The First 'Land Plants'

 

Origin of Life:

Precambrian more than 570 MYA. Several hypothesis

Primordial soup: In 1953 Stanley Miller experimented adding water vapor, carbon, methane and ammonia. The mix was heated and electric sparkles where added. This produced complex organic molecules.Unfortunately it is thought that methane wasn't present during the Precambrian. The origin of organic molecules and life is more likely to be the result of a chance aggregation of molecules.

Procaryotic cells: First form of life were procaryotic cells (cells witout nuclear envelope) and they were heterotrophic (consuming carbohydrates to produce CO2 and energy)

The first photosynthetic cells (autotrophic organisms capable of producing carbohydrate with the sun energy) must have been similar to the cyanobacteria poducing stromatolites and still present on the SW coast of Australia. (Stromatolites are filaments of unicellular bacteria that bind calcium together and form rock like structures).

Plants are multicellular organisms made of eucaryotic cells (containing a nucleus with a nuclear envelope).

The origin of eucaryotic cells could be explained by the phagocytose theory

diagrammatic representation of the origin of a photosynthetic eukaryotic cell from a hetertrophic prokaryote. (Raven, P. et al ' Biology of Plants' 6th ed. 2003. p273)

The apparition of multicellular organisms is not restricted to eucaryotic cells. Some bacterias form aggregates of several cells, they are called protista. Some of those cells are even able to exchange genetic material between them and prefigure a kind of 'sexuality'.

Tree of life:

Bacteria, Archeae and Eukarya (Animals, Fungi, Algae, Plants).

The Three doamins of life and an outline of the history of the acquisition of some of the key components of plant cells. Plants are defined here as the group comprising green algae and land plants. (Kenrick, P. et al 'Fossil Plants' Natural History Museum. 2004:8)

Conquering the earth

Life cycles, mitosis and meiosis

Mitosis: one cell division resulting in two daughter cell.

2n ---> 2n + 2n

Meiosis: two cell divisions resulting in four products of meiosis.

2n --> 2n + 2n --> n + n + n + n

 

General diagramm of a plant life cycle.

Question : Place fecondation, Meiosis and Mitosis on this diagramm.

often plants are refered as embryophytes because the sporophyte (2n) is dependant (growing on) to the gametophyte (n).

But why sexuality appeared ? to promote diversity and exchange of genetic material.

Bryophytes

Bryophytes comprises the mosses but also liverworts and Hornworts.

They are very simple organisms that are water dependant because they have no vessels to transport water in their stems and because they have mobile male gametes that need water to reach the female gametes.

In Bryophytes the gametophyte phase is predominant.

Life cycle of a moss (Phylum Bryophyta, class Brydae). (Raven, P. et al ' Biology of Plants' 6th ed. 2003. p418)

Marchantia polymorpha
Archegonial head
Archegonial head
Sporangium
Thalli with gemmae cups

 

Tracheophytes

Tracheophytes have tracheids (primitive vessels) and roots, they are less dependant on water. The sporophyte is now the dominant phase in the life cycle.

>Protracheophyte:

They are fossil organisms, with a very simple architecture. For example Cooksonia is a one dichotomous branched plant with terminal sporangia.

>Fern allies: Lycophyta, Psilophyta, Sphenophyta, Pterophyta

->Lycophyta: known as clubmosses

-->Lycopodium:

Lycopodiun are homosporous, a dominant sporophyte phase and have leaves or microphylls.

Life cycle of Lycopodium lagopus. (Raven, P. et al ' Biology of Plants' 6th ed. 2003. p436)

 

Lycopodiaceae Lycopodium in Chile, Huperzia selago growing in the Pyrenees. (Note the sporangia all along the stem)
 
Lycopodium magellanicum
 
Huperzia selago
Huperzia selago

 

evolution of micophylls and megaphylls:

The apparition of leaves and stems are linked to the evolution of the stems.

Hypothetical evolution of microphylls and megaphylls.

-->Selaginella:

Selaginellas are heterosporous (micro and macrospores), the gametophyte phase is reduced.

 

Life cycle of Selaginella. (Raven, P. et al ' Biology of Plants' 6th ed. 2003. p440)

->Psilophyta:

-->Psilotum: Very simple, homosporous organisms without micophylls and dichotomously barnched.

Life cycle of Psilotum. (Raven, P. et al ' Biology of Plants' 6th ed. 2003. p446)

Psilotum
Psilotum sporangium
Psilotum sporangium
Psilotum sporangium

-> Sphenophyta:

--> Equisetum: have strobilus and sporangiophore forming like a 'Cone' structure. The spores have elaters attached to them (thickened coiled bands)

Life cycle of Equisetum. (Raven, P. et al ' Biology of Plants' 6th ed. 2003. p450)

 

-> Pterophyta: Ferns

--> Eusporangiate:

---> Ophioglossales: Ophioglossum or Adder's tongue

Ophioglossaceae Botrychium lunaria growing in Kyrgyzstan showing leaf blade and sporangiophyll
Botrychium lunaria

--> Leptosporangiate:

---> Marsileales: Marsilea heterosporous ferns.

---> Salviniales: Azolla and Salvinia heterosporous ferns.

Slaviniaceae Salvinia
Salvinia hairs abaxially
Salvinia sporangium Salvinia sporangium Salvinia megasporangium Salvinia megaspores and microspores.

 

---> Filicales: the Ferns

Life cycle of Polypodium (order Fillicales) a homosporous leptosporangiate Fern. (Raven, P. et al ' Biology of Plants' 6th ed. 2003. p460)

Plant Phylogeny

One hypothesis of embryophyte or plant phylogeny. (Raven, P. et al ' Biology of Plants' 6th ed. 2003. p432)

Seed Ferns and Gymnosperms

Seed Ferns and Gymnosperms give rise to to an important new evolution step. The spores are not released and they become internal gametophytes; the pollen for the microsporangium and the ovule for the megasporangium. The pollen like the spores is not dependant on water and is wind dispersed. The embryo also goes through a new phase the SEED and can be dispersed and kept in dry condition before germinating.

Female and male gametophyte are now located on the same individual but not yet on the same structure; the male and female cones.

Life cycle of a Pine (Pinus: phylum Coniferophyta). (Raven, P. et al ' Biology of Plants' 6th ed. 2003. p478)

The coal age

Timeline (Kenrick, P. et al 'Fossil Plants' Natural History Museum. 2004:8)

Bibliography

Judd, W.S.; Campbell, C.S.; Kellogg, E.A.; Stevens, P.F. 1999. 'Plants systematics, a phylogentic approach' Sinauer Associates Inc. Publ. , Sunderland, 464p.

Raven, P.H., Evert, R.F. 2003 'Biology of Plants' 6th ed. W.H.freeman and company worth Publisher, 944p.

Kenrick, P. & P. Davis. 2004. 'Fossil Plants' Natural History Museum, London. 216p.

Johns, R.J., 1996. 'An Introduction to the Ferns and Ferns Allies' Kew personal communication.

 

Home page

Origin of Life

Tree of life

Conquering the Earth

Life cycles, mitosis and meiosis

Bryophytes

Tracheophytes

Plant Phylogeny

Seed Ferns and Gymnosperms

The coal age

Bibliography