In this article we will discuss about the life cycle of laminaria, explained with the help of suitable diagrams.
Order Laminariales of Laminariaceae:
This order contains the largest and most elaborately organized plant body of all algae. The members of this order, better known as kelps, are predominant plants of cold water.
With the exception of the genus Chorda, the kelps all show a large, coarse sporophyte generation differentiated into holdfast, stipe, and one or more blades. Growth in length results from an intercalary meristem between stipe and blade which adds tissue to both. Secondary growth in diameter of the stipe occurs in some annual and in many perennial species.
The plants have a highly developed internal structure, showing clear tissue differentiation. The sporophyte is the evident plant, the life history always is completed through an alternate, microscopic filamentous gametophyte whose ecological requirements probably are more often the determining factors in the distribution of a given species.
The sporophytes of Laminariales produce superficial unilocular sporangia, either on vegetative blades or on special sporophylls. The sporangia are sometimes in remarkably definitive sori. These mature on succession and fall out of the blade at maturity as an irregular plate, leaving a large hole that rapidly erodes.
The sporangia are associated with unicellular paraphyses, both arising from the same basal cells. Meiosis occurs in the developing unilocular sporangia, which usually release 16 to 64 zoospores (32 in some cases).
The zoospores attach and form a wall after brief motility and germinate within a few hours to grow into haploid gametophytes. The gametophytes, which occur in a 1: 1 ratio, are always dioecious, and this has now been determined in a large number of species in culture. The life history of Laminariales was first worked out by Sauvageau in 1915.
The general appearance of the gametophytes is remarkably uniform throughout the order. All are small, filamentous thalli; of relatively few cells in uniscriate arrangement. The male plants have smaller cells and are more abundantly branched than the female. Although these gametophytes have almost always been studied in culture, it is possible to observe them in nature.
Sexual reproduction in Laminariales is oogamous. The oogonia are terminal or intercalary in position. Each forms a single egg which, though extruded, usually remains attached to the ruptured oogonium. The antheridia occur at the tips of branches or as outgrowths from intercalary cells of male gametophytes.
Each releases a single biflagellate antherozoid, and the plant degenerates after the gametes are shed. The fertilized egg secretes a wall and soon undergoes division to begin the development of the sporophyte, but the female gametophyte persists during the early phases of this growth.
Family Laminariaceae:
Un-branched stipe terminates in a single blade; blade smooth to perforate or imperforate, with or without ribs, often with tufts of hairs on the surface, blade may be fan-shaped spirally twisted and rolled up at the base.
The sporangial sori are produced on the vegetative blades and not on special sporophylls; holdfast may be rhizomatous to discoid or conical, usually holdfast of branched haptera; stipe solid or hollow and with or without mucilage ducts.
Genus Laminaria of Laminariaceae:
This is an alga that has attained the largest size forming an important, element of perennial marine flora. There are many species of Laminaria, all of which have a very wide distribution being conspicuous in colder as well as temperate and tropical waters. They are popularly known as ‘devil’s aprons’. Most species favour quieter water.
To the industrialists they are better known as kelps and are generally employed as a source of food, iodine and fertilizers.
The alga is characterized by a conspicuous sporophyte, which may be even six feet or more in length consisting of a basal discoid or branched holdfast, a stipe, and a lamina which is simple or digitate (Fig. 106A). Growth occurs at intercalary meristem, usually located between the lamina and the stipe (Fig. 106B).
Anatomically both lamina and stipe can be differentiated into different regions, the meristoderm—composed of one or more layers of similar small cells covered externally by mucilage, the outer cortex—much wider and elongated cells with pointed ends, the inner cortex— longer cells with square ends, and the medulla of a mass of tangled threads (Fig. 106C).
Basically lamina shows the same structure as the stipe, only the elements of the medullary region are drawn out owing to the great surface enlargement. The medulla is characterized by the presence of cells modified into elongate hypha-like structures ‘trumpet hyphae’ (Fig. 106D), the transverse cell wall ends of both cells swell out to form bulbs, of which the upper bulb always being larger.
The transverse wall is perforated resembling and behaving like sieve-plate. The trumpet hyphae are still objects of speculation as being suggested to be a storage or conducting tissue, or support. There is a system of anastomosing mucilage ducts which are particularly confined to the stipe.
The sporophyte bears patches of unilocular zoosporangia and paraphyses in sori on both surfaces of the lamina (Fig. 106E & F). The sporangia develop successively in considerable numbers maturing and liberating their contents at the same time.
The zoospores are formed by the repeated division of the diploid sporangial nucleus one of which being meiosis, followed by typical cleavage of the protoplast of the sporangium with ultimate production of uninucleate bits which metamorphose into zoospores. The zoospores are biflagellate with laterally inserted unequal flagella (Fig. 106G).
They, on germination, produce two types of gametophytes of microscopic size.
The male gametophytes are simple filamentous small-celled copiously branched structures (Fig. 106H & I). The antheridia arise singly or in groups at the terminal cells of the projecting threads or as lateral outgrowths from the upper side of the creeping threads.
Each antheridium produces a single pear-shaped laterally inserted biflagellate antherozoid. The male gametophyte disintegrates after the liberation of the antherozoid. The female gametophyte is sparingly branched filamentous structure (Fig. 106J), any cell of which can function as an oogonium bearing one egg.
Prior to fertilization the egg comes out through a narrow apical aperture and remains attached to the wall for a considerable time (Fig. 106K).
The egg is fertilized by an antherozoid when already attached with the oogonial wall and is never set free. The zygote secretes a thin membrane and develops immediately into the sporophyte without undergoing any resting period (Fig. 107A to F).
Laminaria exhibits a heteromorphic alternation of generations, the sporophyte being dominant whilst the gametophyte is much reduced. Meiosis is delayed and takes place during the formation of zoospores in the zoosporangia. The entire process of sexual reproduction is unique of its kind.
Though the egg is not retained within the oogonium it still remains attached with oogonial wall, which indicates a transitional stage between primitiveness and advancement. The spontaneous germination of the zygote when already remaining attached with the female gametophyte without going to rest is also an interesting feature in the life cycle of Laminaria (Fig. 108).
Special features of Genus Laminaria:
1. Gigantic size sporophytic plant body.
2. The plant body differentiated into a distinct holdfast, stipe and lamina.
3. Annual replacement of blade by the growth of meristematic tissue lying between stipe and blade.
4. Complex internal structure of the sporophytic plant body.
5. Zoosporangia in sori developing on the blade.
6. Reduction division during zoospore development in the zoosporangia.
7. Heteromorphic alternation of generations.
8. Predominant sporophyte alternating with much reduced gametophyte.
9. Gametophytes heterothallic.
10. Sexual reproduction oogamous.
11. Fertilization of ovum outside the oogonium but remaining attached to its apex.