Plants > Green Algae
Plants I: Evolutionary History of Plants
Reading: Campbell, Ch. 28, 29 & 30.
Lab Manual, Ch. 3.
This page is intended to go with plant lab I (Chapter 3 in the lab manual). The lab manual gives you the instructions on what to do in lab and what to study. This page will provide images of the plants to help you understand what you see in lab.
The photo above shows Oscillatoria, a cyanobacterium. It grows as long chains of similar-looking cells with no differentiation. This isn't a plant. Sometimes called blue-green algae, the cyanobacteria are photosynthetic prokaryotes. Although they perform photosynthesis, in most respects they are very different from plants.
Since they are prokaryotes, they do not have nuclei or chloroplasts in their cells. Each cell is filled with green photosynthetic pigments. Don't spend a lot of time looking for cellular details; you won't see them under our microscopes. The main reason for you to look at this specimen is to see what it doesn't have; compare these to the green algae in the next section.
Green Algae (Chlorophyta)
Evidence from cell structure, biochemistry, and genetics all shows that green algae are closely related to land plants. One obvious similarity is that green algae are green, and land plants are also green. This is not a coincidence; the similar color is a result of the fact that both groups use similar photosynthetic pigments.
However, green algae are very different from land plants in terms of their anatomy. Green algae are either unicellular, or grow as strings or sheets of similar-looking cells. Unlike land plants, green algae don't have complex organs such as leaves and stems.
The picture at right shows Chlamydomonas, a unicellular green alga. In this picture you see clusters of large oval cells. These cells are stuck together on the slide, but they normally live independently. Inside each cell, you can see some small structures; these are membrane-bound compartments such as the nucleus, vacuole, and chloroplasts. Refer to Campbell for more detail on these structures. You don't need to be able to identify them now; the functions of these intracellular structures will be discussed in Bio 6B. For now, recognizing that there are membrane-bound structures within the cell is important in one key respect: it tells you that these cells are eukaryotic. Prokaryotic cells do not have a system of membranes dividing the cell into compartments (an endomembrane system). Another clue that these are eukaryotes is their large size; at around 50 microns, these cells are much bigger than most prokaryotes.
This picture shows part of a gigantic single cell of Spirogyra. This organism is unusual because the chloroplasts form a long spiral inside the cylindrical cells.
Spirogyra is called a filamentous alga, because it grows as long, filament-like chains of cells. This body plan provides a ratio of surface area to volume, which is important because each cell must independently absorb nutrients and sunlight directly from the environment. Although the cells are linked together, they live more or less independently.
Key points about green algae:
- Photosynthetic eukaryotes.
- Closely related to green plants.
- Many are unicellular, but some have simple multicellular body plans.
- Do not have differentiated tissues other than reproductive cells.
- Because they lack differentiated tissues and other adaptations to life on land, they must live and reproduce in the water.
This page updated September 30, 2013