Describe the morphology of thallus, vegetative and sexual reproduction in Fucus with the help of clear and well labelled diagrams.
Answers
The structure of thallus, vegetative and sexual reproduction in Fucus
Explanation:
Fucus is commonly known as rockweed. They are mainly found in the rocky seacoasts and also in the salt marshes of the northern temperate regions.
Thallus - In Fucus, thallus grows mostly at the tips of the forked shoots and is mostly between 2cm to 50 cm in length. The thallus is mostly irregular in shape with disc-shaped holdfast or haptera. Thallus is mainly perineal in nature. The thallus remains above the water due to the presence of air vesicles or bladder-like structures. The swollen tip of thallus which are known as receptacles lack midrib. This swollen tips are covered with small pimple-like scattered projections with small openings that lead to the opening of the cavities. This pimple-like projections are known as conceptacles. The thallus is diploid in nature and may be either monoecious or dioeceous.
The thallus has a outer layer or peripheral layer called limiting layer and is made up of small cells that contain huge amount of plastids in them. Under limiting membrane, there is cortex which is composed of several layers of elongated mucilaginous parenchyma cells.
The central tissue is medulla which is made up of hypha-like elongated cells. Thallus grows in length due to the presence of apical cell that lies in the depression of the tip of each branch.
Vegetative reproduction - During vegetative reproduction Fucus reproduces only by the method of fragmentation.
Sexual reproduction - Fucus reproduces sexually and the process is heterogamous. The gametes are formed in the oogonia and antheridia in the conceptacles.
Within each conceptacle, many multicellular hairs called paraphyses are present that are mixed with antheridia and oogonia. The oval-shaped oogonium is formed from a single cell on a short stalk. Eight cells are produced by the process of cell division which remains enclosed in a cell wall of two to three layers.
Antheridia are much smaller and more in number than oogonia. Antheridia are produced in groups on short branched hairs. Many pear-shaped cells with two laterally unequal flagella are produced inside the antheridia.
All the eight eggs when gets matured, are released in water which soon gets surrounded by numerous antherozoids that set the non-motile eggs in rotation until an antherozoid penetrates inside the egg and an oospore is formed.
The oospore forms a thin wall and starts dividing immediately without undergoing a resting phase and forms a new Fucus plant.
