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SPIROGYRA 127
may be widened and nearly disappear, leaving the tubes "appar-
ently formed by the male gametangia."
Another matter that must be determined early in the use of
the key concerns the growth of the female, or receptive, gametangia
during conjugation. Do they remain cylindric, or become
slightly enlarged, or greatly distended or inflated ? The two latter
conditions we judge by the enlargement relative to the thickness
of the spore. If the spore just fits the distended part of the gametangium
it is enlarged. If gametangial walls are much more
distended than the diameter of the spore, the gametangium is
inflated. The inflation may be only on the conjugating, or inner,
side, or it may be only on the opposite, or outer, side. In many
species the inflation is on both sides.
The gametangial walls are not distended by the spore walls.
Contrariwise the spore dimensions and forms are often limited by
the gametangial walls. One can find many examples to prove this
where spores formed in small gametangia are variously deformed
by the rigidity of the gametangial wall.
Another interesting fact is that there is no proportional relation
between the combined volumes of the gametangia and the volumes
of the spores in different species. Spore sizes in the same
species, however, are usually larger or smaller depending upon
whether the volumes of the gametangia are larger or smaller than
the average.
It is noteworthy that small spores in large gametangia are not
free to move about as the cell is turned. The spores in all the
Zygnemataceae are more or less fixed in position by an extremely
dilute pectic gel that fills the entire cavity of the "empty" gam-
etangia and sporangia. If these spaces were filled with water alone
the position of the spores could be changed readily.
The last essential feature of the description of a Spirogyra is
the number of spore walls, the color, and surface markings of
each. The kinds of ornamentation are best shown by illustrations.
Not all drawings are equally satisfactory, but some of the most
difficult have been sketched with remarkable accuracy.
Spore wall features should be seen through an oil immersion
lens, so that one can be sure to distinguish between the shadows
of spore contents and actual wall structures. In drawing spores
the contents should be omitted, since they are of no taxonomic
value, while the wall characteristics have great importance.