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Algal cells
It was a huge step in complexity from a virus to a bacterium and it is almost as large a step from the prokaryotic cells of bacteria to the eukaryotic cells of algae. Many of the larger algae are accepted as plants by most people but the absence of highly differentiated cells distinguishes them from the kingdom Plantae and places them in the Protista.

Nevertheless having developed the basic cell pattern of a green alga all that was necessary for the development of higher plants were variations on this pattern.

The Protista
This kingdom includes a tremendous variety of organisms from heterotrophs to autotrophs and unicellular to multicellular. Their origin is "multiphyletic", that is they arose from several lines of prokaryotes, rather than from a common ancestor. They form a kind of link-group between the prokaryotes and other eukaryotes, both animals and plants. Some of them such as Amoeba, Paramecium and Hydra are closer to animals than to plants. The more plant-like include:

  • Oomycota These are heterotrophic and form coenocytic hyphae like fungi, with which they were long included. They are unlike fungi in their mode of reproduction and in having cellulose cell walls. It seems that their similarity to fungi is an example of convergent evolution, rather than a direct relationship. In addition to saprophytes which live in the soil a number of plant diseases are caused by Oomycetes: potato blight, downy mildew, root and collar rots.
  • Euglenophyta Look like a unicellular alga but have a protein "pellicle" like an animal rather than a cell wall. Now thought to be result of capture of a chloroplast by an "animal" cell.
  • Chrysophyta Often unicellular, including the diatoms which are unusual in having silica cell walls and which are an important part of the phytoplankton at the base of the food chain in the sea (a major portion of the earth's photosynthetic productivity).
  • Rhodophyta "Red algae" often abundant in tropical waters and important in the building of coral reefs. Red algae are the source of carrageenan and agar which are used as gelling agents in foods, cosmetics and paints (in addition to tissue culture media!)
  • Phaeophyta "Brown algae" are dominant in temperate seas and include kelp (Laminaria) and rockweed (Fucus). Kelp is harvested for extraction of alginate which is used in toothpaste and many food products. (Is this agriculture or forestry?)

    Fucus vesiculosus or "bladderwrack" 

    Laminaria or "kelp"

  • Chlorophyta "Green algae" include some of the most plant-like organisms. There are many marine types including Ulva or sea lettuce. Unlike the other two groups, this division includes some terrestrial (Protococcus on tree bark) and freshwater (Chlamydomonas, Volvox, Cladophora) organisms. Filamentous green algae are almost weeds, giving trouble in irrigation systems and even on the surface of wet soils and media.

The unicellular alga, Protococcus can often be found growing on the West face of tree trunks, especially in damp locations

Algal blooms tend to form on ponds and lakes especially when there is fertilizer run-off from fields and lawns

Filamentous algal "weeds" in plant nutrient solution

Ulva lactuca

The Rhodophyta, Phaeophyta and Clorophyta constitute the "algae" (singular alga) They include motile unicellular (Chlamydomonas), motile multicellular (Volvox) and static multicellular types (Cladophora, Spirogyra and "seaweeds"). The motile types often use whip like flagellae to propel themselves. The gametes of static multicellular types are free-swimming and also have flagellae.

The larger static types show some degree of cellular differentiation and organization into root-like (holdfast) stem-like (stipe) and even leaf-like (blade) structures.

The smaller free floating or motile forms make up a major part of the phytoplankton (along with Chrysophyta and Cyanobacteria).

In each of these groups of algae there are organisms which show an alternation of haploid and diploid generations. Sometimes the morphology of the haploid gametophyte is distinct from the diploid sporophyte as in Fucus and sometimes they are hard to tell apart as in Ulva.

Life cycle of Ulva lactuca

Lichens are formed by symbiotic association between cyanobacteria or green algae and fungi. The photosynthetic bacteria or algae make the lichen an autotrophic organism. They can take several forms: crustose (encrusting), foliose (leafy) and fruticose (shrubby) and grow on rocks, tree bark or soil often in cold, dry or otherwise inhospitable situations. They are generally intolerant of air pollution and the presence of lichens is an indicator that pollutants (particularly SO2) are absent.


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The Ohio State University
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