f
to end in such a way as to form polygonal and mostly pentagonal
meshes, the size of which varies with the age of the plant. These cells,
which are closely conjoined, but have no passage-ways between them,
are capable of independent life, so that the Hydrodictyon may be looked
npon as an elaborate type of a cell-family, one in which cells are conjoined
in accordance with a definite plan, so as to make a body of definite
shape and size, yet in which each cell is an independent being, drawing
nothing from its neighbours. The cells themselves are cylindrical, with
a thickish cellulose wall, and having no nuclei. Their chlorophyllous
protoplasm is granular, and is placed in tho exterior portion of the cell,
forming thus, within the outer wall, a hollow cylinder, in which are imbedded
starch granules, and whose interior is occupied with watery
contents. The Hydrodictyon cell, when once formed, is capable of growth,
but not of going through the usual process of cell multiplication by
division, so that the adult frond is composed of just as many and, indeed,
the same cells as it had in its earliest infancy.
“ No true sexual reproduction has as yet been discovered in thewater-
nets. There have been described, however, two forms or methods in
which the species multiplies, both of them occurring by means of motile
zoosporoid bodies. ^ In the one case these develop immediately into the
new plant, whilst in the other, before doing so, they pass through a
resting stage. Of the life history of the latter, the miorogonidia, I
have no personal knowledge.
“ The investigation of the production and development of the macro-
gonidia, however, has occupied considerable of the time devoted by
myself to the microscope, and I have seen large numbers of specimens
in almost all the stages of development. I have never been able to
detect any decided motion in the macrogonidia.
“ They are formed in the protoplasmic stratum already alluded to as
occupying the outer portion of the interior of the Hydrodictyon cell.
The first alteration in this, presaging their formation, is a disappearance
of the starch granules, and a loss of the beautiful transparent green
colour. Shortly after this, even before all traces of the starch-grain
are gone, there appear in the protoplasm numerous bright spots placed at
regular intervals; these are the centres of development, around which
the new bodies are to form. As the process goes on, the chlorophyl
granules draw more and more closely around these points, and at the
same time the mass becomes more and more opaque, dull, and yellowish
brown in colour. This condensation continues until at last the little
masses are resolved into dark hexagonal or polygonal plates, distinctly
separated by light, sharply defined lines. In some the origi iial bright
central spot is still perceptible, but in others it is entirely obscured by
the dark chlorophyl. The separation of these plates now becomes more
and more positive, and they begin to become convex, then lenticular,
and are at last converted into free, oval, or globular bodies. When
these are fully formed they are said to exhibit a peculiar trembling
motion, mnbually crowding and pushing one another, compared by A.
Braun to the restless, uneasy movement seen in a dense crowd of
people in which no one is able to leave his place. Whilst the process
just described has been going on, the outer cellulose wall of the Hydro-
dictyon cell has been undergoing changes, becoming thicker and softer
and more and more capable of solution, and by the time the gonidia
are formed it is enlarged and cracked, so that the room is afforded
them to separate a little distance from one another within the parent-
cell. Now the movements are said to become more active—a trembling
jerking which has been compared to the ebullition of boiling water.
There is, however, with this a very slight change of space, and in a very
short time the gonidia arrange themselves so as to form a little net
within the parent-cell, a miniature in all important particulars of the
adult Hydfi'odictyon. Tho primary cell wall now becomes more and
more gelatinous, and soon undergoes complete solution, so that the
new frond is set free in its native element.
“ I t is evident that when the species is mnltiplied in the way just
described the birth of the new frond is consentaneous with the death
of the old cell. But when the Hydrrodiotyon disappear in the fall, it is
months before they reappear in the spring. I t is, therefore, evident
there must be some other method of reproduction. This slow development
of new fronds takes place, according to Pringsheim, by means
of little motile bodies which he calls Dauerschwarmer, which has been
translated in English Chronispores {statospores, Hicks). M, Braun stated
already some years since that sometimes, instead of the Hydrodictyon
producing the ordinary reproductive bodies (macrogonidia), there are
formed in the cells much smaller and more active bodies, the micro,
gonidia. The changes which occur in the production of these are very
similar to those already described as happening when the macro-
gonidia are formed. When the chronispores are formed, however, they,
instead of uniting together, escape in a free, distinct condition with
the water. They are now small ovate bodies, with a large anterior
transparent space, to which are attached a pair of cilia, and their life
and history, according to Pringsheim, is as follows :—For a few hours
they move about very actively in the water, and then, dropping their
cilia, and acquiring an outer cellulose wall, pass into a quiescent stage,
in which they closely resemble Protococcus granules. They are capable
of living in this state for a long time if kept in water. They can also
endure dessication if the light be excluded during the process, but if it
be present, they wither and die, and cannot be revivified.
“ After a longer br shorter period, but never shorter than three months,
according to Pringsheim, they recommence their life, provided they be
in water. For four or five months after this the chief change consists
simply in an increase in size. The dark green protoplasm is arranged
around the exterior of the cell ; within are the more fluid colourless contents,
the whole body still looking like a Protococcus cell. After a size
of about mm. is attained, the endochrome divides successively into
several portions. The external layers of the surrounding wall now give
way in some spot, and allow the inner layers to protrude and form a sort
of hernial sac, into which the several endochrome masses soon pass, at
the same time assuming the well-known characters of J:rue zoospores.
From two to five of these bodies are thus produced out of each original
microgonidium. They are large, ovate, biciliate, and, generally, soon
escaping from the hernial sac, move about actively in the water for a
few minutes. Sometimes, however, they settle down within the generative
utricle. In either case, after a little time, they become motionless,
lose their cilia, and develop into polyhedral cells, which are structurally
remarkable for having their angles prolonged iuto long, horn-like appendages.
Under favourable circumstances, at the end of a few days,
the bright green endochrome of these undergoes similar changes to
those described as presaging the production of the microgonidia, and is
finally formed into zoospores, which, in from twenty to forty minutes,
unite, within the polyhedron or large cells, into Hydrodictyon, which is
finally set free by a solution of the cellulose coat of the polyhedron. The
network thus formed differs in no essential way from that which arises
in the better known way, except that it is composed of much fewer cells.
It is generally a closed sac ; but when the polyhedrou, out of which it is
developed, is small, it is sometimes merely an open network. Its afterhistory
appears to be identical with that of the ordinary HydroâÀctyon
frond.”—Hr. H, C. Wood, “ American F. Water Algce.”
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