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| 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.
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| 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.
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| 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.
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| 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.
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| 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.
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| 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.