21.5: Angiosperms
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Learning Objectives
- Describe the shared derived characteristics of angiosperms.
- Connect these characteristics to the stressors these plants would have faced.
At the end of the Permian period, there was the largest mass extinction this planet has ever experienced. It is estimated that 96% of species that lived at that time went extinct. This event signaled the downturn for some groups and opened up space for others to emerge. The exact timing of the emergence of angiosperms is unknown, so it is difficult to relate their evolution to specific climatic conditions. However, there is relatively new fossil evidence that may place flowering plants as early as the Jurassic period, 174 mya. This was the age of the dinosaurs and coincides with the emergence of the first feathered dinosaurs -- birds! Much like the insects, birds would present interesting opportunities for this new group of plants, working as both pollinators and seed dispersers.
Angiosperms can be distinguished from other plants by a set of specialized characteristics that allowed them to compete in an already full world. The (usually) easiest thing to identify about an angiosperm are its flowers . These collections of modified leaves allowed this group of plants to attract pollinators and increase the chances of successful fertilization. Once pollinated, the fertilized seeds are encased in a protective ovary whose structure can be specialized for different methods of dispersal, such as animal ingestion, animal attachment, flotation, or wind dispersal. This protective ovary and the encased seed(s) are more commonly called a fruit . Inside the developing seeds, angiosperms provide an additional food source to the developing zygote, the endosperm .
Competing with the gymnosperms for access to sunlight was perhaps hopeless, so the angiosperms adapted ways to work smarter, not harder. In the xylem, they evolved large diameter conducting cells for rapid water uptake called vessel elements , though this made them vulnerable to freezing conditions. In the phloem, sieve cells evolved into sieve tube elements , increasingly specialized for transportation of photosynthates.
As you might have guessed from the vast number of species, angiosperms occupy incredibly diverse habitats and span a range of morphologies, from tiny plants floating as a film on the surface of a pond (Figure \(\PageIndex{1}\)) to towering Eucalyptus trees dominating the forests of Tasmania, rivaling redwoods in height.
Selection Pressures and Drivers
- Competition for space . Present day gymnosperms include the tallest, most massive, and some of the oldest organisms on the planet. With this in mind, you can imagine that they would be difficult to compete with. Angiosperms needed to evolve more efficient methods of transporting water and photosynthates, fertilization, and survival of offspring.
- Animals . The primary response to animals that we see in gymnosperms is prevention of herbivory, though there is some insect pollination and animal seed dispersal. While herbivory is still a driver of selection for angiosperms, animals also served as a more efficient method of pollen delivery. Insects and birds could be lured in with sugary nectar or scents and colors that mimicked other resources, then dusted with pollen as they investigated. If the lures were specialized enough, they would continue seeking the same resource, leading them to another plant of the same species. These scents, colors, and nectar resources were produced by structures that also produced pollen and ovules -- the flower. Similarly, fruits allowed for the dual purpose of protecting seeds and co-opting animals as dispersal agents, whether by ingestion or attachment. Some fruits evolved production of sugary tissues and bright colors to attract animals, while others evolved hairs or spines to latch onto their bodies.
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- 2.6.3.1: Flowers
- Flowers are specialized reproductive structures produced by angiosperms. These structures are composed of highly modified leaves in distinct whorls. The sterile whorls, the calyx and corolla, comprise the perianth. Pollen is produced by the androecium and ovules are produced in the gynoecium. Floral formulas are used to describe the composition and morphology of these whorls. An inflorescence involves the production of multiple florets in place of a flower.
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- 2.6.3.2: Angiosperm Life Cycle
- Angiosperms have a complex life cycle. The microgametophyte is reduced to 2 cells, while the megagametophyte is now 7 cells and 8 nuclei. Ovules develop within the ovary or ovaries of the gynoecium. Production of gametophytes and fertilization happens within the flower. The spermatia each fertilize a cell within the ovule (double fertilization), one of which will grow into the embryo. Seeds are protected by the ovary wall, which becomes the fruit, a structure specialized for seed dispersal.