Angiosperm life cycle and diversity



Carpets of spring flowers near Langebaan Lagoon. Photo: C. Voget.

The factsheet in the December issue of Veld & Flora follows on from the factsheet in the September issue, the factsheet in the June 2016 issue, the factsheet in the March 2016 issue and the Factsheets on the Classification of Life in September 2012 and Plant Classification in December 2015. (Click on the highlighted text to download the factsheets.)

Flowers separate the angiosperms from all other plants. Fossil evidence indicates that flowers evolved from spirally arranged leaves and carpels or stamens (the female and male reproductive parts of a plant) so there is some truth in the well-known quote ‘ A flower is a leaf, mad with love’. Although some angiosperm species bear female and male flowers on the same plant (monoecious) and others bear female and male flowers on different plants (dioecious), the typical hermaphrodite (male and female parts together) structure of a flower is quite different to the unisexual cones of most gymnosperms. Flowers also evolved into a myriad of different shapes and structures to tempt different animal pollinators or make use of wind and water for pollination. Like gymnosperms, flowering plants also produce seeds but these differ in that they contain an endosperm, and are entirely enclosed in protective ovaries that mature into the fruit.

A diversity of flowers

The ancestors of flowering plants diverged from gymnosperms over 200 million years ago. By 100–60 million years ago they had replaced conifers as the dominant plants on Earth. Five main groups are recognized: the Basal Angiosperms, Magnoliids, Monocots, Ceratophyllaceae and Eudicots.


Blue Waterlily (Nymphaea nouchali var. caerulea growing at Kosi Bay. Photo: Ricky Taylor, iSpot.

Basal Angiosperms are a group of the most primitive flowering plants, comprising 0.5% of living angiosperms. One of the three orders within the group include members of the Nymphaeales  which are represented in South Africa by the waterlilies.

Stinkwood Tree (Ocotea bullata) flowers. Photo: Gigi Laidler, iSpot.

Magnoliids consist of eight orders comprising 2.5% of living angiosperms today. They include our well-known Stinkwood Tree.

Aristea spiralis, a member of the Liliidae, growing at Cape Point.

Monocots share traits with magnoliids. Both arose between 130 and 125 million years ago from a common ancestor. They comprise about 28% of living angiosperms today and are classified into four superorders, one of which is the Liliidae. South Africa is particularly rich in species from this group.

Ceratophyllum demersum var. demersum growing on the lake margins in Kosi Bay. Photo: Ricky Taylor, iSpot.

Ceratophyllaceae comprises one genus Ceratophyllum with about 30 species – all of which are aquatic and lack roots and their leaves lack stomata.  They comprise 0.012 % of the world’s angiosperms. 

Halfmens (Pachypodium namaquanum) growing in the Northern Cape. Photo: C. Voget.

Eudicots have a distinctive pollen structure, but otherwise they share many traits with the preceding groups. They contain five superorders and comprise some 69% of living angiosperms. South Africa has countless plants in this group including the daisies, ericas and succulents like the enigmatic Halfmens pictured here in its desert habitat in the Northern Cape.

Bear in mind that classification of plants is in a state of flux due to ever-increasing sophistication in molecular studies and the phylogenetic tree of Plantae may well differ in some publications.

Angiosperm life cycle

Almost all land plants reproduce by means of two distinct, alternating life forms: a sexual phase that produces and releases gametes or sex cells and allows fertilisation, and a dispersal phase. The sexual phase is known as the GAMETOPHYTE or haploid (n) generation and the dispersal phase is the SPOROPHYTE or diploid (2n) generation. In angiosperms, as in all vascular plants, the sporophyte phase is the dominant generation.

The gametophyte phase is reduced to a few cells and gametophyes are totally dependent on the sporophyte. Haploid (n) microspores and megaspores (i.e. each spore has a single set of chromosomes) are produced by meoisis. Female megaspores develops into the embryo sac (megasporophyte) that grows inside the ovule, which is retained within the flower on the plant. Male microspores develop into pollen grains (microsporophtes) and are transferred to other flowers – preferably on different plants – during the pollination process. Pollen grains land on the stigma and a pollen tube grows from the pollen grain down the style, eventually reaching the ovary and entering the ovule. The male gamete or sperm (two in the case of angiosperms) then travels along the tube to the embryo sac within the ovule where it fuses with the female gamete (ovum) to form a diploid (2n) zygote (with two sets of chromosomes).

The zygote grows by cell division (mitosis) into an embryo within the seed. In angiosperms the second sperm cell fuses with the large central cell of the female gametophyte, which then develops into the endosperm, a nutrient-rich tissue which provides nourishment to the developing embryo. The seed then germinates and grows into the familiar form of the plant (the mature sporophyte). The sporophyte plants produce flowers in which the haploid (n) megaspores and microspores are produced. These have undergone a process of cell division called meiosis that results in four daughter cells each with half the number of chromosomes of the parent cell. The haploid gametophyte generation takes place within the flowers which will eventually release the diploid sporophyte seeds – and so the cycle continues.  

LINKS TO THE CURRICULUM

GRADE 11 Life Sciences, Strand 1: Diversity, Change and Continuity. Topic: Biodiversity of Plants. Content: Grouping of bryophytes, pteridophytes, gymnosperms and angiosperms.

GRADE 12 Life Sciences Strand 1: Life at Molecular, cellular and tissue level. Topic: Meiosis: the process of reduction division purposes of reduction division (gametogenesis and exceptions: mosses, ferns), Importance of meiosis: diploid to haploid: production of gametes.

Discover more about South African angiosperms by visiting iSpot http://www.ispotnature.org and PlantZafrica http://pza.sanbi.org/.
Download the factsheet by clicking here.

IMAGES for the life cycle diagram are reproduced with thanks to Alice Notten, PlantZafrica (SANBI) and The Protea Atlas Project, SANBI.

TEXT adapted by Caroline Voget from the book The Story of Life and the Environment: An African perspective by Jo van As, Johann du Preez, Leslie Brown and Nico Smit, published by Struik Nature.

Gymnosperm life cycle and diversity

The Clanwilliam Cedar (Widdringtonia cedarbergensis). Photo:Tony Rebelo, iSpot.

The factsheet in the September issue of Veld & Flora follows on from the factsheet in the June 2016 issue, the factsheet in the March 2016 issue and the Factsheets on the Classification of Life in September 2012 and Plant Classification in December 2015.

 

A DIVERSITY OF GYMNOSPERMS

The gymnosperms are cone-bearing, vascular seed plants that do not bear flowers. Their seeds develop either on the surface of scales or leaves, often modified to form cones, or at the end of short stalks as in Ginkgo. There are now only just over 1000 living species of gymnosperm, although there are many more extinct species in the fossil record. They evolved about 365 million years ago and were dominant from 245 to 65 million years ago when the angiosperms took over.  

Gymnosperms were thought to be a natural evolutionary group with one common ancestor (a clade), but with new discoveries in genetics, there is evidence to show that this is not the case. Scientists have yet to work out the evolutionary relationships, but it seems that angiosperms evolved from an extinct group of gymnosperms, although it is not clear which one is their closest relative.

By far the largest group of living gymnosperms are the conifers (pines, cypresses and relatives), followed by cycads, gnetophytes (Gnetum, Ephedra and Welwitschia) and a single living species of ginkgo. In southern Africa we have a few indigenous representatives from the conifers, cycads and gnetophytes although several others are naturalised exotics, garden plants and alien invaders.

An example of a conifer is the Outeniqua Yellowwood (Afrocarpus falcatus). Photo: Shaun Swanepoel, iSpot

Conifers (sometimes called Pinophyta) include pines,  cedars, cypressus, yellowwods, firs and redwoods which are mostly evergreen shrubs and trees with needle-like leaves. In southern Africa we have species in the family Podocarpaceae (our yellowwoods – one species of Afrocarpus and three of Podocarpus) and the family Cupressaceae (three species of cedar –  Widdringtonia  and the African Juniper (Juniperus procera).  

Encephalartos horribilis with T. rex at Kirstenbosch. Photo: C.Voget.

 

There are a hundred known species of cycad worldwide. They appeared about 320 million years ago and reached their peak in the Mesozoic Era where they existed side by side with the dinosaurs. In southern Africa we have several species of Encephalartos and one Stangeria. Having survived for so long, our cycads are seriously in danger of extinction due to human activity.

Welwitschia mirabilis. Photo: Christine Ridge-Shnaufer, with thanks to Colin Ralston, iSpot.

 

Finally, the group of gnetophytes consists of  70 know species in the three genera Gnetum and Ephedra and Welwitschia mirabilis. This group of plants has no close living relatives. Welwitschia mirabilis occurs in the Namib Desert and has evolved to cope with hyper-arid conditions. Two leathery leaves grow continuously from a cone shaped corky stem – becoming rather tattered as it ages. The leaves are grooved which collect and channel condensing fog down to the tap roots which are adapted to store moisture. Male and female plants produce cones that are pollinated by Welwitschia Beetles that only occur on the plants. A truly fascinating living fossil!

Maidenhair Tree (Ginkgo biloba) growing at Kirstenbosch. Photo: Alice Notten. For more about this tree, click here.

There is only one surviving member of the ginkgos, the  Maidenhair Tree (Ginkgo biloba) that occurs naturally in China, but they are easy to cultivate and grace many gardens, including Kirstenbosch. It remains virtually unchanged from 80 million years ago.

GYMNOSPERM LIFE CYCLE

Almost all land plants reproduce by means of two distinct, alternating life forms: a sexual phase that produces and releases gametes or sex cells and allows fertilisation, and a dispersal phase. The sexual phase is known as the GAMETOPHYTE or haploid (n) generation and the dispersal phase is the SPOROPHYTE or diploid (2n) generation. In gymnosperms, as in all vascular plants, the sporophyte phase is the dominant generation.

The gametophyte phase is reduced to a few cells – the embryo sac (female ) and the pollen grain (male). Haploid (n) microspores and megaspores (i.e. each spore has a single set of chromosomes) which are produced in male and female cones. A female megaspore develops into the embryo sac (megasporophyte) that grow inside the ovule, which is retained within the female cone on the tree. Male microspores develop into pollen grains (microsporophtes) and are transferred from the male cones onto the female cones – preferably on different plants – by wind or insects. Pollen grains enter the ovules through a microscopic gap in the ovule coat called the micropyle. Once inside, they mature further and produce sperm cells.

The gametophytes are not free living and are totally dependent on the sporophyte for water, nutrients and protection. However, no external water is needed for fertilisation to take place. Two modes of fertilization are found in gymnosperms. Cycads and ginkgos have motile sperm that “swim” to the egg inside the ovule, whereas the sperm of conifers and gnetophytes are conveyed to the embryo sac along a pollen tube that is formed inside the ovule.

Once fusion of the egg and sperm – or fertilisation – takes place, a diploid (2n) zygote forms with two sets of chromosomes. The zygote grows by cell division (mitosis) into an embryo and eventually forms the seed. The mature seed comprises the embryo and the remains of the female gametophyte, which serves as a food supply, and the seed coat. The seed then germinates and grows into the familiar form of the tree or plant (the mature sporophyte). The diploid (2n) sporophyte gymnosperm plants produce cones in which the haploid (n) megaspores and microspores are produced. These have undergone a process of cell division called meiosis that results in four daughter cells each with half the number of chromosomes of the parent cell. The haploid gametophyte generation takes place within the female cones which will eventually release the diploid sporophyte seeds – and so the cycle continues.  

READ MORE

Articles in back issues of Veld & Flora include “The aerodynamics of wind pollination” by Hans Nieuwmeyer vol 88(2), p.73; “Saving the Clanwilliam Cedar“ by Penny Mustart, vol 99(4), 184-186; “The Cycad Amphitheatre” by Alice Notten, vol. 99(4), 178-179 and “The Winterberg Cycad” by John Donaldson vol 81(2), 36-39. Discover more about South African gymnosperms by visiting iSpot – and searching for ‘gymnosperms’, or PlantZafrica – and searching for Widdringtonia, Podocarpus, cycad and Welwitschia, or Wikipedia.

 LINKS TO THE CURRICULUM

GRADE 11 Life Sciences, Strand 1: Diversity, Change and Continuity. Topic: Biodiversity of Plants. Content: Grouping of bryophytes, pteridophytes, gymnosperms and angiosperms.

GRADE 12 Life Sciences Strand 1: Life at Molecular, cellular and tissue level. Topic: Meiosis: the process of reduction division purposes of reduction division (gametogenesis and exceptions: mosses, ferns), Importance of meiosis: diploid to haploid: production of gametes.

PHOTOGRAPHS for the gymnosperm life cycle diagram are reproduced with thanks to Alice Notten and PlantZafrica (SANBI) for the images of the male and female cones and seeds of the Clanwilliam Cedar (Widdringtonia cedarbergensis ) and to Anthony Hitchcock and iSpot for the use of the image of the Clanwilliam Cedar. 

TEXT adapted by Caroline Voget from the book The Story of Life and the Environment: An African perspective by Jo van As, Johann du Preez, Leslie Brown and Nico Smit, published by Struik Nature and also online sites including Iziko Museum’s website and Wikipedia.

Fern life cycle and diversity

Blechnum attenuatum and other ferns growing on Montagu Pass in the Western Cape. This is the sporophyte phase of the fern life cycle. Photo: Diane Turner, iSpot.

The factsheet in the June 2016 issue of Veld & Flora follows on from the factsheet in the March 2016 issue of Veld & Flora, vol. 102(1), and the Veld & Flora Factsheets on the Classification of Life in vol. 98(3) September 2012 and Plant Classification in vol. 101(4) December 2015.


Fern leaves with sori. Photo: Clare Archer, iSpot.

 A DIVERSITY OF FERNS

Ferns or Pteridophytes are a group of primitive plants. It includes the familiar fern with its graceful fronds, but also encompasses a great diversity of fern-like plants that grow in a variety of habitats from shady, damp forests to deserts. Like Mosses and Liverworts (Bryophytes) they have no seeds, flowers or fruit, and reproduce by means of spores. They mostly grow on dry land but they still need water in order to reproduce. Unlike Bryophytes they have a vascular system (specialized tissue for transporting water and nutrients – xylem and phloem).

Ferns are usually referred to as Pteridophytes, which includes all spore-bearing, vascular plants. In older classifications these plants were informally divided into ‘true ferns’and ‘fern allies’. The ‘fern allies’ comprised the Clubmosses, Spikemosses, Quillworts, Whisk Ferns and Horsetails. New research in molecular biology has necessitated a few changes in fern classification. To reflect evolutionary relationships more accurately, scientists now propose to divide spore-bearing, vascular plants into two formal groups: Lycophytes and Monilophytes.

Lycophytes include the Clubmosses, of which there are three genera in southern Africa: Huperzia, Lycopodiella and Lycopodium, Spikemosses which contain a single genus Selaginella with ten species in southern Africa and the Quillworts, which also contain a single genus, Isoetes, with at least 14 species in southern Africa. This ancient group of plants was once prolific with extinct giant species of Clubmosses growing up to 40 m high.

Monilophytes include the Snaketongue Ferns of which one genus, Ophioglossum, is indigenous to southern Africa, Whisk Ferns of which one species, Psilotum nudum, is indigenous to southern Africa, the Horsetails of which one species, the African Horsetail (Equisetum ramosissimum), is indigenous to southern Africa, the Potato Ferns of which only one species, Ptisana fraxinea, is indigenous to southern Africa, and the Leptosporangiate Ferns, which is the largest and most diverse group that contains around 270 species indigenous to southern Africa.

The African Horsetail (Equisetum ramosissimum subsp. ramosissimum var. altissimum) is the only species of horsetail in South Africa. The spores are borne under sporangiophores in the cone-like structures at the tips of some of the stems. Photo: Clare Archer, iSpot.

Almost all land plants reproduce by means of two distinct, alternating life forms: a sexual phase that produces and releases gametes or sex cells and allows fertilisation, and a dispersal phase. This is an adaption which frees plants from a life under water and allows them to live and reproduce on dry land. The sexual phase is known as the GAMETOPHYTE or haploid (n) generation and the dispersal phase is the SPOROPHYTE or diploid (2n) generation.

In ferns, the dispersal phase culminates in the production of haploid (n) spores (i.e. each spore has a single set of chromosomes) which are released from capsules (sporangia) borne on the underside of fern leaves. These spores germinate and grow into tiny, free-living plants –the prothallus. This is the sexual (or gametophyte) generation of the fern plant. Haploid (n) sexual cells or gametes are formed in the sex organs (the antheridia and archegonia) on the underside of the prothallus. The male gametes (sperm) then swim towards the female gametes (eggs) in the archegonia of the prothallus. Thus, even though ferns mostly grow on land, they still need the presence of water to facilitate fertilisation. Once fusion of the egg and sperm – or fertilisation – takes place, a diploid (2n) zygote forms with two sets of chromosomes. The zygote grows by cell division, in a process called mitosis, into the dispersal (or sporophyte) generation of the fern plant which is the familiar fern that we see growing all about.

The diploid(2n) sporophyte fern plants produce sporangia or capsules on the undersides of their leaves in which haploid (n) spores are produced. The black dots we normally see on the leaf are sori (singular sorus) which are clusters of sporangia. The spores inside the capsules have undergone a process of cell division called meiosis that results in four daughter cells each with half the number of chromosomes of the parent cell. The spores are dispersed and germinate and grow into haploid gametophyte plants – and so the cycle continues.

So although the gametophyte (n) and sporophyte (2n) are two different plants, in ferns the sporophyte is what we see and know as a fern. This is different to the life cycle of mosses, conifers and flowering plants, which are covered in previous and subsequent factsheets.

Some ferns dispense with the sexual phase of the cycle and reproduce vegetatively. See ‘The ferns of the Ntendeka Wilderness’ in Veld & Flora 67(4), 118-120, December 1981.

READ MORE
In the 1981 issue of Veld & Flora, read about ‘The ferns of the Ntendeka Wilderness’ Veld & Flora 67(4), 118–120. In the 1998 issue, read about ‘The ferns of Mariepskop’ Veld & Flora 84(4) 116-117. In the 1994 issue, read ‘The weedy ferns of Ferncliffe: Unusual invaders threaten Natal’s flora’ Veld & Flora 80(3), 88-90.
'To be or not to be a fern ally' by Ronell and Arrie Klopper, Pteridoforum 80: January 2007.

Discover more about South African ferns by visiting iSpot –  and searching for ‘ferns’ or Wikipedia.

LINKS TO THE CURRICULUM

GRADE 11 Life Sciences, Strand 1: Diversity, Change and Continuity. Topic: Biodiversity of Plants. Content: Grouping of bryophytes, pteridophytes, gymnosperms and angiosperms.

GRADE 12 Life Sciences Strand 1: Life at Molecular, cellular and tissue level. Topic: Meiosis: the process of reduction division purposes of reduction division (gametogenesis and exceptions: mosses, ferns), Importance of meiosis: diploid to haploid: production of gametes.

TEXT by Caroline Voget with assistance from Dr Ronell R Klopper, South African National Plant Checklist Co-ordinator, SANBI. The following books and websites were consulted: The Story of Life and the Environment: An African perspective by Jo van As, Johann du Preez, Leslie Brown and Nico Smit, published by Struik Nature and also from online sites including: Iziko Museum’s website, the Online Textbook, Prentice Hall, Wikipedia and The Encyclopaedia of Life.

Bryophyte life cycle

What we see and know as moss is the gametophyte form of the moss plant.

The factsheet in the March 2016 issue of Veld & Flora, vol. 102(1), follows on from the Veld & Flora Factsheets on the Classification of Life in vol. 98(3) September 2012 and Plant Classification in vol. 101(4) December 2015.

Understanding the alternation of generations

The way that almost all land plants reproduce is by means of two distinct, alternating life forms, a sexual phase that produces and releases gametes or sex cells and allows fertilisation, and a dispersal phase – both of which are adaptations to an essentially waterless environment. The sexual phase is known as the GAMETOPHYTE or haploid (n) generation and the dispersal phase is the SPOROPHYTE or diploid (2n) generation.

Mature gametophyte plants produce haploid sex cells (egg and sperm) in sex organs (the male antheridia and female archegonia). These sex cells (also called gametes) fuse during fertilisation to form a diploid (2n) zygote which grows, by means of mitosis (that results in two daughter cells each having the same number and kind of chromosomes as the parent cell), into a new sporophyte plant.

The diploid sporophyte produces haploid (n) spores (i.e. each spore has a single set of chromosomes) by means of the process of cell division called meiosis. Meiosis results in four daughter cells each with half the number of chromosomes of the parent cell. The spores are dispersed and eventually germinate and grow into haploid gametophyte plants – and so the cycle continues

 

Coping out of water

Bryophytes, which include moss, are primitive plants that give us some idea of how the first plants that ventured onto land coped with their new waterless environment. They share many features with other plants, but differ in some ways – such as the lack of an effective vascular system (specialised tissue for transporting water and nutrients – xylem and phloem) which distinguishes them from ferns, conifers and flowering plants. They usually form low-growing, dense cushions on rocks, the bark of trees, and other surfaces including buildings. The plants absorb and lose water depending on their surroundings and they need a film of water in which to reproduce as the sperm need to swim from the male reproductive organs to the eggs in the female organs.

 

Bryophyte life cycle

 

The diagram above shows the life cycle of a typical moss. What you see growing on rocks at Kirstenbosch, or on damp walls on your house, is the gametophyte form of the moss plant. Sexual cells or gametes are formed by male and female gametophyte plants – and the male gamete or sperm needs water in which to swim towards the female eggs in the archegonia of the female plants (see photo below).

In the tufted moss Polytrichum the male sex organs (antheridia) occur within a whorl of leaves and discharge sperm into the water-filled, reddish splash caps. From there raindrops splash the sperm cells onto nearby female plants. Photo: Chris Vynbos, iSpot

Once this occurs, and fusion takes place, a diploid zygote forms with two sets of chromosomes. The zygote develops into the sporophyte which is a stalk growing out from the gametophyte plant that supports a sporangium or capsule in which spores are produced (see photo below).

In bryophytes the sporophyte plant is a stalk growing from the gametophyte plant. The stalk supports a sporangium that produces spores which give rise to new gametophyte plants. Photo: Sally Adam.

The spores are dispersed and grow into the new gametophyte generation. So, although the gametophyte and sporophyte are two different plants, in mosses the sporophyte is always attached to the gametophyte. This is different to the life cycle of ferns, conifers and flowering plants which will be covered in subsequent factsheets.

Download the Factsheet on the Classification of Bryophytes (shown above) here.
Download the Factsheet on The Classification of Plants (above) here.
Download the Factsheet on The Classification of Life here.
Download these articles that are relevant to the study of the classification of moss:
Cocks, Martin 1996. Surviving at the edge of life: The tiny plants that eke out an existence on the frozen continent. Veld & Flora 82(2), p. 46–48.
Online textbook - Bryophytes (Prentice Hall).
Josh Hall Educreations UTube video on Moss and Liverwort lifecycles.