Chlorophytes. (green
algae)
Volvox is a member of the
chlorophyceans and consists of anything from 500 to many thousands
of cells all linked together and can sometimes reach 0.5mm in
diameter so they can easily be seen with the naked eye. This green
alga is common in lakes and ponds and can be found in great numbers
when conditions are at a premium for this organism. The colony
consists of what are often described as chlamydomonas type cells
that are embedded within mucilage; each individual cell is connected
to the nearest one with a cytoplasmic strand. Each individual
cell consists of two flagella (biflagellate) that protrude through
the outer mucilage coat that surrounds the sphere. All these cells
seem to work towards a common purpose moving the cell around the
water finding the right place for maximum absorption of sunlight
energy. According to the textbooks the colony is polarised slightly
with the anterior cells having larger eyespots. It is obvious
that some differentiation is occurring here otherwise the colony
would never get anywhere. The cells at the anterior end with their
larger stigma (eyespot) guide the colony to light while the cells
at the rear can reproduce, is this the first signs of cell specialisation?
Can signals be sent along the cytoplasmic strands to different
parts of the colony? It must be said that Volvox is about as advanced
as a spherical ball of cells can get without changing it's shape,
it appears to be at an evolutionary dead end.When viewing Volvox
care must be taken when placing a cover slip over the preparation
because what usually happens is all the colonies are squashed
flat. Try a hanging drop preparation or place something under
the cover slip so there is enough room for the alga to swim freely.
High powers are not needed to observe this alga in motion.
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Figs 1a, 1b, 1c, 1d.This set of low
power photographs of Volvox are showing the complete hollow ball
of cells. Note the smaller daughter colonies that are beginning
to form within the parent body. When viewing the movement of
Volvox it is best to use a low power or preferably a good stereomicroscope
with zoom features
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Asexual reproduction can occur
when the Gonidia cells (which are non motile) at the posterior
end of the colony start to divide until a certain size is reached,
whereupon they are then released into the outside world by inverting
themselves through a pore that developed earlier at the eight
celled stage. Sexual reproduction occurs when certain cells within
the colony are transformed to produce sperm packets that will
enter the female colony wall by secreting enzymes. Once inside
the colony the sperm packet is broken up and the individual sperms
are then free to fertilise the eggs that are available. The zygotes,
which have a hard and spiny coat, can survive many seasons at
the bottom of the pond waiting for more favourable climatic periods
According to Graham and Wilcox in their excellent book Algae,
male colonies produce a pheromone that induces other Volvox colonies
to become sexually active. The samples that I collected were from
a small pond here in Epping Forest but on my return to the same
area a few weeks later there appeared to be no trace of Volvox
anywhere. This does not mean that they have disappeared, it may
be that they are found only in one very small area of the pond.
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Fig 2. This photograph was taken with
a X40 phase contrast and shows part of the outer sphere with
a daughter cell just to the lower left of the picture. The arrowhead
is showing the flagella that protrude through the mucilage to
propel the Volvox through the water. The cells at the front end
of the sphere have enlarged stigma that steer the colony to assimilate
sunlight in the proper amounts.
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Fig 3. This photograph was taken with
phase contrast and shows the cytoplasmic strands that hold the
individual cells together in the matrix. Take a look at the excellent
photographs of this alga in Freshwater
Algae by John and Hilda Lund.
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An excellent book by Linda
Graham and Lee Wilcox is called Algae and
published by Prentice Hall ISBN 0013-660333-5. The book is designed
to be used by under-graduates and graduates studying aquatic ecology
but can be used by the amateur freshwater biologist to gain a
greater understanding of the subject. The book is supplemented
with many high quality black and white prints and five introductory
chapters. The book costs £18.99. 640 pages.
Volvox two
Introduction to photomicrography.
