Lichens
To be certain, a lichen is not the most conspicuous of plants. Lichens grow in unassuming fashion on rocks, logs and other exposed surfaces in a wide range of habitats around the worlD、To the untrained eye they look like little more than crusty patches that, at first glance, might easily be mistaken for a discoloration of the surface.Even if the average person should happen to notice the lichen’s presence and correctly identify it as some form of life, he is unlikely to go much further in contemplating it. Though almost totally ignored by the layperson, for the botanist, lichens are one of the most fascinating of all plants, and one of the most intensely studieD、
They are the subject of so much scientific scrutiny primarily because a lichen is not just one plant. It is, in fact, a composite organism made up of fungus and algae living together in a close association that is, presumably, beneficial to both. When these two very different plants combine, the result is a unique and very long-lived composite organism that appears, at least on a macroscopic scale, to be a unitary plant. It is an organism that bears no resemblance to either of its constituents when they are observed individually. The separate fungal and algae elements can be recognized only when the body of the plant, called a thallus because there are no stems or roots, is sectioned and examined under a microscope. When viewed this way, the fungus component dominates the picture, as it accounts for nine tenths of the total body mass of the lichen.But, entrapped within it, clearly visible as dark spots, are the algae cells.Essentially, nothing is known of how an amorphous mass of fungi and algae come together to form a highly differentiated, structurally stable body.
Despite all the scientific scrutiny lichens have received, it is still not entirely certain what each member gains from the association.
Some researchers have speculated that the fungi join in the relationship because they are able to consume the algae cells as they die and therefore are guaranteed a food supply.
It is well-known that the chlorophyll- containing algae cells produce food by means of photosynthesis.
There may be some mechanism, still unknown to us, through which this energy source is utilized by the fungus. Fungus possesses no chlorophyll of its own. How or even whether the algae benefit from this association is still less certain, though we can easily imagine that they gain mechanical protection from the elements by being tightly enveloped in the structural fibers of the fungus body. They should also benefit from retention of water between the fibers.
The hardiness of lichens has made them what botanists term "pioneer plants". This refers to their ability to colonize habitats where other plants do not exist. They are common on barren rocky surfaces, where the lack of soil precludes the establishment of most other kinds of plant life. They can even be found in places as hostile and extreme as the interior of theAntarctic continent.Although they are most often associated with far northern or southern environments, they have been found living in sun baked desert soils that are otherwise devoid of life. The most highly specialized lichens are the endolithic species of theAntarctic, which as the name indicates, live inside rocks, forming more or less continuous tissue structures between the rock crystals.
As remarkable in their robustness as lichens are, there is one kind of an environment which they are generally unable to tolerate. Habitats that are heavily affected by pollution are noticeably devoid of lichen

Iodine and Goiter
Iodine is a well-known example of a trace mineral whose lack in the body creates an easily treated disease. When the thyroid gland is not supplied with sufficient iodine to manufacture hormones, it enlarges and forms a goiter or swelling of the neck.At the same time other symptoms, such as fatigue and sluggishness, weight gain, coldness of the body, and depression, may occur.
In the United States goiter was first noted in the Great Lakes region, where in the 1930s, as many as 40 percent of the people in some areas had goiter, due mainly to iodine-deficient soil. This scarcity had been caused by ice age glaciers melting and washing the iodine out of the soil.
The inhabitants of mountainous regions ofEurope had suffered from iodine deficiency for centuries for similar reasons.
It was discovered in the alpine region of Switzerland in the nineteenth century that in areas in which the drinking water contained less than 0.5 micrograms of iodine per liter there was a high incidence of goiter.
In contrast to mountainous or inland regions, areas by oceans or in the vicinity of ocean breezes usually contain enough iodine to prevent this affliction.
As a fairly accurate rule of thumb, if a map is drawn showing the parts of the world where the water supply is deficient in iodine and then superimposed on a map showing the areas where the inhabitants suffer from goiter, the two mapped areas coincide.
To understand how iodine deficiency leads to goiter, it is necessary to look at the underlying physiology of the thyroid glanD、The human thyroid gland is the only place in the body where iodine is stored, and it requires a daily supply of about 150 micrograms of iodine entering the body. When food or water is digested, the iodine it contains is either taken up by the thyroid or eliminated from the body through the kidneys into the urine. When supplies are low, the kidneys still eliminate iodine from the body so the capacity of the thyroid to preserve an adequate supply of raw material is threateneD、
The thyroid gland consists of thousands of balls of cells, called thyroid vesicles, which enclose a space filled with a jellylike protein called thyroglobulin. These cells have an extraordinary ability to trap iodine from the bloodstream, and the efficiency of this trap can be increased if the amount of iodine in the blood circulation decreases. Once the iodine is trapped by the thyroid vesicles it passes into the thyroglobulin, where the actual manufacture of two kinds of thyroid hormones takes place. The hormones are stored here until they travel back through the thyroid vesicles to enter the bloodstream. The thyroid hormones are then taken to every part of the body where they influence the rate at which the chemical processes of every cell proceeD、They have a pervasive effect on the control of oxygen consumption and heat production of the whole body, and they are essential to the healthy growth of body and minD、
The thyroid tries to keep constant the amount of circulating thyroid hormone entering the cells of the body. When iodine supplies are low, the pituitary gland, a small pea-sized gland at the base of the brain, secretes a thyroid-stimulating hormone (TSH) that in turn regulates the power of the thyroid to trap iodine and increase the output of the thyroiD、If the thyroid is continually stimulated by TSH, the cells get larger and eventually the whole gland enlarges with an increase in the number of its cells. The thyroid may have many temporary crises where iodine supplies are not adequate and where swelling in induced unde

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