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REPORT OF THE COMMISSIONER OF FISH AND FISHERIES. 13
Although the species included in this large order differ from one another in size and
habit to an extent that would certainly forbid their being placed together, if we
considered merely the character of the frond, yet they resemble one another very
closely in their mode of reproduction, which is accomplished by means of zoospores.
The Zoosporeæ are divided into four suborders, the Chlorosporeæ, or
Chlorozoosporeæ, as the name is sometimes written, the Phæosporeæ, or
Phæozoosporeæ, the Bryopsideæ, and the Botrydieæ. The former are abundant in
both fresh and salt water. They especially frequent brackish waters and high tidepools.
The mass of the vegetation in brackish rivers is formed of species of this order.
The species are either filamentous or else in the form of green membranes, as in the
sea-lettuces, Ulvæ, which abound in muddy places between tide-marks. The contents
of any of the cells may be transformed into zoospores, which escape from the mother
cell usually at daybreak. The zoospores are of two kinds, microzoospores and
macrozoospores. The latter are produced few in number in the mother cell, and when
they have escaped into the water they are seen to be furnished with four cilia placed
at one end, and with a dark red spot on one side. After swimming about for a short
time they come to rest, the cilia disappear, a wall of cellulose is formed around the
zoospore, which then begins to divide and produce a plant like that from which it
came. The microzoospores are borne in considerable numbers in the mother cell, and
when they escape they are seen to have only two cilia at one end, and a dark red
spot on the side. The microzoospores, after swimming about a short time, approach
one another in pairs, occasionally in threes, which in a short time coalesce so as to
form a body known as the zygospore, or, to use a term first applied by Rostafinski,
the isospore, which has four cilia and two dark red spots. The zygospore swims about
for a short time, then comes to rest, takes on a cellulose wall, and begins to divide in
the same manner as a macrospore. This process of union is called conjugation, and
represents sexuality in its lowest form, it being impossible to say which of the
conjugating bodies is male and which is female. It is only the microzoospores which
come from different mother-cells which conjugate, but it is not quite certain whether
the cells must belong to different individuals. The microzoospores, however, do not
always conjugate. More frequently they do not, but, after swimming about
separately for a short time, lose their cilia and begin to grow just like the
macrozoospores. If one wishes to examine the zoospores, he has only at evening to
put a piece of sea-lettuce into a vessel of salt water, and at daybreak the zoospores
will have formed a green cloud in the water. If the cloud consists of microzoospores,
it will collect in the vessel on the side nearest the light; if composed of
macrozoospores, on the side away from the light. Conjugation was first observed in a
marine species (Ulva) by Areschoug, but had previously been observed by
Pringsheim in a fresh-water species (Pandorina). Since then conjugation of
zoospores has been studied by several observers.