During the course of their evolution, plants have adapted to a land-based existence. Because modern plants occupy numerous, often specialized, ecological niches (e.g., deserts, rainforests, and even aquatic environments), there are many more specific adaptations than those that will be covered in this tutorial.
We will focus on those adaptations that have allowed plants to overcome the challenges of a dry environment and the lack of support for upright growth. These adaptive features include: cuticles, stomata, vascular tissue, gametangia, and seeds. As each of these adaptive features is discussed, keep in mind the transition of early plants from an aquatic to a terrestrial environment and how each feature could enhance the success of plants on land.
A major adaptation to the dry terrestrial environment is the waxy cuticle . Cuticles, composed of wax , are found on the surface of all above-ground parts of the plant. Waxes are a class of lipids that, due to their chemical properties, are maintained as a solid, even at the highest temperatures found in extreme conditions (e.g., deserts). Like all lipids, waxes are hydrophobic and impermeable to water. Of course, plant roots are not covered by cuticle because they are the structures responsible for water uptake and have less exposure to the air than parts of the plant that are above-ground..
The waxy cuticle covering the surface of the plant shoot is an effective barrier to desiccation because it prevents loss of water to the air. Not surprisingly, desert plants have a much thicker cuticle layer than plants growing in wet environments.
Stomata are also an important adaptive feature to the terrestrial environment. Because the cuticle is impermeable, it is necessary for plants to have pores through which gasses can be exchanged with the environment. Carbon dioxide is required for photosynthesis and oxygen is produced during this process. These gasses enter and exit the plant through the stomata. Water can also be lost through stomata, so their opening is regulated by the plant.
Vasculature describes a system of specialized cells found throughout the body of the plant. Vasculature has two functions. First, the specialized cells of vascular tissue allow transport of water and nutrients throughout the plant. This adaptation enables water, absorbed by the roots of the plant, to reach the stem and leaves, and the sugars from photosynthesis, produced in the shoots, to be transported to the roots. Plants with vasculature do not have to depend on living in a moist environment to maintain adequate water throughout the plant.
The second function of vasculature is structural support. Cells of the vascular tissue have secondarily reinforced cell walls that make the tissue rigid. The vascular tissue that runs throughout the plant body, circulating water and nutrients, also forms a "skeleton" that strengthens the roots, shoots, and leaves. Vascular tissue enables plants to grow upright (some to very great heights), while maintaining moisture levels in all parts of the plant.
The evolution of vasculature was a major event in plant history. Plants with vascular tissue do not appear in the fossil record until approximately 425 million years ago, well after the origin of land plants. After this date there was an explosion of plant life, indicating that vascular tissue is a highly successful adaptation to life on land.
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. 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.
Protection of the developing multicellular embryo varies among the different plant lineages. The most primitive group of plants retains the developing embryo through sexual maturity. The diploid embryo is completely dependent on the haploid gametophyte generation. This life cycle is typical of the nonvascular plants. The more-derived plant lineages have further adapted to the terrestrial environment by producing specialized structures for protection and nutrition of the developing embryo. The embryo is enclosed in a seed, which is dispersed from the parent plant long before the embryo reaches maturity. In the derived plant lineages, the haploid gametophyte is greatly reduced because it no longer plays a dominant role in protecting the embryo; in these groups of plants, the haploid gametophyte has become completely dependent on the diploid generation. As you will learn in tutorials discussing seed plants, seeds are a highly successful adaptation to the variable environmental conditions on land. Independent of the parent plant, the seed-enclosed embryo can withstand drying and temperature fluctuations, even the digestive tract of some animals, until conditions are suitable for germination and growth of the embryo to maturity.
The most recent adaptations to the terrestrial environment were the evolution of flowering plants and the production of fruit as a means for seed dispersal. Flowering plants produce their seeds within a fruit that provides a functional "packaging" around the seed(s). The fruit can be edible, such that the digested seeds are then deposited with the feces of the animal that consumed the fruit. Other fruits are suitable for transport on air currents, water currents, or on the fur of different animals.
The first evidence of seed plants in the fossil record occurs approximately 305 million years ago. Seed production enabled plants to reproduce more successfully because the embryos had a much better chance of surviving the dry terrestrial environment than did the embryos of more primitive plants that were still dependent on the parent plant body. Just think about the advantage to plants whose offspring could be widely dispersed and were protected (within the seed) until conditions were suitable for growth. Seed plants are so dominant in the world today that we have to remind ourselves that there are numerous plants in existence that do not produce seeds.
Fruit production by flowering plants is a more specialized adaptation to life on land because it reflects not only the environment, but also the other life forms that exist there. Although the first flowering plants occur in the fossil record only 175 million years ago, the success of fruit production is marked by the huge radiation of flowering plants.