Six most important characteristic features of Order Fucales are mentioned below:
1. They are marine and found in the seas of cold water.
However, Sargassum is found in tropical waters. Most of them are attached to rocks in intertidal zones. Some species of Sargassum are floating.
Image Curtsey: http://upload.wikimedia.org/wikipedia/commons/c/cd/Saccharina_latissima_NOAA.jpg
2. The spores behave as gametes. The motile spores develop in microsporangia (antheridia) and called the antherozoids, and the non-motile aplanospores develop in macrosporangia. (Oogonia), and called the eggs.
3. The anterior flagellum of the swarmer of Fucales is longer than the posterior one whereas in other orders their arrangement is reverse.
4. The plant itself is a sporophyte (2n). The reduction division takes place just prior to formation of gametes.
5. The developing oogonia undergo a reduction division and the haploid eggs are developed. On comparative grounds this is not oogonium but unilocular sporangium.
6. In all Fucales the oogonia have eight nuclei, but the number of developing eggs is different. In Fucus type all eight nuclei develop eggs, this type is most primitive. The other types have been derived from this type. In Ascophyllum type 4 nuclei form eggs and 4 supernumerary nuclei.
In Pelvetia type there are two eggs and six supernumerary nuclei. In Hesperophycus type there is one functional large egg and other eggs with 7 degenerating nuclei. In Cystoseira type there is one egg and 7 supernumerary nuclei. In most advanced Sargassum type there is one egg and 7 degenerating nuclei.
There are about 40 genera and 350 species in this order.
Classification:
Seven families are included in this order.
1. Fucaceae, e.g., Fucus, Pelvetia.
2. Himanthaliaceae, e.g., Himanthalia.
3. Cystoseiraceae, e.g., Cystoseira.
4. Sargassaceae, e.g., Sargassum, Turbinaria.
5. Hormosiraceae, e.g., Hormosira.
6. Durvilleaceae, e.g., Durvillea.
7. Ascoeiraceae, e.g., Ascoseira.