FUNGI
Fungi are non green plants ie they do not contain any chlorophyll. They need moist conditions for growth and can thrive as well in darkness as in sunlight. They are primitive plants with no proper cellular organization as found in the flowering plants. They occur in many forms and are found almost every where, ranging in size from single-celled organisms, namely yeasts to giant multicellular structures such as mushrooms.
Moulds, which are often associated with decaying organic food substances, are common examples of fungi. Mucor, Rhizopus and penicillin are common moulds which are widespread in occurrence.
Together with two other types of fungi known as mushrooms and toad-stools, they are example if saprophytic organisms, ie. organisms which feed on dead or decaying organic matter. There are several fungi which are parasitic in habit. Phytophthora, puccinia, claviceps and others live as parasites in certain green plants and derive their food requirements from their hosts.
Many fungi can reproduce asexually by means of tiny spores. Some of them need light for forming spores. Some fungi can also reproduce sexually.
RHIZOPUS NIGRICANS
Rhizopus nigricans belongs to a group of fungi known as the "black moulds". It is also known commonly as the "bread mould" because it freely on moist, stale bread. It appears as a delicate black mass.
STRUCTURE: Rhizopus forms a mass of soft, closely-women white, silky threads. This mass, which is known as a mycelium is a vegetative part of the fungus. Each silky thread-like structure is a hypga.
The mycilium grows not only on the surface of the bread but into it as well. The bread is known as the substrate. The mycilium consists of three sorts of hyphae: stolons which grow horizontally on the substrate, rhizoids or root-like hyphae which arise at points where the stolons come into contact with the substrate, and sporangiophores.
The rhizoids are much-branched hyphae which penetrate into the substrate. They are able to digest and absorb organic food. The sporangiophores are erect unbranched hyphae arising from the stolons at the same point where rhizoids are formed. They grow vertically upwards and give rise to the reproductive structures called sporangia.
Each hypha is non septate ie it has no cross walls dividing it up into cells. Its wall is composed of proteins and carbohydrates. The interior of each hypha contains granular cytoplasm in which lie numerous nuclei.
The hypha Is, therefore, described as multi nucleate. The cytoplasm also contains abundant reserve food substances in the form of oil droplets, vacuoles of varying sizes and granules of glycogen.
MODE OF LIFE: Rhizopus is said to have a saprophytic mode of nutrition, ie, it feeds on dead or decaying organic matter. It has no chlorophyll and cannot synthesize its own food. Therefore, it resorts to feeding on complex organic matter. The hyphae in the substrate produce enzymes which diffuse out of the hyphal walls into the surrounding organic matter. These enzymes digest the organic matter and the digested, soluble end products then duffuse into the hyphae, where they are assimilated into the cytoplasm. This sort of digestion, which occurs outside the body of the organism, is described as extracellular digestion.
In Rhizopus, respiration is a simple process of gaseous exchange by diffusuon between the atmosphere and the cytoplasm in the hyphae. While oxygen diffuses into the hyphae, carbon dioxide is given off into the atmosphere.
REPRODUCTION: Rhizopus produces by two methods– sexual and asexual reproduction. The former occurs more frequently than the latter. As mentioned earlier, the vertical unbranched hyphae, or sporangiophores, bear the reproductive structures. For asexual reproduction to occur, the tip of each sporangiophore enlarges to form a more or less spherical structure, the sporangium.
Within the sporangium lie a large number of tiny, rounded reproductive units called spores. Each spore has a few nuclei and a resistant wall. The central part of the sporangium is occupied by a dome shaped structure called columella.
The wall of the sporangium gradually darkens as the spores within ripen. Although the wall is thick, it is brittle due to the presence of calcium salts which are deposited in it from the cytoplasm of the sporangiophore.
Finally, the wall ruptures and the spores are liberated. As they are very light, they can be carried far away by the wind. Sometimes a large number of these spores are carried on the bodies of insects.
When these spores fall on some suitable substrate, each one germinates. Germination follows the absorption of water by the spore.
The wall of the spore ruptures and a delicate hypha grows out of it. The hypha elongates very rapidly and branches repeatedly to give rise to a new mycelium.
Sexual reproduction rarely occurs and is possible only when the hyphae of two different mycelia come into contact with each other. When two spores produced by two different mycelia fall close to each other on the same substrate, each one germinates and produces a number of new hyphae. In Rhizopus nigricans, if sexual reproduction is to occur, the hyphae of one mycelium must be of a plus strain while those of the other must be of a minus one. This means that though the hyphae of the two mycelia are similar structurally, they are physiologically so different that they can be considered to be opposites. However, they cannot be considered as male or female since there is no morphological distinction between the gametes or structures which produce them. In several species of related fungi, there is no distinction into plus and minus strains. Here, sexual reproduction takes place between morphologically and physiologically identical strains. When two such hyphae come into contact with each other , each one gives out a short side branch, the progamentagium at the point of contact.
These side branches grow towards each other and finally meet. A cross-wall is formed a short distance behind the tip of each progamentagium and the portion which is cut off in this way from the rest of the plant contains dense cytoplasm and several nuclei. This structure is known as a gamentangium. The portion which connects the gamentangium with the rest if the mycelium, ie the basal portion of the side branch, is known as the suspensor.
After some time the cross-walls at the point of contact of the two gamentangia dissolve to form a single cell. This allows the contents of the two gametangia to mix.
The nuclei fuse together in pairs and the whole structure increases in size. Its outer wall darkens in colour, thickens and develops spines.
This structure is known as the zygospore.
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