Plants Before Flowers: Focus on Cycads No
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Tropical Topics An interpretive newsletter for the tourism industry Plants before flowers: focus on cycads No. 59 December 1999 All thanks to the bacteria Notes from the When the Earth came into existence, about 4600 million years ago, there was little or no oxygen in the atmosphere of our planet. Indeed, life owes its origins Editor to an apparently lethal combination — a thin atmosphere composed largely of hydrogen, ammonia, methane and carbon monoxide, hot oceans, ultraviolet rays As we get caught up in Millennium which bombarded the earth’s surface in the absence of an oxygen (ozone) shield Fever it's interesting to look back at and electricity in the form of abundant lightning. Laboratory experiments have the past. However, 2000 years is shown that this combination eventually produces simple sugars, amino acids and just a blink of an eye compared with nucleic acids — the basic requirements for protein synthesis. the 4600,000,000 years for which planet Earth has existed and the Eventually, after further interactions, feed themselves. We have a lot to 3500,000,000 years life has been DNA (deoxyribonucleic acid) thank them for. Not only did they slowly evolving. appeared. Able to replicate itself and produce oxygen, but this also led to to cause formation of amino acids, this the creation of an ozone (O3) shield The recent TV series ‘Walking with was the essence of life. Simple bacteria against lethal amounts of ultraviolet Dinosaurs’ has also encouraged us were the first proper life forms to rays. However, by excluding the very to look back. Settings for the emerge from this process, about 1000 rays which kick started the whole million years after the formation of the process, these bacteria also ruined the computer animations had to be planet. They started off feeding on chances of life ever beginning again in chosen carefully. There were to be simple carbon compounds but when the same way. no grasses — they weren’t around these stores began to run low, a light- in those days. Instead tree ferns, harvesting compound — chlorophyll At some stage, many many millions of cycads and conifers — the plants — was developed which enabled some years later, these primitive life forms on which dinosaurs fed and which of these bacteria to create food from became more complex and more formed a backdrop to their dramas sunlight. This process required organised. It is likely that some simple — were needed. hydrogen, a gas available in abundant organisms engulfed others and, quantities from volcanic activity. instead of digesting them, formed a These plants have since been However, this restricted the bacteria to partnership with them — perhaps largely overwhelmed by the hugely volcanic zones, until one enterprising rather like corals and zooxanthellae. successful flowering plants which branch of the family developed a Eventually the partners merged, evolved after them. Many have method for extracting hydrogen from forming one corporate body. become extinct but, nonetheless, an even more abundantly available source — water. They also discovered sex. Instead of many have persisted as a reproducing simply by dividing into fascinating link with the distant Waste products from this process identical copies, as life forms became past. This Tropical Topics looks at were to have far-reaching more sophisticated those links and focuses on perhaps consequences for the further they found a way of the most prominent ‘dinosaur development of life on Earth. When H mixing and plants’, the cycads. In future issues is removed from H2O, oxygen — the O matching their genes we will look at other primitive plants — remains. Casually discarded by the through the which still share our world today — bacteria as a waste product, this production of simple the ferns, conifers, mosses, lichens essential element of our modern air eggs and sperm — and liverworts. began to build up in the atmosphere. a step which allowed variation The fashion for photosynthesising and, thereby I would like to thank Gary Wilson caught on in a big way and, as accelerated of the Queensland Herbarium for millennia came and went, huge evolution. his valuable assistance with this amounts of oxygen were produced by issue. vast numbers of microscopic bacteria all busily splitting water molecules to Plants step on to land Algae were the first proper plants and it was from green algae that all the land plants eventually developed. However, leaving the water was no easy step. The body structures and the reproductive systems of early life forms, both plant and animal, depended on this watery environment. Sexual changes Structural changes Algae have complicated sex lives involving alternate As plants left the water they had to generations, one produced sexually and the other adapt physically to their new asexually. The mature plant divides asexually, by surroundings. The first problem producing spores. These grow into different plants which, in was finding enough water. turn, produce male and female cells. If, by swimming through water, these meet and Although they live on land, mosses fuse sexually they can then produce a mature plant and the cycle starts are totally dependent on moist again. surroundings. They have no proper Spore capsule roots so the first adaptation to the This pattern of alternate generations was retained in early environment came from those land plants such as mosses. Mosses produce female plants, club mosses, ferns and cells (eggs) attached to stems and sperm which are free horsetails, which evolved true to swim through the moisture in the plant towards the eggs. roots. These could probe the When fertilised, the egg germinates while still attached to the ground below the plants and tap parent plant and produces the asexual generation — a capsule full into the water film surrounding soil of spores on a long thin stem. The capsule splits and the spores are particles, thus allowing the plants distributed by the wind to grow into other moss plants. to move further from the coast. This pattern appears also in ferns. Spores are produced, often in clusters on This water also had to be the backs of the leaves, and are blown away to develop initially as tiny plants retained in the plant so those called thalluses. The underside of the thallus produces male and female cells, which ventured on to land the sperm again swimming through ground moisture to fertilise the eggs, needed a watertight skin. Ferns resulting in a mature fern plant. Spore were the first plants to develop clusters true leaves. Large spreading green All these methods of sexual reproduction required water, limiting the sexual stages structures, intended to gather as to moist habitats, close to the ground. However, as plants moved on to land and as much light as possible, they had a they grew taller, they needed to liberate themselves from this dependence. waxy cuticle ‘skin’. Pores in the Thalluses were being shaded out and, on the ground, were being cropped by cuticle allowed the exchange of animals. gases with the surrounding atmosphere and stomata Cycads represent an important step in the sexual liberation controlled the size of the pores, of plants. By producing fertilised seeds on one of the according to conditions. parents they eliminated the necessity for alternating generations. They were among the first plants to Water, and nutrients, had to be reproduce above ground level. Male cells, in the form of circulated internally through the pollen, were either blown or carried by insects from the plant now that it was no longer cone on the male plant to the cone on the female plant. possible to absorb them, as algae There sperm and egg united through a drop of water inside do, from their surroundings. the ovary in the female cone — a last link to the watery Without the physical support of ancestors — and a seed was formed. water, they also had to hold themselves up. Both problems were Conifers perfected waterless fertilisation, the male spore uniting overcome by the development of directly with the egg at the end of its journey without first going for a the vascular system — a system of swim. With both male and female cones produced on the same plant, woody vessels which provided chances of fertilisation were increased, wind blowing pollen from the rigidity as well as a circulation male cone to the female one. system. Only mosses and Bunya cone liverworts, among today’s land The first flowers were produced in west Gondwana, now west Africa, plants, are non-vascular. a trend which was to catch on in a big way. Although cycads had put to good use the insects’ appetite for their pollen, flowering plants With newfound structural strength, specialised in using animals as couriers for their sexual needs by the plants started gaining height, providing attractive flowers and excess pollen and nectar in payment. those which grew tallest, like the tree ferns, being Of even more significance was the development in these plants of rewarded with the ‘double-fertilisation’, a process by which not only the ovary but also the lion’s share of ovary wall was fertilised so the seed was encased in an endosperm — a fruit — the sunlight for which provided food and protection for the seed. Magnolias and waterlilies are photosynthesis. among the earliest flowering plants. Ancient swamp forests were Flowering plants competed very efficiently with their predecessors, not least because dominated by now they were able to grow faster. Nonetheless, conifers remain the dominant species in extinct giant club many temperate areas and cycads hang on in sometimes substantial numbers. mosses and giant horsetails growing on *Different sexual strategies did not evolve directly from one plant type to another. vast trunks to 45m in Evolution took different paths, with changes taking place on different branches of height.