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5. What organs osmoregulate?
6. What part do excretory systems play?
VI. On the basis of the text explain that the statement “Life began in water” is
true.
Text 4
I. Read the text and state what information it carries.
BACTERIA
The Bacteria constitute an important group of Thallophita whose
relationships are very obscure. The majority of species are colorless unicellular
plants of extremely small size. Some are so small as to be almost, if not quite,
invisible under the highest powers of microscope. They occur in almost every
possible situation, and live under the most varied conditions.
They are not killed by cold, and some can survive for many months at the
temperature of liquid air.
Very little is known regarding the internal structure of the cells, and it is a
matter of doubt whether or not they possess a true nucleus. The cell-wall
appears to consist of a protein, and may be regarded merely as a differentiated
outer layer of the cytoplasm.
Multiplication of the cells by division, under normal circumstances,
ensues with great rapidity, often taking place once every hour, or even more
frequently.
Bacteria survive unfavorable conditions by the formation of resting
spores, produced within the cells by a localized concentration of the greater part
of the protoplasm. Bacterial spores are often very resistant to extremes of
temperature and can sometimes withstand prolonged boiling.
Most sorts of Bacteria live a free life a in water, earth, etc. Many exert a
very beneficial action in promoting decay and maintaining the circulation of
nitrogen and carbon dioxide in nature, while a small number are extremely
harmful, living as parasites on higher animals and plants, and causing such
diseases as typhoid, cholera, plague, etc.
The identification of the pathogenic forms is of great importance. Owing,
however, to the small size of the individuals, it is as a rule impossible to identify
the species by microscopic examination alone. One has therefore to resort to the
various reactions of Bacteria under conditions of culture.
On various artificial media, such as extract of meat, decoctions of fruit,
etc. growth is very rapid, and a single organism soon multiplies to such an extent
that the mass of its offspring is visible to the naked eye.
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