Download PDF

TAXONOMIC CHARACTERISTICS 5
the opposite side. The bulges may be rounded, or cylindric with
shoulders at both ends, or spindle-shaped. In any event the
resulting forms of the expanding cell walls are remarkably uniform
in a given species. In other species many of the nonreproducing
cells enlarge and become distended or even bullate. These
changes in cell form are definitely the result of hereditary factors.
In some environments one may also find vegetative filaments
composed of variously rounded or distorted cells, even with rhi-
zoidlike outgrowths, in extreme instances. These collections, in
my experience, have always been gathered from ponds with warm
water and low oxygen pressure. Since these effects were noted on
several of the species present, the causes are probably environ-
mental.
The following key to the genera is based primarily on the
characteristics of the chromatophores, and secondarily on the
changes that take place in the reproductive cells during spore
formation.
In the subsequent keys to the species grouped under each genus
stress is laid on the characteristics of the spores. Consequently one
must be quite clear about the nature of these several kinds of
reproductive structures.
The simplest, most universally present, and probably the most
primitive type of propagating cell is the akjnete (PI. I, Figs. 1-3).
This is usually just a vegetative cell, the walls of which have been
thickened by additional layers of cellulose, or cellulose and pectose.
These cells survive long periods of drought, high and low tem-
peratures, as well as the effects of these factors on the concen-
tration of solutes in their immediate environments. Akinetes
germinate readily and initiate new filaments. Only one species
(Zygiiema sterile) has been found to reproduce by this method
alone. There are probably many others but they are not so easy
to identify as this one, which happens to be the largest species of
Zygnema in the Ohio valley.
There is a prevalent notion that akinetes are formed when
conditions are unfavorable. It has been our experience that they
develop both when conditions seem most favorable and when
least favorable. In cultures, they appear both at high temperatures
and low temperatures. The causes for the initiation of akinetes
are best stated as unknown.
At high altitudes and latitudes the species of Zygnemataceae