In this article we will discuss about the classification of sargassum.
Family Sargassaceae:
Plants possess flat costate branches or transitional stages to cylindrical branches bearing numerous macroscopic spine-like projections, turbinate foliar organs, or costate leaves, usually with cryptostomata and buoyant air-bladders. Conceptacles are in ordinary branches or in special receptacular branchlet systems. Sexual reproduction is oogamous. Each oogonium produces a single egg.
Genus Sargassum of Sargassaceae:
It is principally confined to warmer seas in the tropical zone, and is commonly known as ‘gulf weed’. It is one of the most conspicuous and common of the sea-weeds being very abundant along the Indian coast growing attached to rocks below low-tide level along the East and West Coasts of India. The co-called Sargasso weed is found as large floating mass in the Sargasso Sea near the West Indies.
The bushy habit of the plant body has a great resemblance with the seed plants as its body is composed of stem-like stipe and leaf-like blades. The predominant sporophyte is attached to the substratum by means of a more or less irregular, warty, solid, parenchymatous base from which arises a stipe-like structure which grows into the main axis bearing alternate branches.
The branches bear short, phyllode-like structures or leaf-like expansions commonly known as ‘leaves’, with or without midrib depending on species (Fig. 113A). Besides this, the secondary branches are also sterile and reduced to air-bladders (Fig. 113B to D). There occur tertiary branches which are fertile finger-like in appearance, called receptacles (113C) whose development varies with species.
In these receptacles are embedded flask-shaped conceptacles.
In certain species of Sargassum, the thallus is free-floating, where it perpetuates itself by fragmentation. Structurally, the thallus is very similar to that of Fucus being differentiated into a peripheral layer of compact, columnar cells containing abundant plastids performing the function of photosynthesis.
Inner to it is the cortex of many cells whose walls are gelatinized (Fig. 113E). These cells probably perform the function of storage. The central zone is the medulla of loosely arranged cells. Medulla is perhaps the food-conducting region. Growth of the thallus is carried by a three-sided apical cell situated at a depression at the apex of the thallus.
Sargassum reproduces vegetatively by fragmentation and by the growth and decay of the thallus.
Antheridia and oogonia are borne in flask-shaped conceptacles that are embedded in the receptacles. Some species are monoecious and others dioecious. The monoecious species may have both antheridia and oogonia developed in the same conceptacle or in different conceptacles on the same thallus. Whereas, the dioecious plants are either male or female.
The two types of plants are distinguishable from the nature of the receptacles, the male receptacle being smooth and the female spinous.
In the early stages, the development of male and female conceptacles is very identical as it initiates from a single initial cell which by repeated division and enlargement gives rise to a conceptacle.
The oogonia are sessile and borne directly on the wall of the conceptacle (Fig. 113G). The uninucleate young oogonium is filled with dense cytoplasm. Its nucleus has a very prominent nucleolus. The nucleus enlarges and undergoes three successive divisions with the first division meiotic, resulting eight haploid nuclei, only one survives and other degenerate.
The entire content of the oogonium rounds up to form an uninucleate ovum. Each mature oogonium after emergence through the ostiole, remains attached to the conceptacle wall by means of a ‘long gelatinous stalk which is formed by the elongation of the thickened apex of the oogonium (Fig. 114A).
The antheridia occur in large numbers and are crowded in each coaceptacle. They are borne in clusters on the lower branches of the branched paraphyses which again arise from the wall of the conceptacle. The mature antheridium is a small ovoid structure with a thick mucilaginous wall. It produces 64 pear-shaped biflagellate antherozoids.
Meiosis takes place during the development of antherozoids. The mature antherozoids are liberated in sea-water.
The liberated antherozoids swim towards the female conceptacle and several of them surround each oogonium. The antherozoids get attached to the oogonial wall by their flagella. One of the attached antherozoids after penetrating through the oogonial wall enters the ovum. Ultimately fusion of male and female nuclei takes place.
Thus, the fertilization is effected while the ovum is still confined within the oogonium which again is attached to the conceptacle Wall by a gelatinous stalk.
The zygote starts germinating immediately after formation and while already enclosed in the oogonium, without undergoing any reduction division of the diploid nucleus. The oogonium also remains attached to the wall of the conceptacle. Ultimately the zygote is liberated out by the gelatinization of the oogonial wall, rests on some solid object and then divides transversely.
The upper cell gives rise-to the erect portion and the lower to the rhizoids of the diploid plant (Fig. 114B to D).
In certain species of Sargassum there are found sterile flask-shaped structures borne on primary branches and on leaf-like expansions. These structures look like the conceptacles and are known as cryptoblasts (Fig. 113F). They have their early stages of development very similar to those of conceptacles.
These cryptoblasts do not bear any sex organs, colourless unbranched hairs arise from the floor of the cryptoblasts. These hairs secrete mucilage and project in tufts through the openings of the cryptoblasts. These openings are called cryptoblamata.
Sargassum exhibits certain special features of advancement. The complex diploid thallus is very predominant being differentiated into main axis with branches on which leaf-like structures are developed. There is great internal differentiation of tissue with fairly well division of labour. Meiosis is gametic.
Haploid generation is restricted only in the gametes. Oogamous sexual reproduction is accompanied with retention of ovum within the oogonium even up to the early stage of development of embryo (Fig. 115).
Some Indian species of Genus Sargassum:
Sargassum caryophyllum J. Ag.; S. duplicatum J. Ag.; S. suiartzii (Turner) C. Ag.
Special features of Genus Sargassum:
1. Diploid plant body of bushy habit having a main axis producing branches bearing leaf-like structures and air-bladders, a special floating device.
2. Internal differentiation of tissues of the plant body with division of labour.
3. Absence of any kind of spores.
4. Sexual reproduction oogamous.
5. Sex organs (antheridia and oogonia) developing in conceptacles borne on the same or different plants.
6. Reduction division during the development of gametes in the antheridia and oogonia.
7. Formation of a single egg in an eight-nucleate oogonium, the rest of nuclei degenerate.
8. Fertilization taking place while the egg is still enclosed in the oogonium which is liberated out in water but remains attached to the female conceptacle by means of a gelatinous stalk—a condition much more advanced than Fucus.
9. Absence of any alternation of morphological generations correspondingly with chromosomal alternation of generations.
10. The diploid plant body instead of producing sporangia and spores produces gametangia and gametes—a condition encountered also in Fucus.