 |
| 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. |
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.
