Some of the Most Important Modes of Reproduction found in Phaeophyceae are as follows !
The reproduction takes place by 1. Vegetative, 2. Asexual and 3. Sexual methods.
1. Vegetative reproduction:
Vegetative propagation takes place by fragmentation of the thalli. In the case of Sargassum this type of reproduction is very prolific. In S. natans it is found abundantly. This is a free floating species.
In some of the cases special fragments known as ‘propagula’ are developed. They occur in Sphacelaria.
The ‘adventitious buds’ develop in many species of Fucus. They develop by the division of meristematic cells in young plants. Each bud develops into a new plant.
2. Asexual reproduction:
Asexual reproduction takes place by means of zoospores and aplanospores formed inside the sporangia.
(a) By zoospores:
The formation of zoospores is most common in all the members of Phaeophyceae except in Dictyota and Fucus. The zoospores are pyriform and biflagellate. The anterior flagellum is larger than the posterior one except in Fucales. In Dictyotales single flagellum is found on the zoospore. The zoospores are produced inside the zoosporangia, which may be of two types, i.e., unilocular sporangia and multilocular or pluriiocular sporangia.
The unilocular sporangium may be terminal or intercalary in position. In each sporangium 64 or 128 zoospores are produced. At the time of formation of zoospores reduction division takes place and zoospores are haploid. They germinate into haploid thalli (gametophytes). In some cases as Punctariales, Sphacelariales, Chordales, Ectocarpus, Pyaeliella, etc., the zoospores behave as gametes and unite in pairs. This is the example of origin of sex.
The pluriiocular sporangia are always terminal in position. All the cells of sporangium are diploid and give rise to diploid zoospores. Such zoospores will give rise to sporophytes. The multilocular sporangia are unknown in Fucales and Laminariales.
(b) By aplanospores:
Sometimes, in unilocular sporangia instead of producing zoospores the aplanospores may also be produced. They are non-motile and without flagella, e.g., Dictyotales. The first division is always reductional. The aplanospores are always less in number. In Dictyota 4 aplanospores and in Zonaria 8 aplanospores are produced per sporangium.
3. Sexual reproduction:
This ranges from isogamy to oogamy.
This takes place by the fusion of two similar gametes. This is found in many Phaeophyceae such as Ectocarpales, Sphacelariales, Dictyosiphonales, etc. The gametophytes may be monoecious or dioecious.
Here the fusion of two dissimilar gametes takes place. The examples of this type are found in some members of Ectocarpales, Cutleria, Soranthera, etc. If fusion fails the female gametes may develop parthenogenetically.
In majority of Phaeophyceae the sexual reproduction is oogamous. The species may be homo or heterothallic. The male sex organs are called the antheridia and the female sex organs are called the oogonia. In Dictyotales the antheridia are multicellular structures. Each cell of antheridium gives rise to a spermatozoid. In Desmarestiales and Laminariales the antheridia are unicellular and each antheridium produces a single spermatozoid.
Usually each oogonium produces a single ovum or oosphere, but in Fucus eight eggs are produced in the oogonium.
The eggs liberate in the water and only then the fertilization takes place so that the; rtilization is external.
In Fucales the sex organs develop inside conceptacles which develop on special reproductive oranches called the ‘receptacles’.