Archive for category Medical Science
Although there are freshwater species the majority of algae live in the sea and the various seaweeds found at low tide are familiar examples. A lot of the most primitive of these plants are single-celled and invisible to the naked eye, whereas the large, more advanced seaweeds are made up of countless cells which may be divided into groups or tissues performing different tasks. Such plants may have primitive roots. These algae are very similar to other plants in the way they obtain their food, for they contain chlorophyll and photosynthesis takes place in the same way as it does in the advanced forms of plant life. Although many are green, quite a number have pigments in their cells which are stronger colours than green, giving the algae red, brown or even bluish colours. Their method of increase can be either by simple division of the original plant or sexual reproduction. There are thousands of different species of algae and it would be impossible here to describe even a small proportion of them, but one or two examples can be taken to show what life-forms exist in this group. Some of the tiny single-celled species are capable of movement and such a plant is Chlamydomonas, which is found in ponds and ditches. It is roundish in shape and is built up in the same basic way as other cells. At one end are two thread-like arms which project through the cell wall. These are called flagella, and by waving them around the cell can move in the water. The cell contains a nucleus and chloroplasts for manufacturing food, and in addition an orange-coloured object known as the eye-spot. This helps the chlamydomonas find its way towards brighter light, and in doing so gives the chloroplast a better chance to produce food. Other single celled algae do not go through life individually, but group themselves into a calony and hundreds or thousands together may just be visible without a microscope. These types are interesting in that they all use their flagella together and move as a whole colony towards light. Sometimes in bright weather the rate of reproduction is so fast that the water in ponds may be coloured green by their presence, and can easily be seen. The hard green covering often seen on older wooden fences is formed by large numbers of a small land alga known as Pleurococcus. It can withstand long periods of drought but like those which live in water it only becomes very active when there is plenty of moisture and it is warm. Many of the other algae in water can also form what are called resting stages so they can withstand adverse conditions such as the drying out of the pond in which they live. One of the commonest alga in ponds and slow steams is the plant which sometimes forms the slimy green masses familiar to all who spend some time near the water. This is called Spirogyra and is a long thread like plant formed from short cylindrical cells joined end to end. Among the most advanced of the algae are the common forms of seaweed called wracks which inhabit rocky costs all over Europe. Fucus vesiculosus commonly called bladder wrack, has a portion as its base which is adapted to clasp on to rocks and prevent the plant being washed away. This part of the plant has no powers of absorbing foods and is entirely an anchor. The stem is cylindrical at first then higher up it is flattened out and has built in bladders of air which act like water wings and keep the plant upright in the water. The whole plant is very slippery, being covered with a jelly like substance and it is rubbery to withstand the buffeting of waves. Reproduction is either by the simple breaking off of sections of the stem which in these plants is called the thallus, or by the formation of two kinds of special cells, one of which is capable of movement like chlamydomonas plants.These special cells are set free when the tide is in and the mobile ones swim to the others and they fuse together forming a spore. This can germinate and develop into another plant.
Fungi represent a very important part of the thallophytes and are separated from the rest by many definite characteristics. They have no chlorophyll and no starch is present in their cells and it is easy to see from the example of a few familiar toadstools that they are quite different in actual make-up from all other plants. They do not live on their own manufactured food, but rely either on the decaying remains of other plants and animal life(these types are called saprophytes)or on living plants or animals(parasites). The fungi may be single celled plants such as yeast, which is used for baking bread and brewing beer, or multi-celled plants such as the mushroom. In these more advanced forms the main part of the plant is an intricate web of threads known as hyphae, the whole web being called the mycelium. This often lives underground or inside the host plant on which the fungus is living. The hyphae threads run about either in or between the cells of the plant and have the power of dissolving the cellulose of the cell walls and living on the contents. The saprophytic ones act in a similar way on dead organic material and are not generally harmful to plants or animals. These parasites on the other hand cause man a great deal of trouble and expense in trying to get rid of them. Some fungi attack animals, including man, an example being the fungal disease athlete’s foot. With the larger fungi, the hyphae sometimes come above ground and form a special structure which can produce spores. This is the fruiting stage of the plant and the mushroom and toadstool are examples of this. The hyphae have another property which is important to the fungus and this is the formation of a hard tuber like body called sclerotium(again a resting stage) which is capable of existing for a long time without actually growing, or doing damage in the case of harmful fungi. This stage makes it difficult to get rid of some fungal diseases for they can withstand a lot of adverse treatment, and then germinate when conditions are suitable. As fungi lack chlorophyll they cannot make their own food by photosynthesis. They must therefore absorb carbohydrates from the plant or animal matter on which they live. Having got s source of supply of carbohydrates they can themselves convert these substances into the more complicated ones needed to make cellulose for cell walls, and for proteins and protoplasm. The last two can only be formed if the fungus has a source of nitrogen and other more complex chemicals from an outside source, so for this reason a lot of fungi are rather specialized in their choice of where to grow. Like the animals, fungi are dependent upon green plants for their food and could not live without them. So it is that the first living matter to colonise a new piece of ground will be something which can manufacture its own food, without depending on any other life form and it must therefore be a green plant whether one celled or multi celled. When this is established, fungi and animals will appear either to grow on or eat the plant or live on the organic matter which occurs when it dies.
Some tiny plant organisms produce substances which are capable of killing bacteria, and penicillin and streptomycin are two examples of these. The organisms producing these useful antibiotics are in fact fungi, and are cultivated by man in order to obtain large quantities for treating diseases. Viruses are organisms which are very much smaller even than bacteria and very little is known about them. They seem to be on the borderline between the living and non-living although they do have the ability to increase and spread. They cause many diseases to plants and animals and in man are thought to be responsible for such things as colds, influenza and measles. In plants they produce disorders such as wrinkling and yellowing of leaves. They seldom kill plants, but completely upset their normal way of life, and because they can ruin crops are a problem to man. Unlike a lot of bacterial and fungal diseases in plants, they cannot be killed by chemical sprays
Bacteria also come within the division of thallophytes that are dependent on green plants, for they are a group of minute organisms without any cholorophyll. They are parasites or saprophytes and are usually single celled often with the power of movement in the same way as chlamydomonas. They increase by straightforward division of an old cell into two new ones, or they can also form resting spores. These consist of cells which have specially tough cell walls, and in this stage the bacteria is very resistant, in some cases even to boiling water. These spores are very numerous and are present in the soil, the air and water. In fact they are present virtually everywhere unless special precautions are taken to kill them. During the resting stage they are not increasing at all but when conditions are suitable for their growth the cell wall breaks down and the bacteria become active again. Parasitic bacteria are often harmful such as anthrax in animals tetanus in man and soft roots in plants but quite a number do exist on other living organisms without doing them any harm. Saprophytic bacteria are often useful for they help in rooting down dead material. They are very active in for example a compost heap. Some are used to covert alcohol into vinegar. One group of bacteria (nitrogen fixing bacteria) are especially important as they live in swellings in the roots of plants of the legume family (peas and beans) and have the power of extracting nitrogen from the surrounding air in the soil and making it into a form that can be used by plants. This is why plants of this family, especially clover, are often grown in fields and then ploughed in to improve the soil. Other bacteria in the soil (nitrifying bacteria) can covert plant and animal remains into a form that plants can use. So by the very valuable action of these tiny life forms a continuous process is set up of turning waste and dead materials back into food for other plants. This called the nitrogen cycle.