parts of the plant.

Question 3

Features of land plants

Answer 3

Defining features of embryophytes (land plants)
A protected embryo (separates them from green algae, around 490 Mya)
A Waxy cuticle to reduce water loss
A Multicellular sporophyte
Gametangia (enclosing the gametes)
Thick walled spores (Sporopollenin)

Other characteristics:
‘Sun screen’ pigments e.g. anthocyanins
Stomata
Vascular tissue
Mycorrhizal partnerships (e.g. essential for orchids)
Seeds
Flowers and fruits

How do plant cell walls differ from the cell walls of algae?
See diagram in notes

Question 4

All land plants show alternation of generation

Answer 4

All land plants show alternation of generations:

1) Mitosis: The gametophyte produces haploid gametes by mitosis.
2) Fertilisation: Gametes fuse to form a zygote.
3)The zygote develops into a diploid sporophyte.
4) Meiosis: the sporophyte produces haploid spores by meiosis
5) Mitosis: Spores germinate and divide to form the haploid gametophyte. (cycle returns to 1)

Question 5

The plant kingdom

Answer 5

The plant kingdom:

Non-vascular plants [bryophytes]
- Division Hepatophyta [liverworts]
- Division Bryophyta [mosses]
- Division Anthoceratophyta [hornworts]

Vascular plants (tracheophytes), without seeds
- Division Lycopodiophyta [club-mosses and allies]
- Division Monilophyta [horsetails, ferns]

Vascular plants (tracheophytes) with seeds (spermatophytes)
Gymnosperms
- Division Cycadophyta [cycads]
- Division Ginkgophyta [Ginkgo]
- Division Gnetophyta
- Division Coniferophyta [conifers]

Angiosperms [flowering plants
- Basal angiosperms
- Core angiosperms [Mesangiospermae]
^ Including Magnolids, Monocots and Eudicots

see notes for evolutionary phylogenetic tree

Question 6

Nonvascular plants

Answer 6

Liverworts, mosses and hornworts
- Lack true leaves, stems and roots (vascular system)
- Restricted to where water is readily available
- Often mat-forming – water moves by capillary action
- Most only few cm tall
- Cuticle usually very thin
- Sporophytes depend on gametophyte
- Aglaophyton major , a rhyniophyte, with mycorrhizal associations - importance?
- Remaining clades of vascular land plants all have tracheids
^ Tracheids have strengthened walls (lignin) and are
connected to one another by pits.
^ Speed up the flow of water and provide rigidity.

Question 7

Monilophyta

Answer 7

-ferns and horsetails
- Ferns have circinate vernation – clusters of spores in sori under fronds
- Fern lifecycle is less dependent on water than the non-vascular plants (see diagram)

Question 8

The evolution of leaves

Answer 8

see diagram in notes
1) Microphylls
A Sporangium evolved into a simple leaf-like structure

2) Megaphylls
A branching stem system became progressively reduced and flattened.
Flat plates of photosynthetic tissue developed between
branches, the end of these branches evolved into the
veins of leaves.

Question 9

Homospory to Heterospory and the development of seed plants: general evolutionary trends

Answer 9

General evolutionary trends:
- reduction in size and duration of the haploid gametophyte phase
- evolution of heterospory –two types of spores generate two types of gametophytes.
- In seed plants, gametophytes develop partly or entirely whilst attached to, and dependent on, the sporophyte. Further reduces dependence on water for reproduction

see diagram

Question 10

The fossil record of seed plant evolution

Answer 10

Seeds provide long-term protection for the embryo:

1,300-year-old seed of sacred lotus (Nelumbo nucifera), recovered from a dry lake bed in NE China in 1995 has been germinated successfully (Shen-Miller, 2002)

2000 year old date seeds ( of Phoenix dactylifera L.) from Masada (Israel) germinated and grown into 120 cm high plant (Sallon et al., 2008)

Question 11

Pollination is a hallmark of the seed plant

Answer 11

e.g. Pine (gymnosperm) pollen has wings for wind distribution

e.g. Hibiscus (angiosperm) pollen has a textured surface to adhere to pollinators (usually birds)

Question 12

Seed plant characteristics

Answer 12

Seed plant characteristics
Five Derived Traits of Seed Plants:

1)Reduced gametophytes
Microscopic male and female gametophytes (n) are nourished and protected by the sporophyte (2n)

2) Heterospory
Microspore (gives rise to a male gametophyte)
Megaspore (gives rise to a female gametophyte)

3) Ovules

4) Pollen
Pollen grains make water
unnecessary for fertilization

5) Seeds
Seeds survive better than unprotected spores and can be transported long distances

Question 13

Seed development in gymnosperms

Answer 13

in Pinus sp.
Female cones (megastrobili) ovules are fertilised by Male cones (microstrobilis) pollen

The seed coat is derived from integument which surrounds the megasporangium (ovule) food supply is provided to the embryo by the female gametophyte tissue

Seeds contains 3 generations

Question 14

Angiosperms (flowering plants) : have seeds in a vessel or container

Answer 14

Angiosperms produce flowers and fruits this has major reproductive advantages.

Flowers contain the sex organs. Nearly all species reproduce sexually, many asexually as well.

Male gametophyte is reduced to the pollen grain and female gametophyte to an embryo sac embedded in the ovule, in host sporophyte tissue.

Double fertilisation, results in triploid nutritive tissue, the endosperm - unique to angiosperms.

Ovules and seeds are enclosed by a carpel (modified leaves bearing the ovule) which provides additional protection.

Angiosperms also have specialised vessel elements and fibres in their xylem tissue and companion cells in phloem tissue.

Question 15

Seed development in angiosperms

Answer 15

1) The microspore undergoes mitosis forming a tube cell and a generative cell
2) In the ovule three of the four meiotic products degenerate
3) The embryo sac is the female gametophyte. After three mitotic divisions, it contains 8 haploid nuclei, seven of which take part in double fertilisation
4) The pollen grain is transferred to the stigma (pollination)
5) The pollen tube grows towards the embryo sac
6) One sperm cell fuses with the egg cell
7) The second sperm cell fuses with the central cell then the polar nuclei fuse with the sperm nucleus
8) The fruit is derived from the ovary wall and aids in seed dispersal - the seed develops into a flowering angiosperm and process returns to step 1

Question 16

A complete flower has 4 concentric groups of organs

Answer 16

Carpals
Stamens
Petals
Sepals

Question 17

Gymnosperms and angiosperms both have tracheids.
Only angiosperms have vessels

Answer 17

see diagrams of vessels and tracheids in notes for comparison

Question 18

Evolutionary relationships between angiosperms

Answer 18

(see phylogenetic tree)

All angiosperms have:
Carpels; endosperm; seeds in fruit; reduced gametophytes; double fertilization; flowers; phloem with companion cells

All angiosperms except Amborella have:
Transitional tracheid vessel elements

All angiosperms except Amborella and waterlilies have
Vessel elements

Magnolids, monocots and eudicots have:
Carpel margins fused by tissue connection

Monocots and eudicots have:
Sepals and petals of two whorls

Monocots have
Single cotyledon

Eudicots have
Pollen with three grooves

Question 19

Monocots vs eudicots

Answer 19

Around a quarter of angiosperm species are monocots (ca. 74 000species)
… and just over 70 % of angiosperm species are eudicots(ca. 210 000 species)

                                   Monocots                         Dicots  Embryos:                    one cotyledon                 2 cotyledons Leaf venation:           usually parallel               usually net-like Vascular bundles:  complex arrangement/ ring arrangement Roots:                            fibrous system           taproot usually  Floral parts:                 in multiples of 3 /  in multiples of 4 or 5

Question 20

What’s so special about the angiosperms?
The range of habitats where they can thrive!

Answer 20

Largely a result of their interactions with animals…
Angiosperms produce flowers to attract animal pollinators

Interactions with pollinators: scent, colour and food rewards see :
https://heatherkelly.blog/2015/04/03/why-are-flowers-pretty/
https://heatherkelly.blog/2015/04/21/beetles-friend-or-foe/

Other features that make them adaptable:

Flowers have double fertilisation, resulting in a triploid nutritive tissue, the endosperm.
Ovules and seeds are enclosed by carpel, which develops into a fruit. Allows for greater protection and easier dispersal of seeds.
Vascular tissues are more complex, including vessel elements, companion cells and fibres, as well as tracheids

Question 21

Promoting out-crossing: dioecious plants are self-incompatible

Answer 21

see diagram - flowers on one plant cannot self pollinate as they are incompatible for fertilisation with their own pollen.
Goat Willow for example is male or female.

Question 22

Summary

Answer 22

Land plants have evolved a range of characteristics which help them thrive on land.

These include:

Changes in cell wall structures from glycoproteins and polysaccharides(such as carrageen and agar) to polysaccharides (such as cellulose,hemicelluloses and pectin)

Life cycles which feature the alternation of multicellular gametophyte(haploid) and sporophyte (diploid) generations

A cuticle, gametangia, protected embryo, protective pigments, thick spore walls, stomata and mutualistic associations with fungi. Tracheophytes also have a vascular system.

A sporophyte which has become larger than the gametophyte over time and eventually encloses it.* Pollen which has become increasingly independent of water for fertilisation

Interactions with other species and physical mechanisms which promote recombination of genetic material

Seeds to offer long term protection for the embryo and, in the case of angiosperms, fruits which add further protection and aid seed dispersal.