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25.2 Green Algae: Precursors of Land Plants
The kingdom Plantae used to have all photosynthetic eukaryotes classified as members.
The protist supergroup Chromalveolata now houses the brown and golden algae.
They don't have many of the structural and biochemical qualities of plants because they don't capture light energy and fix CO2.
The plants, along with the red and green algae, are in the protist supergroup Archaeplastida.
Unlike land plants, green algae have the same carotenoids and chlorophyll a and b as land plants.
Both green algae and land plants have starches in them.
Their cells have a variety of shapes, as do their cell walls and land plants.
Which of the green algae is included among the plants?
The colonial alga Volvox is one of the chlorophytes.
The charophytes include desmids, Spirogyra, Coleete, and Chara.
There are green plants in both groups.
Plants are multicellular, which adds to the ambiguity of green algae classification.
Some green algae, like Volvox, form colonies and others, like Ulva, are multicellular.
Spirogyra is a collection of colonial cells.
The majority of members live in fresh water, brackish water, or even in snow patches.
A few green algae can live on the soil if it is covered with a thin film of water.
Dry spells give an advantage to algae that can survive water stress.
Spirogyra and desmids are included in Charophyta.
They include (c) Chlamydomonas and (d) Ulva.
The land plants are a sister group of the charophytes.
Cells in charophytes and the land plants divide along cell plates called phragmoplasts, in which microtubules parallel to the spindle serve as guides for the vesicles of the forming cell plate.
In the chlorophytes, the cell plate is arranged in a way that the microtubules are parallel to the spindle.
The charophytes and the land plants have intercellular channels that allow the transfer of materials from cell to cell.
Intercellular connections do not persist in mature multicellular forms in the chlorophytes.
The charophytes and the land plants show apical growth from the tips of the plant.
Land plants and charophytes are now part of a new monophyletic group.
Green algae reproduce both asexually and sexually by producing gametes during fertilization.
In a single-celled organisms such as Chlamydomonas, there isn't a problem after fertilization.
Both Ulva and Chlamydomonas produce flagellated gametes.
The Charales, the Zygnematales, and the Coleochaetales are suggested to be the closest relatives of the land plants.
The Charales can be traced back hundreds of millions of years.
They vary in size from a few millimeters to a meter in length.
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