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The most old plants
In spring and in summer especially, one can see, on the banks of rivers,
lakes or sea, of filamentous heap of green, yellowish or brown colour,
floating on the water or immersed. It is Algae, plant bodies that evolved
from forms of tiny dimensions, such as those that confer on the stagnant
waters their characteristic green colour, either those that, in the
oceans, constitute the phytoplancton. It is about plant bodies capable of
making photosynthesis because they are provided with chlorophyll - one
estimates that Algae carry out more half of all the photosynthesis which
is done on Earth - and that establish the base of the food chain in the
aquatic circles. The phytoplancton, for example, (the set of tiny bodies
autotrophes which move freely in the water) is the food of the zooplancton,
constituent animal of the plankton, on which feed so much the small Fishes
(which are afterward eaten by the bigger Fishes) than the great Mammals,
as the whales.
Algae present extremely variable dimensions: some are tiny whereas others
can reach dozens metres in lengths.
Even though their calorie value is low, Algae are rich in vitamins and in
mineral salts, and constitute a food for the people, especially in far
East, where they are used in the preparation of soups, jellies and ices,
that they make more compact and homogeneous. Algae are also much used in
the industry of cosmetics, medicines, paper, etc. They serve also of
fertilizer because they enrich the in substance organic ground and put at
disposal of plants mineral salts necessary for their growth.
If they can be exploited by the man in the profit, Algae do not provoke
less some damages of it, as it is case when, as a result of an excessive
proliferation, they pollute some aquatic circles by phenomena of
decomposition. The impressive "bloom" of some sorts which
produce toxic matter pulls sometimes the death of numerous Fishes.
Ecology
Algae constitute the base of the food chain in the aquatic biomes, where
they constitute the most important source of oxygen. Without them, the
oceans, lakes and all the other plans of water would become quickly
intolerable for any animal, because the oxygen would be consumed without
being renewed, and, consequently, would not be enough any more to support
their vital processes. Algae are spread in the aquatic circles (sea,
lakes, rivers, brooks etc.) and in the not aquatic circles, Provided that
they are wet. They grow, consequently, on the ground, on the sand, on the
cliffs, on the trunks of trees and even on the dry grounds of deserts,
where they develop only one or two times a year, after rainy season.
Besides, some unicellular Algae live in the body of animals - toweling,
hydras or Mollusks - so as to establish with them a report of mutual
advantages ( mutual symbiosis).
Protophytes : unicellular Algae
Various types of Algae according to their morphological characteristics
are distinguished. The most simple are the unicellular Algae, which float
on the water, move by means of flagella or are immovable and settle on
capital. According to the recent classification of alive bodies in five
administrations, Algae must be enclosed in the administration of Protistes,
which includes all the unicellular bodies autotrophes ( Protophytes) and
heterotrophes (Protozoons). Protophytes can live in colonies - their cells
are regrouped by means of a gelatinous substance but keep their
independence. These colonies compose more or less important, sometimes
visible structures in the bare eye. In their the most evolved forms, all
the cells of the colony have no same function. Some reach a precise
specialization, as those that are appointed to the reproduction and assure
the development of new individuals.
Vital cycle
To Algae, as to many other plants, vital cycle is characterized by the
alternation of generations, that is by the succession, in the reproductive
cycle, two very different generations: the gametophyte and the sporophyte.
The gametophyte is the generation which produces gametes and which
presents haploïds cells ( n ), that is endowed with a chromosome luggage
haploïde. The sporophyte represents the generation which produces spores
and possesses a chromosome luggage diploïd (2n), that is that it
possesses a double number of chromosomes.
In the vital cycle of a very common green Alga, named Ulva, or sea
lettuce, the sporophyte and the gamétophyte is identical. They present a
thalle flattened in blade, of a thickness of two cells only and of a width
more of 2 m, which settles in the substratum by means of a suction cup
placed in the base. The diploïd sporophyte (2n) produces spores, haploïds
cells ( n ) endowed with four flagella, from spore cases, diploïds cells
specialized which divide by meiosis. In pervading suited conditions,
spores germinate, producing the gametophyte ( n ) where specialized cells,
gametanges, engender, by mitosis, haploïds gametes ( n ) endowed with two
flagella.
Sexual reproduction takes place when two gametes identical, but of
opposite sign (that is belonging to two different sexual types) merge to
form a diploïd cell , the zygote. The sporophyte develops from this cell.
All the Algae reproduce also in an asexual way. This type of reproduction
is very important, especially for the most primitive forms, for which it
represents the only way which assures the development of new individuals.
To Protophytes, asexual reproduction is done by simple division of a cell
in two equal parts. To Algae, on the other hand, it takes place by
fragmentation, when a more or less big part of the thalle gets loose and,
autonomous time, develops and engenders a new individual.
Classification
This widely exceeds the competences of the cellular biology but not to
leave you on your famine, we detail the classification of algae below.
Algae constitute a particularly diverse division represented by bodies
characteristics of which are extremely variable. For that reason, they are
regrouped in different classes on the basis of the nature of their
pigments, spare substance that they accumulate and structure of their
cellular wall. Although they are called Algae, the blue Algae, or
Cyanobacteria is not a part of this division, because their cellular
structure is the typical structure of procaryots, and, consequently, they
are a part of an administration with part, the administration of Moneres.
Algae (without taking into account Protophyts) divide there :
1) Green algae ( Chlorophyts). They live especially in the aquatic
circles, essentially in the circles of fresh water, even though some forms
are navy. They are endowed with a filamentous thalle, in blade or
tridimensional. They seem to be at the origin of the upper plants because
they are endowed with the same molecules of chlorophyll, starch, their
foodstuff of reserve and of cellulose, the main constituent of their
cellular wall.
The Alga Spirogyra is a green Alga pluricellulaire filamentous which, in
spring or in autumn, forms in small stream of the masses écumeuses
floating. This Alga is constituted of joined cells of which each possesses
a chloroplast rolled up in spiral, where from the name of the Algae.
2) Brown algae ( Chromophyts). Most of them live in the cold seas of the
moderate regions. They present a filamentous thalle, in shape of blade or
band, sometimes even more complex. Their name is due to the fact that they
contain of the fucoxanthine, a pigment which hides the green of the
chlorophyll returning these brown Algae. One of the most known brown Algae
is Sargassum. It forms floating masses developed in a zone of the
Atlantic Ocean called exactly Sargasso Sea.
3) Red algae ( Rhodophyts). It is a question for the main thing seaweeds
that live fixed to cliffs or to another Algae. They are endowed with a
filamentous thalle and are particularly plentiful in the tropical regions.
They are found in a depth bigger than the one in that evolve the other
groups, because they possess particular pigments capable of absorbing
brilliant radiations reaching the deep waters and that are not absorbed
during their passage in the water. It is phycobilines, of red colour,
that, with another pigments, hide the green colour of the chlorophyll and
set in the Algae of the different colours. Their cellular walls are
endowed with a stiff internal layer and with a layere more external
constituted of agar. The agar is a very interesting substance from
economical point of view because it is used in the composition of
medicines, cosmetics or capsules of culture of microbodies.
Among the unicellular Algae, that is Protophytes, those that are the most
spread in the nature is Diatomes. They can be suspended in the air, where
they are transported by winds on the glaciers, the cliffs, the ground or
the monuments. They constitute the phytoplancton, that is all the plant
micro-organisms suspended in the aquatic environment which represent the
main source of food for the sea animals and of fresh water. As another
tiny forms of Flogged autotrophes, they are spread in seas and in fresh
water. The cellular wall of Diatomes is soaked with siliceous material,
the same material which constitutes the glass, so as to form a pod endowed
with two valves which fit the one in the another one as a box and its lid.
This stiff pod is provided with small protuberances, with holes or with
tiny canals which form characteristic drawings of every sort. When
Diatomes dies, the cellular internal mass degenerates and pods accumulate
on the bed of sea, lakes or ponds. And so forms the diatomite (or
Tripoli), used notably as abrasive and as insulation.
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