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Origin of Green Plants
The prevailing theory of plant evolution concerns the adaptation of green algae from a water-dependent to a land lifestyle. Long chains of sugar called cellulose make up the cell walls of both the Chlorophyta and plants, leading biologists to examine the theory that green algae and plants shared a common ancestor. The first true plants may have evolved from a certain type of green algae, probably the charophytes.
Early plants lived almost completely out of water, yet they did not wander far from water sources. The plants eventually grew to higher heights, forming stems and leaves in the process. They developed mutual associations, or symbioses, with certain fungi. The fungi inhabited the roots of plants, providing early plants with essential soil nutrients to carry out photosynthesis, the process by which organisms combine carbon dioxide and water with the help of light energy to produce glucose sugar, their food. Plants then provided the fungi with food.
Independence from Water
Green algae lived mainly in the water and did not have to cope with a lack of water. About 450 million years ago, the first green algae began their transition to a terrestrial habitat, leading to a set of adaptations that made land a habitable environment. First and foremost, these early plants devised methods of preventing their tissues from drying out, or desiccating. The waxy cuticle layer kept water in plant tissues from escaping into the environment. Because major portions of the organism no longer directly contacted water, plants developed vascular tissues that transported water from root systems to the stems and leaves where photosynthesis occurred. Land plants evolved stoma, openings on the leaves and stems that allowed carbon dioxide and oxygen to flow freely into and out of the plant cell. Finally, reproductive structures such as seed and pollen mostly rely on other avenues such as animals and the air for dispersal.
Photosynthetic Pigments
Green algae species and plant species all carry out photosynthesis, making them autotrophs, organisms that make their own food. Autotrophs that engage in photosynthesis use certain pigments to capture light energy. Plants and green algae both use chlorophylls a and b. These two types of chlorophyll absorb the greenish portion of light, giving members of Chlorophyta and true plants their greenish hues. Contrastingly, brown algae contain chlorophyll c while chlorophyll d occurs in red algae, according to the University of the West Indies.
Chloroplast Evolution
Lynn Margulis, a biologist who found proof for the endosymbiotic theory, theorized that the chloroplasts of both green algae and plants arose from a single source: the cyanobacteria. The endosymbiotic theory explained that in the past, anomalies occurred in which certain unicellular organisms devoured cyanobacteria without digesting them. The autotrophic cyanobacteria continued to carry out photosynthesis within the larger organisms to provide both with energy, and mutually beneficial associations began. Over time, the larger organisms incorporated the cyanobacteria so completely that the smaller cells became completely dependent on the larger organisms and lost all other survival functions except for photosynthesis. Upon careful examination of chloroplasts in both green algae and plants, scientists hypothesized that the larger organisms became the ancestors of green algae and plants while the cyanobacteria developed into chloroplasts.
Multicellularity
Single cells usually define the members of Chlorophyta. However, some species do display a simple form of multicellularity. Plants, on the other hand, all exhibit multicellularity. Dr. Stephen M. Miller, Department of Biological Sciences at the University of Maryland, analyzed several studies regarding the evolution of multicellularity in green algae such as Volvox. He found that Volvox, more of a colonial species than a true multicellular organism, evolved basic multicellularity due to a simple gene mutation.
Single Volvox cells bear a striking resemblance to a related, unicellular algae species, Chlamydomonas. Dr. Miller, merging the studies of several scientists, theorized that Volvox and Chlamydomonas shared a common, unicellular ancestor in their recent pasts. The ancestor line split at this time, with one line giving rise to the present-day, unicellular Chlamydomonas. The other line had undergone some sort of genetic mutation that caused subsequent generations to clump together to form colonies with some individuals specialized for mobility and others for reproduction. Although the plant kingdom did not directly descend from the Volvox colony, plants probably evolved multicellularity in a similar way.