The transition from an aquatic to a terrestrial environment was also marked by adaptations in plant reproduction. In the charophyte ancestor of modern plants, gamete production, fertilization, and development of the embryo were highly dependent on the aquatic environment. Gametes were dispersed by water currents and were maintained in a hydrated state until fertilization occurred. The zygote and growing embryo developed free from the parent organism because there was no threat of drying out. The move to land required protection from desiccation of gametes and embryos, as well as a new means of gamete and embryo dispersal.The major adaptation of plants to the terrestrial environment (with respect to reproduction) was the production of gametes and the development of embryos within gametangia. The gametangium (-ium, singular; -ia, plural, from Latin) can be male or female, and is the site of gamete production. The female gametangium produces egg cells and the male gametangium produces sperm. A protective chamber, formed by a single layer of sterile cells, prevents the gametes from drying out by reducing or eliminating their exposure to air. Egg cells are maintained in the female gametangium, but the sperm must leave the male gametangium and travel to the egg for fertilization to occur (Fig. 10). Some groups of modern plants have retained the primitive characteristic of flagellated sperm and still are dependent on water for dispersal of male gametes; however, the majority of modern plants do not have motile sperm and have developed nonwater-based methods of dispersal (e.g., wind and insect pollination).In all plants, fertilization occurs within the female gametangium, where the zygote begins to develop into the embryo. Because all plants retain the developing embryo within the gametangium, they are referred to as embryophytes. Protection of the growing embryo is especially important in the terrestrial environment because the waxy cuticles, stomata, and vascular tissue present in mature plants are not well developed in the embryonic plant.
Plant Evolution and Diversity Questions
Bring on the tough stuff
1. What structures did early land plants evolve? How did these help them live on land instead of water?
2. Knowing that convergent evolution can cause plants to be similar, what do you predict would have a better chance of becoming invasive in the desert: a plant from Arizona, or a plant from Maine?
3. Why can’t we just split angiosperms into monocots and dicots like people used to do?
4. Lycophytes were tall trees during the Carboniferous period. Today, the only lycophytes that exist are small, herbaceous plants. What happened?
5. What is the advantage of having flowers?
Plant Evolution and Diversity Answers
1. Answer: Early land plants evolved cuticles, which are waxy layers that keep water from evaporating. Since air is not as wet as water, plants that had a mechanism for keeping water in were well-situated to live on land. The earliest land plants were still limited in their growth because they did not have well-developed roots or leaves. Later, seedless vascular plants evolved roots that anchored them and helped them draw nutrients from the soil. Big leaves offered the plants more area to intercept the Sun’s rays and make sugars from photosynthesis.
2. Answer: Although we can never predict what a plant will do in a new environment or predict invasions, the plant from Arizona has a better chance of becoming invasive. In fact, it has a better chance of surviving period, since it is probably already adapted to desert conditions.
3. Answer: The past division of the angiosperms into monocots and dicots was based on the number of cotyledons plants have, but it was also believed to be an evolutionary split of those two types of plants. We know now that some of the plants with two cotyledons, the Magnoliids, are actually more primitive than the monocots and belong in a different group.
4. Answer: The Carboniferous period was characterized by a warm and wet climate. Toward the end of the period, the Earth’s climate cooled. The tall lycophytes died out. We can only guess why, but for some reason, they couldn’t survive being tall in a dry climate. The smaller lycophytes were more resilient to the climate change, and their descendants are today’s lycophytes.
5. Answer: Flowers attract animals with their colors, scents, and nectar. These animals inadvertently take some of the flower’s pollen as they drink nectar and transfer pollen to other flowers in the same species when they travel around looking for more nectar. Animal pollinators can efficiently take pollen from one plant to another and improve the odds of successful fertilization. Successful fertilization leads to seeds, and seeds lead to plants, so flowering plants were able to spread all over the world.