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CHAPTER ONE
GENERAL TAXONOMIC CHARACTERISTICS
The Order ZYGNEMATALES G. M. Smith 1933
The plants belonging to this order of the Chlorophyta differ
from all other green algae in the absence of free swimming flagel-
lated gametes and spores; sexual reproduction is consummated by
amoeboid gametes through, or within, a tube, pectic sheath, or an
enveloping pectic mass.
The plants consist of single cells, of loose cell aggregates within
a pectic gel, or of unbranched or very sparsely branched, and
usually undifferentiated, filaments. The cell walls have an outer
pectic layer and an inner cellulose layer. The spore walls consist
of at least three layers, the inner and outer of which are of cellu-
lose, and the median wall of cellulose with varying amounts of
chitinous deposits. The median wall may be colored yellow,
brown, or blue.
The chromatophores may consist of axial or parietal plates, or
spirally arranged parietal ribbonlike structures, or of two axial
stellate bodies often highly diversified among the desmids.
The zygospores have a dormant period during which there is
a fusion of the gametic nuclei followed by a reduction division.
At the time of germination the four resulting nuclei may each
become the center of a new cell, or two or three of the nuclei may
disintegrate and only two or one sporelings emerge from the
spore wall.
The Zygnematales are naturally divided into three families
which have, briefly, the following characteristics:
FAMILY I. ZYGNEMATACEAE
These are filamentous plants, usually unbranched, and have
cylindric cells. The walls of the vegetative cells are unsegmented
and without pores. The chromatophores are either axial
more or less stellate, ribbonlike or platelike bodies; or are parietal
platelike or spirally arranged ribbonlike bodies. Conjugation of