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What you’ll learn to do: Classify seedless plants
An incredible variety of seedless plants populates the terrestrial landscape. Their decomposition created large deposits of coal that we mine today.
Current evolutionary thought holds that all plants—green algae as well as land dwellers—are monophyletic; that is, they are descendants of a single common ancestor. The evolutionary transition from water to land imposed severe constraints on plants. They had to develop strategies to avoid drying out, to disperse reproductive cells in air, for structural support, and for capturing and filtering sunlight. While seed plants developed adaptations that allowed them to populate even the most arid habitats on Earth, full independence from water did not happen in all plants. Most seedless plants still require a moist environment.
9.2 Soil-Plant Interactions
Soil plays a key role in plant growth. Beneficial aspects to plants include providing physical support, water, heat, nutrients, and oxygen (Figure 1). Mineral nutrients from the soil can dissolve in water and then become available to plants. Although many aspects of soil are beneficial to plants, excessively high levels of trace metals (either naturally occurring or anthropogenically added) or applied herbicides can be toxic to some plants.
Figure 1. Soil-Plant Nutrient Cycle. This figure illustrates the uptake of nutrients by plants in the forest-soil ecosystem. Source: U.S. Geological Survey.
The ratio of solids/water/air in soil is also critically important to plants for proper oxygenation levels and water availability. Too much porosity with air space, such as in sandy or gravelly soils, can lead to less available water to plants, especially during dry seasons when the water table is low. Too much water, in poorly drained regions, can lead to anoxic conditions in the soil, which may be toxic to some plants.
Seedless plants, like these horsetails (Equisetum sp.), thrive in damp, shaded environments under a tree canopy where dryness is rare. (credit: modification of work by Jerry Kirkhart)
An incredible variety of seedless plants populates the terrestrial landscape. Mosses may grow on a tree trunk, and horsetails may display their jointed stems and spindly leaves across the forest floor. Today, seedless plants represent only a small fraction of the plants in our environment yet, three hundred million years ago, seedless plants dominated the landscape and grew in the enormous swampy forests of the Carboniferous period. Their decomposition created large deposits of coal that we mine today.
Current evolutionary thought holds that all plants—green algae as well as land dwellers—are monophyletic that is, they are descendants of a single common ancestor. The evolutionary transition from water to land imposed severe constraints on plants. They had to develop strategies to avoid drying out, to disperse reproductive cells in air, for structural support, and for capturing and filtering sunlight. While seed plants developed adaptations that allowed them to populate even the most arid habitats on Earth, full independence from water did not happen in all plants. Most seedless plants still require a moist environment.
Chapter 19 Introduction – Seed Plants
The lush palms on tropical shorelines do not depend on water for the dispersal of their pollen, fertilization, or the survival of the zygote—unlike mosses, liverworts, and ferns living within the same terrain. These palms are seed plants, which have broken free from the need to rely on water for their reproductive needs. The seed plants play an integral role in all aspects of life on the planet, shaping the physical terrain, influencing the climate, and maintaining life as we know it. For millennia, human societies have depended on seed plants for nutrition and medicinal compounds. Somewhat more recently, seed plants have served as a source of manufactured products such as timber and paper, dyes, and textiles. As an example, multiple uses have been found for each of the plants shown above. Palms provide materials including rattans, oils, and dates. Grains like wheat are grown to feed both human and animal populations or fermented to produce alcoholic beverages. The fruit of the cotton flower is harvested as a boll, with its fibers transformed into clothing or pulp for paper. The showy opium poppy is valued both as an ornamental flower and as a source of potent opiate compounds.
Alternation of Generations: The Moss Life Cycle
To better understand alternation of generations in nonvascular plants, study the moss life cycle in Figure 3. It depicts the life cycle of a moss and helps to distinguish the haploid and diploid stages. Note the prominent form of the moss, the gametophyte, which is haploid (1n). This is the plant body that is most often observed. In the figure there are separate male and female gametophytes however, the gametophyte can also be bisexual (male and female gametangia are located on the same plant body). In step 1, the gametophyte is the generation that produces gametes sperm are produced in the male gametangium, the antheridium (plural, antheridia), and eggs are produced in the female gametangium, the archegonium (plural, archegonia). In step 2, the motile sperm has reached the egg, which is retained in the archegonium and fertilization takes place. (Remember, water is required for fertilization because the flagellated sperm must swim to the egg.) In step 3, the diploid (2n) zygote undergoes mitosis and begins to develop into the embryo (also 2n). In step 4, the embryo matures into the sporophyte, the diploid (2n) plant body. The sporophyte is the small, brown, stalked structure that one sometimes sees held above the main body of the moss. In step 5, meiosis takes place in the sporangium of the mature sporophyte and haploid spores are produced. In steps 6 and 7, the haploid spores are dispersed and each spore undergoes mitotic cell division to create a haploid multicellular gametophyte. The prominent haploid gametophyte is then ready to produce gametes (back to step 1).
An important feature of the moss life cycle is that the developing embryo is retained on the gametophyte plant body. This is an adaptation to the terrestrial environment because the embryo is protected from desiccation throughout its development into the sporophyte. (Remember the description of plants as embryophytes?)If you think about haploid and diploid stages in terms of the animal life cycle, it will be difficult to make sense of the plant life cycle. As noted above, the plant life cycle includes alternation of generations, with a multicellular haploid stage. Review again, step 5 of the life cycle (where meiosis takes place). Note that meiosis does not occur again when gametes are produced. Once you recognize these differences, you should begin to feel more comfortable thinking about plant life cycles.
Figure 3. Bryophyte Life Cycle. (Click image to enlarge)
Using the energy carriers formed in the first steps of photosynthesis, the light-independent reactions, or the Calvin cycle, take in CO2 from the atmosphere. An enzyme, RuBisCO, catalyzes a reaction with CO2 and another organic compound, RuBP. After three cycles, a three-carbon molecule of G3P leaves the cycle to become part of a carbohydrate molecule. The remaining G3P molecules stay in the cycle to be regenerated into RuBP, which is then ready to react with more CO2. Photosynthesis forms an energy cycle with the process of cellular respiration. Because plants contain both chloroplasts and mitochondria, they rely upon both photosynthesis and respiration for their ability to function in both the light and dark, and to be able to interconvert essential metabolites.