20.3: Selection Pressures and Drivers
- Page ID
- 29600
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An important aspect of understanding the life history traits of the bryophytes is understanding the challenges of living in a terrestrial environment.
- Sun exposure. Sunlight provides the power that drives our biosphere, but some wavelengths of sunlight can be damaging to cellular structure and even DNA. High frequency wavelengths, such as ultraviolet (UV), X-rays, and gamma rays can penetrate outer protective layers like skin, through cell membranes, and causing damage to DNA, proteins, and other biomolecules. Fortunately for organisms on Earth, almost all of these wavelengths are filtered by the atmosphere before they reach us, though some UV rays still make it through. These last UV rays are filtered out for aquatic organisms, but terrestrial organisms need adaptations to protect against UV radiation. Humans have skin with melanin pigments. Terrestrial plants have an epidermis and carotenoid pigments.
- Desiccation. Transitioning from a completely aquatic environment to a terrestrial one leads to challenges of drying out, also known as desiccation. Temperatures are more extreme outside of the water and evaporation from tissues into the relatively dry air is constant. Terrestrial plants quickly adapted a waxy covering on the epidermis, called a cuticle. This water-tight covering required the evolution of simple pores, and eventually stomata, to allow gas exchange with the outer environment. Because these plants lack vascular tissue, water can only be transported around the organism via osmosis. Thus, these plants must keep all tissues close to water access.
- Lack of a soil environment. The first organisms to move onto land would have found a relatively barren, rocky landscape. Soils did not yet exist. The rocky substrate experienced physical weathering from rain and wind that would help break it down. Chemical weathering through acidic rain or the interaction of water with compounds in the rock could also assist in breakdown. However, up to this point, contributions from organic matter would be minimal. Bryophytes lack true roots, instead producing structures called rhizoids whose function is anchorage. There are genes present in bryophytes, as well as some fossil evidence, that indicate bryophytes likely had mycorrhizal relationships with fungi that helped them acquire nutrients in this new landscape.
View the bryophytes on display. Compare and contrast the overall morphology of bryophytes to the plants you can see outside. Why do you think the bryophytes all share such a similar growth form?