Wednesday, March 17, 2010

From Green Algae to Land Plants

There is an ample amount of evidence that land plants evolved from green algae: one in particular being that both green algae and land plants have peroxisome enzymes to help minimize losses from photorespiration. What more evidence is there that land plants evolved from green algae? What challenges did land plants face as they moved away from the water? What adaptations were necessary for them to survive?


  1. Some common characteristics between plants and green algae that show plants evolved from green algae is that both have starch as their reserve of energy. Both organisms have a cell wall made of cellulose microfibrils as well as photosynthetic pigments (chlorophyll a &b) which allow them to use photorespiration to obtain the energy they need. When land plants moved out of the water, they faced the challenges of drying out, receiving water and nutrients, receiving sunlight, gas exchange, support for the plant, and protection. Plants evolved roots in order to get water and nutrients from the ground and use their xylem to transport water. The waxy cuticle on leaves prevents plants from drying out. Plants used stomata for gas exchange (in order to get carbon dioxide from their environment in order to use it for photosynthesis). Around the stomata, there are guard cells which help to control gas exchange and prevent water loss. CAM plants have mesophyll cells that store organic acids made from carbon dioxide taken in during the night until the daytime, which allows the plants to take in CO2 during the night when it is the coolest out which prevents excess water loss. Plants have xylem and roots as well as cellulose in their cells walls in order to support them, which allow them to receive sunlight because the plant can overcome gravity and maintain an upward position. Plants have apical meristems as well as phytochromes which both play a role in plants growing toward sources of light in order to have maximum exposure to sunlight. Plants have also evolved toxins and thorns to protect themselves from herbivores that might eat them. Plants have evolved ovaries, fruits, and seeds to eliminate the need to have water to reproduce as well as to help protect the reproductive parts of the plant.
    Campbell 8th Edition

  2. When we talk about the evolution of land plants in class, we list them in the following order: Bryophytes, Liverworts and Hornworts, Pterophyta, Gymnosperms, and Angiosperms. The reason for this order deals mainly with physiological complexity and fossil record. Mud cores in river beds show pollen and organic fragments of past flora (C. Whitloc. "Doing Field Work in the Mud" JSTOR). If we trace physiological similarities and differences from green algae through angiosperms, the evolutionary relationship becomes apparent.

    Algae lives in an aqueous environment. In order to reproduce, algae utilizes flagellated sperm. The earlier land plants would have lived near water. Bryophytes, worts, and pterophyta all live in moist areas and have flagellated sperm. Pollen that moves through the air only appears in Gymnosperms.

    Another radical change for terrestrial plants was the need for an anchor. While algae may float and receive nutrients through the water, land plants face the challenges of wind and having their nutrients come from the ground. The anchoring problem seems to have been solved first. Bryophytes, liverworts, and hornworts have rhizoids to grip surfaces such as rocks. True roots are seen starting with pterophyta. As plants moved farther away from water and into more arid environments, it became a selective advantage to be able to absorb nitrates, phosphates, etc through the ground rather than through water.

    Being able to stand up straight is a competitive advantage that aids in photosynthesis. This is why plants gradually became taller and have branching leaves (trees). The challenge of standing up has met by lignin.

    There are many other differences between aquatic and terrestrial environments for plants. The challenge of intense UV is met by flavenoids. Annie covered the threat of drying out and herbivores very well. Every example of a terrestrial challenge is explained by the evolution of algae through the land plant progression. Since each type of plant is a successful species, there is living record of this transition alive today.