After the plant has attained a certain stage in its growth, if it be
attentively watched, a marked change will be observed near the ends of the
filaments. The chlorophyl appears to assume a darker hue, and the granules
become more densely crowded. This appearance increases until the extremity
of the tube appears almost swollen. Soon the densely congregated granules
at the extreme end will be seen to separate from the endochrome of the
filament, a clear space sometimes, but not always, marking the point of
division. Here a septum or membrane appears, thus forming a cell whose
length is about three or four times its width, and whose walls completely
inclose the dark green mass of crowded granules (Fig. 1, b). These contents
are now gradually forming themselves into the spore or "gonidium," as
Carpenter calls it, in distinction from the true sexual spores, which he
terms "oospores." At the extreme end of the filament (which is obtusely
conical in shape) the chlorophyl grains retract from the old cellulose
wall, leaving a very evident clear space. In a less noticeable degree, this
is also the case in the other parts of the circumference of the cell, and,
apparently, the granular contents have secreted a separate envelope
entirely distinct from the parent filament. The grand climax is now rapidly
approaching. The contents of the cell near its base are now so densely
clustered as to appear nearly black (Fig. 1, c), while the upper half is of
a much lighter hue and the separate granules are there easily
distinguished, and, if very closely watched, show an almost imperceptible
motion. The old cellulose wall shows signs of great tension, its conical
extremity rounding out under the slowly increasing pressure from within.
Suddenly it gives way at the apex. At the same instant, the inclosed
gonidium (for it is now seen to be fully formed) acquires a rotary motion,
at first slow, but gradually increasing until it has gained considerable
velocity. Its upper portion is slowly twisted through the opening in the
apex of the parent wall, the granular contents of the lower end flowing
into the extruded portion in a manner reminding one of the flow of
protoplasm in a living amoeba. The old cell wall seems to offer
considerable resistance to the escape of the gonidium, for the latter,
which displays remarkable elasticity, is pinched nearly in two while
forcing its way through, assuming an hour glass shape when about half out.
The rapid rotation of the spore continues during the process of emerging,
and after about a minute it has fully freed itself (Fig 1, a). It
immediately assumes the form of an ellipse or oval, and darts off with
great speed, revolving on its major axis as it does so. Its contents are
nearly all massed in the posterior half, the comparatively clear portion
invariably pointing in advance. When it meets an obstacle, it partially
flattens itself against it, then turns aside and spins off in a new
direction. This erratic motion is continued for usually seven or eight
minutes. The longest duration I have yet observed was a little over nine
and one-half minutes. Hassall records a case where it continued for
nineteen minutes. The time, however, varies greatly, as in some cases the
motion ceases almost as soon as the spore is liberated, while in open
water, unretarded by the cover glass or other obstacles, its movements have
been seen to continue for over two hours.
The motile force is imparted to the gonidium by dense rows of waving cilia
with which it is completely surrounded. Owing to their rapid vibration, it
is almost impossible to distinguish them while the spore is in active
motion, but their effect is very plainly seen on adding colored pigment
particles to the water. By subjecting the cilia to the action of iodine,
their motion is arrested, they are stained brown, and become very plainly
visible.
After the gonidium comes gradually to a rest its cilia soon disappear, it
becomes perfectly globular in shape, the inclosed granules distribute
themselves evenly throughout its interior, and after a few hours it
germinates by throwing out one, two, or sometimes three tubular
prolongations, which become precisely like the parent filament (Fig 2).
Eminent English authorities have advanced the theory that the ciliated
gonidium of _Vaucheria_ is in reality a densely crowded aggregation of
biciliated zoospores, similar to those found in many other confervoid algæ.
Although this has by no means been proved, yet I cannot help calling the
attention of the members of this society to a fact which I think strongly
bears out the said theory: While watching a gathering of _Vaucheria_ one
morning when the plant was in the gonidia-forming condition (which is
usually assumed a few hours after daybreak), I observed one filament, near
the end of which a septum had formed precisely as in the case of ordinary
filaments about to develop a spore. But, instead of the terminal cell being
filled with the usual densely crowded cluster of dark green granules
constituting the rapidly forming spore, it contained hundreds of actively
moving, nearly transparent zoospores, _and nothing else_. Not a single
chlorophyl granule was to be seen. It is also to be noted as a significant
fact, that the cellulose wall was _intact_ at the apex, instead of showing
the opening through which in ordinary cases the gonidium escapes. It would
seem to be a reasonable inference, I think, based upon the theory above
stated, that in this case the newly formed gonidium, unable to escape from
its prison by reason of the abnormal strength of the cell wall, became
after a while resolved into its component zoospores.