These plants attach with such firmness to rocks that when torn away, portions of the rock often come away also. Because the blade portion is much damaged by buffeting wave action, it is replaced by new growth in the spring. Each year, a zone of dividing meristematic cells where the stipe and the blade meet renew the blade. While the blade is annual, the stipe is perennial.
When the plant is mature enough to produce sporangia, dark patches called sori appear on the surface of the blade. Sori are collections of sporangia. Meiosis takes place in the production of zoospores. These zoospores are minute, bitlagellated cellso f two types: those that produce male gametophytes and those that produce female gametophytes. One-half of the zoospores are of the one type, one-half are of the other. The microscopically sized gametophytes actually evaded detection until Each gametophyte is composed of only a few cells.
The antheridia that are borne on the male gametophytes each produce a single sperm cell. Likewise, the oogonia of the female plants each house a single egg. Sperm cells swim to the egg cell at the opening of the oogonium. The zygote thus formed grows into the familiar sporophyte plant.
Figure Lakinaria: at left, the conspicuous sporophyte generation consisting of a holdfast, b stipe, and c blade with d sporogenous tissue k.
An enlargement of a sporangium intermingled with paraphyses. Algae are further divided into the several phyla: Euglenophyta , Chrysophyta diatoms , Pyrrophyta dinoflagellates , Chlorophyta green algae , Phaeophyta, and Rhodophyta.
It used to be the phylum consisting of organisms commonly referred to as brown algae. At present, the brown algae are now members of Phaeophyceae, which is one of the taxonomic classes under the new phylum, Ochrophyta.
However, apart from this class, other phyla include some algal species that are brownish in color as well. They are phyla Dinoflagellata dinoflagellate s and Bacillariophyta diatom s. However, members of Phaeophyceae are more popularly referred to collectively as the brown algae.
In contrast to the two phyla whose members are single-celled, Phaeophyceae consists of algal species that are multicellular. Phaeophyceae is a clade comprised of olive green to brown multicellular algae. The size could range from a tiny tuft of few centimeters to the giant kelp of over 50 meters long. Macrocystis pyrifera giant kelp is considered as the largest algae. Their characteristic greenish-brown color is attributed to and depends on the amount of fucoxanthin.
Apart from this pigment, chlorophyll a and c2 are also present. Similar to other algal groups, brown algae manifests alternation of generations.
The sporophyte is often the more visible form. Most of the brown algae except for the Fucales reproduce sexually by sporic meiosis. Those that are capable of asexual reproduction reproduce by means of motile zoospores. The body thallus of Phaeophyceae lack vascular tissues as seen in tracheophytes. Thus, brown algae do not have true roots, stems, and leaves.
The root-like structure of the brown algae is referred to as the holdfast. Three examples of life cycles are considered following. We will use Ectocarpus to represent the isogamous form of sexual reproduction in brown algae. The center circle of figure shows asexual reproduction while sexual reproduction is shown in the larger circle. Although the sporophyte is 2n and the gametophyte is n, the two generations look alike.
Two types of sporangia are produced on the sporophyte : unilocular sporangia having one compartment and plurilocular sporangia having many compartments. The unilocular sporangia each at first contain a single cell. This cell goes through repeated divisions, the first of which are meiotic and, thus, reduce the chromosome number. These first divisions are followed by a number of mitotic divisions, which generate a number of biflagellated haploid zoospores.
The zoospores then grow into gametophytes. The gametophytes, of course, are haploid. Plurilocular gametangia are produced on the gametophytes.
Biflagellated isogametes are produced in these gametangia by mitosis. The gametes fuse in pairs, thus producing zygotes that can then grow into new sporophyte plants. While the preceding is clearly an example of alternation of generations , Ectocarpus can go through a reproductive cycle without alternation of generations.
Look again at the sporophyte plants in figure and note the plurilocular sporangia. These contain many cells that mature into diploid zoospores, which, instead of growing into gametophytes, become new sporo-phytes. Because there is no gametophyte generation in this sequence, alternation of generations is bypassed. Some species of Ectocarpus are monoecious, having both plus and minus gametangia on the same plant. Other species are dioecious, requiring a fusion of gametes from different plants to form zygotes.
Haploid Meiospores Released. Figure Ectocarpus life cycle. At right are sporophyte generations exhibiting two types of sporangia: c unilocular and j plurilocular. In the unilocular form, a , meiosis occurs, b , followed by mitotic divisions, c. Haploid meiospores d are released, producing gametophytes, e and f.
This service is more advanced with JavaScript available. Advertisement Hide. Open Access. First Online: 04 January Download conference paper PDF. In the life cycle of many brown algae, sporophyte and gametophyte generations exist independently, and sexual reproduction male and female gametes, or sperm and egg and asexual reproduction zoospore or tetraspore after meiosis connect the two generations.
Swarmers, namely flagellated motile cells gametes, sperm, and zoospores , have two heterogeneous flagella Fig. The acronema, which is composed of two central microtubules of axonemes, makes the tip of each flagellum. It has remained unclear how mastigonemes regularly attach only to the AF during flagellar elongation Fu et al.
Open image in new window. These brown algal pheromones are volatile, lipophilic, and fragrant and have a low molecular mass with unsaturated C 8 and C 11 hydrocarbons with biogenetically related structures. Male gametes actively surround the sexual pheromone that female gametes secrete soon after their settlement on the substratum Fig.
Motile female gametes never gather around a settled female one. Therefore, it is clear that the pheromone receptor must naturally exist in the male gamete, not in the female one. In the case of isogamous brown algae, such as E. Those authors reported chemo-thigmo-klinokinesis, which means that the pheromone has two effects for attracting male gametes around settled female gametes: 1 reducing male gamete velocity by a thigmotactic response and 2 increasing beating frequency of the PF of male gametes in proportion to the pheromone concentration.
Unfortunately, the pheromone receptor in male gametes has not yet been identified. Acknowledgments We thank Drs. Andersen RA Biology and systematics of heterokont and haptophyte algae.
Baldauf SL The deep roots of eukaryotes. Boland W The chemistry of gamete attraction: Chemical structures, biosynthesis, and a biotic degradation of algal pheromones. J Exp Biol —66 Google Scholar. Oxford Science, Oxford, p Google Scholar. Function of the mastigonemes of Ochromonas. Jekely G Evolution of phototaxis. Kawai H A flavin-like autofluorescent substance in the posterior flagellum of golden and brown algae.
Kawai H Green flagellar autofluorescence in brown algal swarmers and their phototactic responses.
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