SPIROGYRA.
=283.= In our study of protoplasm and some of the processes of
plant life we became acquainted with the general appearance of the
plant spirogyra. It is now a familiar object to us. And in taking up
the study of representative plants of the different groups, we shall
find that in knowing some of these lower plants the difficulties of
understanding methods of reproduction and relationship are not so great
as they would be if we were entirely ignorant of any members of the
lower groups.
[Illustration: Fig. 128. Thread of spirogyra, showing long cells,
chlorophyll band, nucleus, strands of protoplasm, and the granular wall
layer of protoplasm.]
=284. Form of spirogyra.=—We have found that the plant spirogyra
consists of simple threads, with cylindrical cells attached end to
end. We have also noted that each cell of the thread is exactly alike,
with the exception of certain “holdfasts” on some of the species. If
we should examine threads in different stages of growth we should find
that each cell is capable of growth and division, just as it is capable
of performing all the functions of nutrition and assimilation. The
cells of spirogyra then multiply by division. Not simply the cells at
the ends of the threads but any and all of the cells divide as they
grow, and in this way the threads increase in length.
=285. Multiplication of the threads.=—In studying living material
of this plant we have probably noted that the threads often become
broken by two of the adjacent cells of a thread becoming separated.
This may be and is accomplished in many cases without any injury to the
cells. In this manner the threads or plants of spirogyra, if we choose
to call a thread a plant, multiply, or increase. In this breaking of a
thread the cell wall which separates any two cells splits. If we should
examine several species of spirogyra we would probably find threads
which present two types as regards the character of the walls at the
ends of the cells. In fig. 128 we see that the ends are plain, that is,
the cross walls are all straight. But in some other species the inner
wall of the cells presents a peculiar appearance. This inner wall at
the end of the cell is at first straight across. But it soon becomes
folded back into the interior of its cell, just as the end of an empty
glove finger may be pushed in. Then the infolded end is pushed partly
out again, so that a peculiar figure is the result.
=286. How some of the threads break.=—In the separation of the
cells of a thread this peculiarity is often of advantage to the plant.
The cell-sap within the protoplasmic membrane absorbs water and the
pressure pushes on the ends of the infolded cell walls. The inner
wall being so much longer than the outer wall, a pull is exerted on
the latter at the junction of the cells. Being weaker at this point
the outer wall is ruptured. The turgidity of the two cells causes
these infolded inner walls to push out suddenly as the outer wall is
ruptured, and the thread is snapped apart as quickly as a pipe-stem may
be broken.
[Illustration: Fig. 129. Zygospores of spirogyra.]
=287. Conjugation of spirogyra.=—Under certain conditions, when
vegetative growth and multiplication cease, a process of reproduction
takes place which is of a kind termed sexual reproduction. If we select
mats of spirogyra which have lost their deep green color, we are likely
to find different stages of this sexual process, which in the case of
spirogyra and related plants is called _conjugation_. A few threads
of such a mat we should examine with the microscope. If the material
is in the right condition we see in certain of the cells an oval or
elliptical body. If we note carefully the cells in which these oval
bodies are situated, there will be seen a tube at one side which
connects with an empty cell of a thread which lies near as shown in
fig. 129. If we search through the material we may see other threads
connected in this ladder fashion, in which the contents of the cells
are in various stages of collapse from what we have seen in the growing
cell. In some the protoplasm and chlorophyll band have moved but little
from the wall; in others it forms a mass near the center of the cell,
and again in others we will see that the contents of the cell of one
of the threads has moved partly through the tube into the cell of the
thread with which it is connected.
=289.= This suggests to us that the oval bodies found in the cells
of one thread of the ladder, while the cells of the other thread were
empty, are formed by the union of the contents of the two cells. In
fact that is what does take place. This kind of union of the contents
of two similar or nearly similar cells is _conjugation_. The oval
bodies which are the result of this conjugation are _zygotes_, or
_zygospores_. When we are examining living material of spirogyra in
this stage it is possible to watch this process of conjugation. Fig.
130 represents the different stages of conjugation of spirogyra.
=290. How the threads conjugate, or join.=—The cells of two
threads lying parallel put out short processes. The tubes from two
opposite cells meet and join. The walls separating the contents of the
two tubes dissolve so that there is an open communication between the
two cells. The content of each one of these cells which take part in
the conjugation is a _gamete_. The one which passes through the tube to
the receiving cell is the _supplying gamete_, while that of the
receiving cell is the _receiving gamete_.
[Illustration: Fig. 130.
Conjugation in spirogyra; from left to right beginning in the upper
row is shown the gradual passage of the protoplasm from the supplying
gamete to the receiving gamete.]
=291. How the protoplasm moves from one cell to another.=—Before
any movement of the protoplasm of the supplying cell takes place we can
see that there is great activity in its protoplasm. Rounded vacuoles
appear which increase in size, are filled with a watery fluid, and
swell up like a vesicle, and then suddenly contract and disappear.
As the vacuole disappears it causes a sudden movement or contraction
of the protoplasm around it to take its place. Simultaneously with
the disappearance of the vacuole the membrane of the protoplasm is
separated from a part of the wall. This is probably brought about by a
sudden loss of some of the water in the cell-sap. These activities go
on, and the protoplasmic membrane continues to slip away from the wall.
Every now and then there is a movement by which the protoplasm is moved
a short distance. It is moved toward the tube and finally a portion of
it with one end of the chlorophyll band begins to move into the tube.
About this time the vacuoles can be seen in an active condition in the
receptive cell. At short intervals movement continues until the content
of the supplying cell has passed over into that of the receptive cell.
The protoplasm of this one is now slipping away from the cell wall,
until finally the two masses round up into the one zygospore.
=292. The zygospore.=—This zygospore now acquires a thick wall
which eventually becomes brown in color. The chlorophyll color fades
out, and a large part of the protoplasm passes into an oily substance
which makes it more resistant to conditions which would be fatal to the
vegetative threads. The zygospores are capable therefore of enduring
extremes of cold and dryness which would destroy the threads. They
pass through a “resting” period, in which the water in the pond may be
frozen, or dried, and with the oncoming of favorable conditions for
growth in the spring or in the autumn they germinate and produce the
green thread again.
=293. Life cycle.=—The growth of the spirogyra thread, the
conjugation of the gametes and formation of the zygospore, and the
growth of the thread from the zygospore again, makes what is called a
complete _life cycle_.
=294. Fertilization.=—While conjugation results in the fusion of
the two masses of protoplasm, fertilization is accomplished when the
nuclei of the two cells come together in the zygospore and fuse into a
single nucleus. The different stages in the fusion of the two nuclei of
a recently formed zygospore are shown in figure 131.
In the conjugation of the two cells, the chlorophyll band of the
supplying cell is said to degenerate, so that in the new plant the
number of chlorophyll bands in a cell is not increased by the union of
the two cells.
[Illustration: Fig. 131. Fertilization in spirogyra; shows different
stages of fusion of the two nuclei, with mature zygospore at right.
(After Overton.)]
=295. Simplicity of the process.=—In spirogyra any cell of the
thread may form a gamete (excepting the holdfasts of some species).
Since all of the cells of a thread are practically alike, there is no
structural difference between a vegetative cell and a cell about to
conjugate. The difference is a physiological one. All the cells are
capable of conjugation if the physiological conditions are present. All
the cells therefore are potential gametes. (Strictly speaking the wall
of the cell is the _garnetangium_, while the content forms the gamete.)
While there is sometimes a slight difference in size between the
conjugating cells, and the supplying cell may be the smaller, this is
not general. We say, therefore, that there is no differentiation among
the gametes, so that usually before the protoplasm begins to move one
cannot say which is to be the supplying and which the receiving gamete.
=296. Position of the plant spirogyra.=—From our study then we
see that there is practically no differentiation among the vegetative
cells, except where holdfasts grow out from some of the cells
for support. They are all alike in form, in capacity for growth,
division, or multiplication of the threads. Each cell is practically
an independent plant. There is no differentiation between vegetative
cell and conjugating cell. All the cells are potential gametes.
Finally there is no structural differentiation between the gametes.
This indicates then a simple condition of things, a low grade of
organization.
=297.= The alga spirogyra is one of the representatives of the
lower algæ belonging to the group called _Conjugatæ_. Zygnema with
star-shaped chloroplasts, mougeotia with straight or sometimes twisted
chlorophyll bands, belong to the same group. In the latter genus only
a portion of the protoplasm of each cell unites to form the zygospore,
which is located in the tube between the cells.
[Illustration: Fig. 132. Closterium.]
[Illustration: Fig. 133. Micrasterias.]
[Illustration: Fig. 134. Xanthidium.]
[Illustration: Fig. 135. Staurastrum.]
[Illustration: Fig. 136. Euastrum.]
[Illustration: Fig. 137. Cosmarium.]
=298.= The desmids also belong to the same group. The desmids
usually live as separate cells. Many of them are beautiful in form.
They grow entangled among other algæ, or on the surface of aquatic
plants, or on wet soil. Several genera are illustrated in figures
132-137.