Class Phaeophyceae (Brown Algae).

Brown algal walls are composed of cellulose fibrids and the commercially important mucopolysacaride alginic acid.  There are few, if any, unicellular or colonial browns.  The simplest are branching filaments, and the largest are the kelps.  Simple filaments show little cellular differentiation while the Kelps demonstrate a high level of cellular differentiation. More complicated forms have cortex and medulla. Cortex is the outer layer of thallus with photosynthetic cells. Medulla is the inner layer of non pigmented cells (storage + transfer). Growth is either diffuse or at the meristem. Meristen is the region where new cells are produced, these can be defined as apical or terminal or intercalary (within a branch).

Alternation of Generations (diplohaplontic)

In some cases the sporophyte (usually diploid) and gametophyte (usually haploid) are isomorphic (similar in size and shape).   Simple branch forms look most alike while the complex forms look most different.

Order Ectocarpales.   It has one of the simplest thalli among the brown algae and a life cycle with isomorphic generations. It is a common filamentous brown, found growing on rocks or large algae.

Definition: Heterotrichous- filamentous, two types of branches; basal branch on subtrate and erect branch usually more openly branching.

Definition: Physodes- vesicles containing polyphenolic compounds, probably unpalatable to most grazers.


The Ectocarpus life cycle involves an alternation of generations. Both phases are filamentous and have a similar appearance, but gamethophytes may be more restricted in their distribution and seasonality than sporophytes. Reproductive structures form at the ends of branches. Sporophytes form two types of sporangia.


1. Plurilocular sporangia- large number of cuboidal cells, each cell develops into zoospores (2N) each develops into sporophyte (clone).  More common at warmer temperatures

2. Unilocular sporangia- one enlarged cell undergoes meiosis, then several mitosis occur to 32 or 64 (1N) zoospores.   More common at lower temperatures.

      Note: Zoospores can be 1N or 2N, flagelated + grow to mature form
    1. Plurilocular gametangia- cuboidal cells (1N) except merge by sex to form zygote which grows to 2N sporophyte .
Figure 10. Plorilocular gametangium cycle (fig. 4.23 pag. 111).

Some specialized goups such as Sphacelariales have more complex filaments and branches form propagules. This is an specialized branches that break-off and settle reestablish, multi-cellular and vegetative cells. They show a regular growth pattern.

Dictyotales algae are more common in tropical oceans than in colder waters. The common genus Dictyota has a ribbon-like thallus with a regular dichotomus branchin pattern. Besides, it may have 3 layer of 3 cells thick ribbons fan shape thallus encrusted with CaCo3; e.i. Padina.

Class Phaeophycea- Heteromorphic generation.

Primitive brown algae are filaments with isomorphic generations. Brown algal evolution has shown the followings trends:

Evolutionary path; filaments with isomorphic generations in early forms

  1. Develop alternation of heteromorphic generations.
  2. Increasing complexity of the sporophyte a. size + specialization.
  3. Reduction of gametophyte to microscopic filaments
  4. Loss of plurilocular spornagia on the macrothallus
Figure 11. Life cycle of ectocarpus sporophyte (fig 4.30 pag. 115).

Pseudoparenchymatus sporophyte (orders chordaniales and desmarestiales)

Definition: Mass of filaments growing together so that so that they filaments are difficult to distinguish.

Definition: Penaphyses- stenile filaments associated with reproductive filaments.

Many are epiphytes on large algae.

Paraenchimatus macrothalli (orders dictyosiphonales + scytosiphonales)

Dictyosiphonales contains brown algae that are multiseriate filaments (e.i. Dictyosiphon or form ribbon-like blades (Petalonia)

Multiseriate filaments

Figure 12. (figures 4.28a and 4.29a pag. 114)

Tarte Thallus


Scytosiphonales- blades

Kelps (oder laminariales)

Parenchymatous- cell division in 3D

Kelps (Order Laminaria)

The large sporophytes of the Kelp with parenchymatous construction show greater differentiation into specialized tissues and overall structural complexity than any other algae.

Figure 13. Laminaria saccharina (fig. 4.35 pag. 119)

Thallus has differentiate into several layers

Figure 14,15,16 (figures 4.39abc, 4.40, 4.41ab pp. 121,122)

Other genera show greater specialization

  1. Long stip with multiple blades (fronds)
  2. Gas blades for flotation
  3. Stitt, erect stipes
  4. Separate reproductive blades called sporophylls
Macrosystis- longest kelp (algae) up to 60m. form a whole layer of sieve cells between cortex and medulla for transport.

No free living N phase

Order fucales, durvillaeales

Quite different, smaller but, dichotomass branchiss of blades

  1. Growth at the end of the blade (branches) around epical cell
  2. Reproduction regions called receptacles at end of blade (branches)
Conceptacles beneath receptacles spherical cavity with opening to outside

Coametangial branches (form inside)

Antheridia or oogonia may or may not be in same conceptacle or on different plants drying off receptacles may force out gamete external fusion, new zygote settles.