Growth form (habit) of different species of algae varies considerably among and within algal divisions. Some Divisions, such as the Euglenophyta, Bacillariophyta, and Dinoflagellata consist entirely of unicellular forms. Other Divisions, especially the Phaeophyta and Rhodophyta consist almost entirely of multicellular forms (except in the spore stage of the life cycle), often with appreciable body development and differentiation of parts. The growth form of algae is related to their life style. Unicellular algae are often members of the phytoplankton, floating freely in oceans, lakes, and streams. Many unicellular algae have flagellae, enabling them to move (albeit slowly) through the water column. Multicellular algae are usually attached to the bottom in the ocean, lakes, or streams. Some multicellular algae have complex forms, often producing wide photosynthetic surfaces that are analogous to leaves.

(A) Levels of Complexity within the Chlorophyta and other Divisions

The Chlorophyta are particularly diverse in growth form. Some members are unicellular throughout their life, while at the other extreme some Chlorophyta grow to be 8 meters long. It is possible to trace the increasing complexity in algal structure through certain members of the Chlorophyta. Material is available in lab for this purpose. Before examining these growth forms, it is important to remember that, while you will consider models of increasing structural complexity, the genera that are present in lab are not necessarily related in any evolutionary order.

(1) A Unicellular Form

Examine live or prepared specimens of Chlamydomonas (C & V fig. 4.23) or other unicellular green algae. What cellular components are present in these cells? What features are particularly necessary for unicellular algae to exist in an aquatic environment

(2) Colonial Forms

(a) Examine live or prepared specimens of Gonium (C & V fig. 4.24), Pandorina, and Pediastrum, few-celled colonial forms. Can you suggest a model for the evolution of a unicellular form like Chlamydomonas to a few-celled colonial form? What advantages might be conferred on colonials relative to unicells? Are there any differences between the individual cells of the colony in Gonium or Pandorina and the Chlamydomonas unicells

(b) Examine material of Volvox (C & V fig. 4.25-4.29), another colonial alga. How does Volvox differ from the colonial genera observed in the previous section? Can you envision any trends in increased complexity from unicellular algae to colonial forms like Volvox?

(3) Filamentous Forms

Examine live and prepared material of Spirogyra (C & V fig. 4.37-4.38), Cladophora, Ulothrix (C & V fig. 4.30-4.31), and Oedogonium (C & V fig. 4.32-4.36). These genera are filamentous forms, where cell division in an organism is generally in only one plane (except where the filaments branch). Note that these genera are not closely related and represent four very different evolutionary lines (orders) within the Chlorophyta; nevertheless, they have in common a growth form that is a definite departure from unicellular or colonial forms. Can you suggest a model whereby a filamentous form may have evolved from a unicellular ancestor? Compare the structure of a colonial form, such as Volvox or Gonium, and a filamentous form, and consider how each growth form fits its environment. In what environments does a filamentous form have an advantage over colonial forms?

(4) Thallose Forms

Examine live material of Ulva (C & V fig. 4.42), the sea lettuce. This growth form, termed thallose, differs from filamentous in that cell division occurs in two planes, resulting in the flat body of the alga. What environmental constraints are expected with this growth form? The thallose habit is also present in other algal divisions (e.g., the Rhodophyte genus Porphyra).

(5) A Siphonaceous Form

Examine live material of the common marine alga Enteromorpha. While appearing superficially like a filamentous form, its body actually consists of branching tube-like (siphonaceous) sections. Does the Latin name of this genus give you a clue as to its growth form? This habit is yet another example of algal structural diversity.

(6) Phaeophyta, the most complex Algal Forms

For a consideration of the ultimate in structural diversity in algae, examine the live specimens of the large kelps of the Phaeophyta (C & V fig. 4.45-4.49). Note the different structures ("organs") that are present. These kelps generally have regions that are crudely analogous to roots in their ability to anchor the organism to the substrate. They also have stem-like regions and laminar leaf-like regions, as well as special gas-filled structures (floats) that facilitate flotation near the surface of water. Consider the different functions of each structure. Where in the environment do you expect to find giant kelps? What factors might restrict their distribution to their particular range?