34.1:
Introduction to Plant Diversity
Before plants colonized land from the sea, the continents were barren. Now millions of species of microbes, plants, and animals live on land, and it was the evolution of land plants that made all of this possible.
To understand this staggering transformation, it is first necessary to understand what exactly plants are, and how they evolved to meet the challenges of life on land.
There are certain characteristics that all plants share, from the tallest trees to the smallest moss. First, all plants are multicellular eukaryotes.
Second, plants produce the photosynthetic pigment chlorophyll in organelles called chloroplasts, enabling them to produce their own food using energy from the sun. Third, all plants have cells that are surrounded by walls made of cellulose.
Lastly, all plants have a life cycle characterized by the alternation of generations, defined as the transition between haploid and diploid multicellular stages over the life cycle. Here, generations refers to two different multicellular phases in the life cycle.
One phase is the haploid gametophyte. The gametophyte produces gametes by mitosis, which fuse during fertilization to form a diploid cell, which then undergoes mitosis to develop into a sporophyte. The sporophyte, in turn, produces haploid spores by meiosis, completing the cycle by giving rise to new gametophytes.
Although all plants share these characteristics, there are stark differences among the diverse plant lineages in how some of these characteristics are expressed.
Land plants fall into three major groups, the nonvascular plants, the seedless vascular plants, and the seed plants. Each of these groups contains many thousands of species.
As evolutionary time progressed, the three groups evolved different traits which characterize them – from vascular tissue, to the evolution of fruits. Together, these adaptations enabled plants to dominate most terrestrial biomes.
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.