Evolution of Land Plants

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Pages:  6
Wordcount:  1640 Words
Date:  2021-03-19

Question 1. Sporophyte generation of Polypodium, as typical fern, is photosynthetic and dominates gametophyte. It has vascular tissue and roots. The structure at the bottom side of the frond, which contains sporangia protected by a layer of cells (indusium), is called a sorus. Sporangia cells undergo meiosis to yield haploid spores. Each spore divides by mitosis to form usually heart-shaped gametophyte. It is much smaller than sporophyte and doesnt have true roots, anchored to the ground by rootlike projections called rhizoids, but it is also photosynthetic and independent of the sporophyte. Gametophyte differentiates both archegonia (female organ) and antheridia (male organ). Process of fertilization requires water for sperm, which have flagella, to reach the archegonia and fertilize eggs.

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Pinus is heterosporous, meaning that two kinds of spores (male microspores and female megaspores) are produced by the same mature sporophyte. Female cones (ovulate cones) contain two ovules per scale. Megasporangium in each ovule undergoes meiosis, which results in formation of four haploid megaspores. But only one megaspore survives and develops into female multicellular gametophyte with archegonium which produces egg cell. Microsporangia in pollen cone (staminate cone) undergoes meiosis forming haploid microspores, which develop into pollen grains (male gametophytes). Pollen grain contains tube cell, which then form pollen tube and generative cell.

Pollination is mediated by air or animals in the spring. Pollen tube grows through micropyle from tube cell to reach egg. The pollen tube develops slowly and this process may take up to two years after pollination. Generative cell in the pollen grain produces two sperm: one of them travels through pollen tube to fertilize egg and another dies. Seed develops into young sporophyte tree.

In Pinus lifecycle, a typical conifer, the sporophyte (diploid, 2n) phase is the longest phase. Gametophytes (haploid, 1n) are reduced in size. This phase may take more than one year between pollination and fertilization.

Thus, both Pinus and Polypodium are characterized by alternation of generations (the diploid sporophyte alternates with the haploid gametophyte phase). They have dominant sporophyte phase and small gametophytes. The process of fertilization of Polypodium is much faster than Pinus. The gymnosperms do not need water for fertilization, which then results in seed formation. The main difference lies in the fact that ferns do not have seeds, whereas gymnosperms do not reproduce by spores. Most ferns are homosporous, whereas gymnosperms are heterosporous.


Boundless. (2016) Life Cycle of a Conifer. Boundless Biology. Boundless. Retrieved from https://www.boundless.com/biology/textbooks/boundless-biology-textbook/seed-plants-26/gymnosperms-159/life-cycle-of-a-conifer-622-11843/Gilbert, S. F. (2000) Plant Life Cycles. Developmental Biology. 6th edition. Sunderland (MA): Sinauer Associates;

Kenrick, P., Crane, P. R. (1997). The origin and early evolution of plants on land. Nature. 389: 33-39

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Willis, K., McElwain, J. (2002). The Evolution of Plants. Gosport, Hampshre: Ashfort Colour Press Ltd.

Question 2. First vascular land plants (also called tracheophytes) were seedless. They appeared in the middle of the Silurian period (437-407 million years ago) and were represented by the rhyniophytes and possibly lycophytes. They use vascular tissue (xylem and phloem) to transport inorganic and organic substances throughout plant. They use haploid and unicellular spores instead of seeds for reproduction.

They are not spread all over the world because fertilization depends on water. Sperm requires water to reach the egg. Thats why they are abundant in damp environments, such as rainforests and swamps. Seedless vascular plants are characterized by the alternation of generations. The diploid sporophyte (dominant phase) alternates with the haploid gametophyte phase.

There are four groups of living seedless vascular plants: Pterophyta (ferns), Psilophyta (whisk ferns), Arthrophyta (horsetails) and Lycophyta (club mosses).

Ferns include about 12,000 living species and they seem to be the closest relatives to the seed plants. They originated at the Devonian period (about 350 million years ago) and became wide spread during the next 50 million years. More than 70% of modern ferns occur in tropics. Most ferns are homosporous.

Psilophyta and Arthrophyta have many features in common with Pterophyta. They all form archegonia and antherida and require water for the fertilization. Their sporophytes are much bigger than gametophytes, and they both are photosynthetic.

Lycophyta are the most abundant in the tropics and moist temperate regions. Some tree like club mosses became extinct about 270 million years ago. Modern club mosses are either homosporous or heterosporous.


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Bennici, A. (2008). Origin and early evolution of land plants: Problems and considerations. Communicative & Integrative Biology, 1(2), 212218.

Boundless (2016). Seedless Vascular Plants. Boundless Biology. Boundless. Retrieved from HYPERLINK "https://www.boundless.com/biology/textbooks/boundless-biology-textbook/seedless-plants-25/seedless-vascular-plants-157/seedless-vascular-plants-613-11833/"https://www.boundless.com/biology/textbooks/boundless-biology-textbook/seedless-plants-25/seedless-vascular-plants-157/seedless-vascular-plants-613-11833/

Ingrouille, M. (1992). Diversity and Evolution of Land Plants. London: Chapman & Hall.

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Willis, K., McElwain, J. (2002). The Evolution of Plants. Gosport, Hampshre: Ashfort Colour Press Ltd.

Question 3. The second land flora, called Coal Age Flora, appeared at least 350 million years ago at Paleozoic era. The Earth became warmer, which led to great spread of forests and swamps. Oxygen levels increased during that time, while carbon dioxide levels decreased. The main Early Carboniferous plants were the Equisetales, Sphenophyllales, Lycopodiales, Lepidodendrales, Filicales, Medullosales and the Cordaitales, which dominated through the period.

The period of second land flora was called Carboniferous period (about 358.9 million to 323.2 million years ago). During this time remains of dead vascular land plants formed most of the coal people use today for energy. Some biological or environmental factors inhibited the complete decay of plants, resulting in great amount of coal.

The evolutionary lag hypothesis suggests that in those times fungi were not involved n process of lignin degradation. On the other hand, evidence from Nelsen et al. (2016) claims that there were enough lignin-degrading organisms during the Carboniferous. Accumulation of coal during this period was "likely the result of a unique combination of ever wet tropical conditions and extensive depositional systems". Large amounts of plant material were unable to be processed quickly, which led to the formation of coal seams.

During the Carboniferous Period swamps were dominated by lycopods with dense, spirally arranged leaves (in the modern world they are represented only by club mosses), rivers and stream edges by several members of the group Equisetum (nowadays there are only horsetails).

Increasing of the oxygen level to 35% (today there is only 21% of oxygen in atmosphere) and simultaneous reduce of carbon dioxide in the atmosphere to record level led to the beginning of the Ice Age. About 10% of all living organisms disappeared after that.


Bennici, A. (2008). Origin and early evolution of land plants: Problems and considerations. Communicative & Integrative Biology, 1(2), 212218.

Cleala, C.J., Uhl, D., Cascales-Minanac, B., Thomas, B. A., Bashforth, A. R., King, S. C., Zodrow, E. L. (2012). Plant biodiversity changes in Carboniferous tropical wetlands Earth-Science Reviews.114(12): 124155

DiMichele, W. A., Pfefferkorn, H. W., Gastaldo, R. A. (2001). Response of Late Carboniferous and Early Permian Plant Communities to Climate Change. Annual Review of Earth and Planetary Sciences. 29: 461 -487

DiMichele, W. A., Pfefferkorn, H. W., Gastaldo, R. A. (2005). Plant Biodiversity Partitioning in the Late Carboniferous and Early Permian and Its Implications for Ecosystem Assembly. 56(4): 3249

Nelsen, M. P., DiMichele, W. A., Peters, S. E., Boyce, C. K. (2016). Delayed fungal evolution did not cause the Paleozoic peak in coal production. Proc Natl Acad Sci U S A. 113(9):2442-2447.

Willis, K., McElwain, J. (2002). The Evolution of Plants. Gosport, Hampshre: Ashfort Colour Press Ltd.

Question 4. Gymnosperm is a group of first seed-producing plants that includes division Pinophyta (conifers, the largest group of living Gymnosperms), Gnetophyta and Cycadophyta and genus Ginkgo (a single living species from division Ginkgophyta). All seed plants are heterosporous, meaning that two kinds of spores (microspores and megaspores) are produced by the sporophyte. Therefore, they are monoecious plants. Visible part of the plant represents sporophyte rather than gametophyte. Sporangia of seedless plants typically lack an integument, which forms the seed coat in gymnosperms.

The word "gymnosperm" came from the Greek word gymnosperms, meaning "naked seeds".

Gymnosperms appeared in the late Carboniferous and remained dominant through most of the Mesozoic era (about 252.2 million to 66 million years ago). But the earliest seed-like bodies were found in rocks of the Upper Devonian Series (about 382.7 million to 358.9 million years ago). Gymnosperms replaced lycophyta from tropical rainforests. Today there are about 66 living genera of Gymnosperms.

The evolution of seed plants appears to be the result of two distinct rounds of whole genome duplication events, which occurred at 319 million years ago.

Since the Cretaceous Period (about 145 million to 66 million years ago) gymnosperms have been gradually displaced by the more recently evolved angiosperms. The earliest recognized group of seed plants are members of extinct division Pteridospermatophyta. They originated in Devonian period and flourished particularly during the Carboniferous and Permian periods. Modern division Cycadophyta arose from Pteridospermatophyta in Permian period, although some fossils suggests that early cycads existed during the preceding Carboniferous.

Some earliest conifers (class Cordaitopsida) appeared first toward the end of the Carboniferous Period. They were trees with trunks similar to those of extant conifers and with long strap-shaped leaves. Modern families of conifers began to appear in the Mesozoic Era.


Gymnosperm. (2016). In Encyclopaedia Britannica Online. Retrieved from http://www.britannica.com/plant/gymnosperm

Jiao, Y., Wickett, N. J., Ayyampalayam, S., Chanderbali, A. S. et al. (2011). "Ancestral polyploidy in seed plants and angiosperms". Nature. 473(7345): 97100.

Bennici, A. (2008). Origin and early evolution of land plants: Problems and considerations. Communicative & Integrative Biology, 1(2), 212218.

Wellman, C. H., Osterloff, P. L., Mohiuddin, U. (2003). Fragments of the earliest land plants. Nature. 18;425(6955): 282-285

Willis, K., McElwain, J. (2002). The Evolution of Plants. Gosport, Hampshre: Ashfort Colour Press Ltd.

Question 5. Angiosperms (also called flowering plants) appeared at the early Cretaceous Period (the earliest fossil identified as an angiosperm is dated to about 125 million years ago) and became dominant near the end of the period. Creation of modern flowering plants is theori...

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