Spirogyra
Introduction:
Spirogyra is a genus of filamentous, green algae. They consists of long threads of cylindrical cells
that form an un-branched chain. The cells are identical in structure and are thus colonial. Their
unusual helical chloroplast structure makes Spirogyra an interesting algae to observe under the
microscope but it can be challenging to draw. The helical or spiral chloroplasts also exlain the
name ‘Spirogyra’.
There are about 400 different species of Spirogyra with cell diameters ranging from 10-100 µm.
Individual filaments containing hundreds of cells connected end-to-end may be well over 20cm
long.
Where to find Spirogyra
Spirogyra can be found in freshwater, both running and still, where it forms masses of ‘pond slime’.
Often it is found in polluted waters where there is a high nutrient content and abundant light.
Bubbles of oxygen produced by photosynthesis are often caught in the mass of threads.
There are other filamentous green algae that also form masses of green slime like Spirogyra and
you will not know if a collected sample is in fact Spirogyra until it is examined microscopically. In
some cases, other species of algae can be mixed in with the Spirogyra. Spirogyra is free-floating
and not usually attached to rocks or other objects within the stream. Although root-like attachments
have been reported they are not commonly seen and the threads tend to be tangled up in aquatic
plants.
Because the cell wall of Spirogyra is covered with a layer of mucus, it is slimy to the touch and
prevents the strands being tangled. Larger species can also feel slightly coarse when handled.
Spirogyra can be found in ponds (left) slow moving water
(centre) or moderately fast streams (right)
Mounting Spirogyra for microscope
observation.
Spirogyra is best handled with tweezers, tease
the strand out on your slide and use a drop of
water that the spirogyra has been growing in as a
mounting fluid.
A common mistake made by many students is to
put too many strands of the algae onto their
microscope slides. The filaments lay over each
other and make observation difficult.
Cell structure
Within a sample of Spirogyra you will find filaments
with differences in the number of chloroplasts within
the cell and the closeness of the individual spirals of
the chloroplast. To make observations easier it is
best to concentrate on a filament where the
chloroplast spirals are widely separated (see arrows
in the 2 photographs right and below).
These photos are taken at Medium Power (x100)
below and High Power (x400) right. The Spirogyra
below has also been lightly stained with Iodine.
As you observe the cells of Spirogyra remember to continually change your focus adjustment. As
you do so different parts of the cells will come into focus and you will effectively see the cells in
three dimensions. The next 3 photographs illustrate this:
focused on the top of the
chloroplast spiral.
Chloroplast obscures the
nucleus
focused on the central
nucleus, note that both
the top and the bottom of
the chloroplast are out of
focus.
focused on the lower
spiral of the chloroplast,
note that one spiral is
obscured by the nucleus
which is above it.
All 3 photographs at High
Power (x400)
Descriptions of cell parts
Cell wall and cell shape
Spirogyra cells are cylindrical in shape. They are connected end-to-end forming and un-branched
filament.
The ratio of cell length : cell width is between 4:1 to 6:1. i.e. cells are about 4 - 6 times as long
as they are wide. Be sure that any diagram you draw reflects this.
The cell wall is actually made up of two layers. This is not easy to see and can best be seen when
cells are slightly plasmolysed. When this occurs the cell membrane pulls away from the inside of
the cell wall. Remember that the cell wall is being observed side-on and the two layers will not
always be obvious. High Power (x400).
2 layers of the cell wall
can just be seen here
Position of cell
membrane
Space between cell
membrane and cell wall
The outer cell wall layer is made of cellulose and the inner one is made of pectin. On the outside is
a very thin layer of mucilage, which helps the filaments slide over each other and avoid tangling.
End walls
Close examination of the end walls between adjoining cells shows that they are not always flat but
tend to be dished. Ensure your drawing shows this shape. High Power (x400).
nucleus
nucleus
End wall between adjacent cells
Nucleus and suspending cytoplasmic strands
The nucleus of Spirogyra can be seen without stain but staining does make it more obvious.
It is located exactly in the middle of the cell in an area that is occupied by the large central vacuole.
This does not mean however that the nucleus is in the vacuole, it is in fact suspended there by a
number of strands of cytoplasm. Staining makes these visible.
In these photos the
Spirogyra cells
have been stained
with Iodine. The
cytoplasmic strands
that suspend the
nucleus are visible
in all of these cells.
(All 3 photographs
High Power, x400)
Cytoplasm and vacuole
From all the preceding photos, it is obvious that the cytoplasm of Spirogyra cells forms a thin layer
just inside the cell membrane and that the bulk of the cell volume is occupied by a single large
vacuole.
As well as this, there is also a layer of cytoplasm surrounding the nucleus and strands of cytoplasm
that appears to suspend the nucleus in its central position within the cell and the vacuole.
Chloroplast and pyrenoids
It is the shape of the chloroplasts that have given Spirogyra its name.
The number of chloroplasts can vary from 1 to as many as 10. However, 3 or 4 is probably the
most common and most of the photographs in this booklet show 3 chloroplats.
The chloroplast form a series of helixes within the cytoplasm (think of the shape of DNA which has
2 helical strands)
In these photographs, iodine stain has darkened the
pyrenoids. These bodies within the ribbon-like
chloroplast are responsible for starch production. It
is their high starch content that has reacted with
iodine to produce the dark colouration.
Note also the edge of the chloroplasts, which is
scalloped rather than a smooth. The green colour of
the chloroplasts is due to the presence of chlorophyll
and has not been entirely obscured by the dilute
Iodine stain that has been used.
Both photographs High Power (x400).
Spirogyra diagrams:
The first two diagrams are provided so students can see how the structure of a Spirogyra cell more
clearly. No labels are included, for these refer to the complete diagram on the next page.
1.
Cytoplasm and nucleus
Note how the central nucleus is held by strands of cytoplasm.
2.
Chloroplasts and cytoplasm
The chloroplasts and pyrenoids form spirals within the cytoplasm which lines the inside of
the cell. This is located just inside the cell membrane. There are 2 chloroplasts drawn here
but more are commonly present.
3.
Complete cell