34.1: Introduction to Plant Diversity
From Water to Land
Kingdom Plantae first appeared about 410 million years ago as green algae transitioned from water to land. This land was a relatively uncolonized environment with ample resources. Terrestrial environments also offered more light and carbon dioxide, required by plants to grow and survive.
However, the stark differences between land and sea posed a formidable challenge to early colonizing species prompting many new adaptations that have resulted in the wide variety of plant forms observed today.
One early adaptation was the development of an outer waxy coating, called a cuticle. Cuticles serve to protect plants from desiccation, by trapping moisture inside. However, this adaptation prevented the direct exchange of gases across the surface of plants. As a result, pores developed on the outer surfaces of plants that allowed the absorption of carbon dioxide and release of oxygen.
Additional structures were necessary to facilitate the transport of water and nutrients from soil to the superior portions of the plant. As a result, vascular tissue developed that not only serves to transport water and nutrients to all areas of the plant but also provided structural support as stems grow taller and stronger.
To accommodate reproduction on land, terrestrial plants developed gametangia - reproductive structures that protect gametes and embryos from the harsh environment outside the plant. In males, this structure is called the antheridia, while in females, it is called the archegonia.
Different strategies evolved to facilitate the transport of sperm from the antheridia to the eggs within the archegonia. These include sperm swimming from one structure to the next, being carried by the wind, or being transported by pollinators like bees and birds. The specific mode used is unique to each classification of plants. Following fertilization, eggs are retained within the archegonia to protect and nourish the developing embryo, or sporophyte.
Another major reproductive adaptation was the generation of seeds. Though not all terrestrial plants are seeded, seeds are advantageous for many reasons. Without these structures, plants require moist environments to transport gametes from one place to another. Often in seedless plants, male and female spores are approximately the same size and both travel. However, seeded plants generally contain small male spores adapted to be highly mobile, called pollen grains, which travel to female gametophytes to deposit sperm directly to the egg. Once fertilization occurs, a seed forms that contains the plant embryo and a supply of nutrients.
These adaptations have created plant species well adapted to life in terrestrial environments.
Major Lineages of Plants
Though countless varieties of plants now exist, all can be divided into one of three groups: non-vascular, vascular seedless, and vascular seeded. Non-vascular plants are the most ancestral and least complex, including mosses, liverworts, and hornworts. Next, the vascular seedless plants include ferns and horsetails, and were the first group to evolve a vascular transport system. The last group, vascular seeded plants, includes all remaining species. This group is the most diverse and occupies the broadest range of habitats, and is split into two major sub-groups, angiosperms, and gymnosperms. Angiosperms include all flowering and fruiting plants, with pollen carried by the wind or transported by pollinators. Gymnosperms are non-flowering plants, including conifers, cycads, and ginkgo trees. These species produce bare seeds not protected by fruit and pollen carried by wind.