Gnetophyta is a plant division containing only 3 genera and approximately 80 species. It isn’t known when plants in this division first evolved, but somewhere between 140 and 250mya. Although gnetophytes are gymnosperms, with no true flowers or fruits, they have some features in common with flowering plants:
- Vessel elements in the vascular system not seen in other gymnosperms
- Both Welwitschia and some Gnetum species are pollinated by insects
- Flower like structures on male cones of Welwitschia
- Nectar – produced on the tip of the cones rather than in a flower
All gnetophytes are evergreen and woody, and may be trees, vines or in the case of Welwitschia, difficult to classify. These plants have not been studied much and it is tricky trying to find out information about them. For example, although they are mostly considered dioecius (male and female cones on separate plants) all three genera sometimes produce bisexual cones, containing both stamen and ovules, but it isn’t really understood why, or if these cones can then reproduce.
Gnetophyta Family Tree
There are 30-35 species of Gnetum, including two trees, many vines, and shrubs.
One tree, Gnetum gnemon, reaches 15-20m tall, and does not have fruits, but a fruit-like juicy covering for the seeds, which, like fruit, are edible to birds and aid in the spreading of seed.
Many Gnetum have seeds and leaves that are also edible to humans. Leaves of Gnetum have network of veins, something seen in dicotyledonous flowering plants, but no earlier evolved plants. All are dioecious. Gnetum are thought to be the first plants to be insect pollinated, by now extinct scorpion flies.
There is only one species of Welwitschia and it only grows in the deserts of Namibia and Angola. Despite sometimes growing 10m wide (although more commonly 4m wide), Welwitschia has just two strap like leaves that grow continuously. The longest recorded leaves were 37m long, but most leaves break up in the harsh desert environment and become tatty and brown at the ends. Unlike Gnetum, the veins are parallel, as seen in monocotyledonous flowering plants as well as some ferns and cycads. Welwitschia probably live 1000-2000 years, although this is difficult to know for sure. The female cones produce drops of nectar to entice insects to pollinate them. They have a single tap root grows deep into the sandy desert soil in search of water.
* Photo from The Encyclopedia of Earth with some more technical details about Welwitschia
I recently bought some Welwitschia seeds to see how they would grow. I planted them in a pipe to give space for the deep tap roots, 2:1 sand to compost. Within a week, three had germinated. Two died a few weeks later, I believe because I didn’t take into account that the single root only takes water from deep in the soil, so watering from above was pointless. I spray with fungicide every week or so. As can be seen from the pictures, Welwitschia has two cotyledons that start out orange and turn green.
There are about 50 species of Ephedra. They have slender stems with needle like leaves and small, sometimes brightly coloured, cones. They grow in dry areas in the Northern hemisphere, such as North Africa, Europe and North America. Ephedra looks very much like a gigantic version of psilotum (see previous blog about ferns) and can grow up to 3m. Some are monoecious.
The Evolution of Attracting Insects
While researching the previous blog about Ginkgophyta I learnt about terpenoids. Terpenoids are chemicals produced by both primitive plants (eg mosses and ferns) and flowering plants, the last group of plants to evolve. However, the function of terpenoids has altered as the plants have evolved. Terpenoids attract certain beneficial insects that feed on other insects that are harmful to the plant and this is an advantage to all plants, however, in later plants, Cycadophyta, Gnetophyta and Magnoliophyta, the insects attracted are also used to pollinate the plants and it was presumably because of the existence of terpenoids that such a partnership of plants and insects was able to form. Insect pollination is a far more efficient means of transporting pollen than wind, because an insect seeks out another plant, often a specific insect becomes an exclusive visitor to a specific plant. In the case of Welwitschia, growing in the desert, there may be many kilometres between plants, an awful lot of pollen would need to be produced in the hope of it being carried on the wind. Using insects to transport the pollen is akin to getting the postman to post a letter through the letterbox of the person you want to reach, instead of throwing a thousand leaflets down the road they live in, in the hope they pick one up.