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4
ZYGNEMATACEAE
gametes is by means of a tube between the gametangia, or sometimes
by a mere opening between the adherent gametangia. The
zygospores are formed either in the tube or within one of the
gametangia.
FAMILY 2. MESOTAENIACEAE
Vegetative cells are solitary, cylindric or spindle-shaped, or
sometimes loosely united into filaments within an amorphous
pectic gel or pectic sheath, and without wall pores. Chromato-
phores are stellate, platelike, or ribbonlike and spirally arranged.
Conjugation of gametes occurs through tubes of cellulose, or
within a pectic gel.
FAMILY 3.
DESMIDIACEAE
Vegetative cells are solitary, or sometimes are united into
simple filaments. Vegetative cells have highly diversified and
sometimes bizarre forms, but all have transversely segmented
walls and wall pores. In most genera the cells have a median con-
striction or isthmus between the two nearly symmetrical halves.
There may be one or several chromatophores in each semicell
with a nucleus in the isthmus. During cell division, after the
nucleus divides, the isthmus elongates and the two semicells are
separated by a median wall. On both sides of this wall new semi-
cells develop from the isthmus. Conjugation is by gametes that
emerge from their respective cell walls and unite within an amorphous
pectic sheath or in a conjugating tube. (For further details
see G. W. Prescott, "Desmids," Botanical Review, 14 [1948],
pp. 644-76.)
The Family ZYGNEMATACEAE Smith 1933
The species belonging to the Zygnemataceae are probably more
numerous, and are more generally distributed over the earth than
those of any other family of filamentous green algae. The tangled
green masses of algae floating on ponds, ditches, and slow streams
anywhere are sure to contain representatives of this group.
All the species have cylindric cells during the period of vege-
tative growth. During the reproductive period, however, the cell
walls of some species grow and change their size and form in a
most astonishing manner. In some species the gametangia alone
are affected. The receptive gametangia in particular may become
distended on all sides, on the conjugating side only, or only on