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The life cycle of the brown algae, Laminaria, begins when sporangia undergo meiosis, producing 1n zoospores. The zoospores undergo mitosis, producing multicellular male and female gametophytes. The female gametophyte produces eggs, and the male gametophyte produces sperm. The sperm fertilizes the egg, producing a 2n zygote. The zygote undergoes mitosis, producing a multicellular sporophyte. The mature sporophyte produces sporangia, completing the cycle. A photo inset shows the sporophyte stage, which resembles a plant with long, flat blade-like leaves attached to green stalks via bladder-like connections. Both the blade and stalks are submerged. Sporangia are associated with the leaf-like structures.
Several species of brown algae, such as the Laminaria shown here, have evolved life cycles in which both the haploid (gametophyte) and diploid (sporophyte) forms are multicellular. The gametophyte is different in structure than the sporophyte. (credit “laminaria photograph”: modification of work by Claire Fackler, CINMS, NOAA Photo Library)

Which of the following statements about the Laminaria life cycle is false?

  1. 1 n zoospores form in the sporangia.
  2. The sporophyte is the 2 n plant.
  3. The gametophyte is diploid.
  4. Both the gametophyte and sporophyte stages are multicellular.

The water molds, oomycetes (“egg fungus”), were so-named based on their fungus-like morphology, but molecular data have shown that the water molds are not closely related to fungi. The oomycetes are characterized by a cellulose-based cell wall and an extensive network of filaments that allow for nutrient uptake. As diploid spores, many oomycetes have two oppositely directed flagella (one hairy and one smooth) for locomotion. The oomycetes are nonphotosynthetic and include many saprobes and parasites. The saprobes appear as white fluffy growths on dead organisms ( [link] ). Most oomycetes are aquatic, but some parasitize terrestrial plants. One plant pathogen is Phytophthora infestans , the causative agent of late blight of potatoes, such as occurred in the nineteenth century Irish potato famine.

The photo shows a mucous-like mass, covered in white fuzz, hanging from a rock.
A saprobic oomycete engulfs a dead insect. (credit: modification of work by Thomas Bresson)

Rhizaria

The Rhizaria supergroup includes many of the amoebas, most of which have threadlike or needle-like pseudopodia ( [link] ). Pseudopodia function to trap and engulf food particles and to direct movement in rhizarian protists. These pseudopods project outward from anywhere on the cell surface and can anchor to a substrate. The protist then transports its cytoplasm into the pseudopod, thereby moving the entire cell. This type of motion, called cytoplasmic streaming    , is used by several diverse groups of protists as a means of locomotion or as a method to distribute nutrients and oxygen.

The micrograph shows a semi-round cell with long, hair-like projections extending from it.
Ammonia tepida , a Rhizaria species viewed here using phase contrast light microscopy, exhibits many threadlike pseudopodia. (credit: modification of work by Scott Fay, UC Berkeley; scale-bar data from Matt Russell)

Take a look at this video to see cytoplasmic streaming in a green alga.

Forams

Foraminiferans, or forams, are unicellular heterotrophic protists, ranging from approximately 20 micrometers to several centimeters in length, and occasionally resembling tiny snails ( [link] ). As a group, the forams exhibit porous shells, called tests that are built from various organic materials and typically hardened with calcium carbonate. The tests may house photosynthetic algae, which the forams can harvest for nutrition. Foram pseudopodia extend through the pores and allow the forams to move, feed, and gather additional building materials. Typically, forams are associated with sand or other particles in marine or freshwater habitats. Foraminiferans are also useful as indicators of pollution and changes in global weather patterns.

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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