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126 ZYGNEMATACEAE
cells, or the thickening may be unequally distributed and modify
the usual cell form. (PI. I, Fig. i.) Aplanospores are formed by
the contraction of the contents of vegetative cells and the growth
of a new and distinctive spore wall.
The "spore wall" is really a complex of three to five walls, one
or more of which may be variously sculptured or ornamented.
The outer wall is of cellulose and is usually colorless and trans-
parent. Sometimes the outer wall is made up of two colorless
layers, either of which may be thick and sculptured. The median
spore wall at maturity is distinguished by its yellow to chestnut
brown color resulting from chitinous deposits in or on the cellu-
lose. This is the spore wall most frequently sculptured and ornamented.
In some species the median wall also consists of two
distinct chitinous layers. The inner spore wall is a thin cellulose
layer lining the median wall and seldom seen, except when the
spores are crushed. The median and the outer walls have a suture,
more or less distinct, encircling the spore. At the time of germi-
nation these walls split along this line, while the inner wall
enlarges with the protoplast and becomes the wall of the one
celled sporeling. The walls of zygospores are similar to those of
aplanospores.
The forms of Spirogyra spores are of three general types:
ellipsoid (like an American football), ovoid (like a watermelon),
and lenticular (a compressed spheroid). These forms may be
slightly modified by elongation or compression but the curvatures
of the polar ends remain the same. Usually there is no difficulty
in recognizing the spore form of the 155 species with ellipsoid
spores, and the 77 species with ovoid spores. There are 13 species,
however, in which there is greater variation, and the form may
be on either side of the border line between ovoid and ellipsoid.
These are usually cared for in the keys by listing under both types.
Compressed-ellipsoid, and compressed-ovoid spores are rare but
do occur. The deviation of "lenticular" spores (30 species) from
the form of a sphere may be as slight as an orange, or as great as
a pocket watch. Obviously such spores are nearly circular in face
view, and ovoid in side view.
Conjugating tubes are usually formed by both gametangia
(228 species) but in 31 species they are outgrowths of the male
gametangia. In a few species in which the receptive gametangia
become greatly distended the part of the tubes formed by them