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i6 ZYGNEMATACEAE
specimens with mature spore walls can be named with any degree
of satisfaction. Not infrequently the ornamentation can be determined
only by crushing and separating the several layers of the
spore wall. In the matter of dimensions, the student should
remember that every species is a complex of clones with cells of
slightly different sizes and proportions. These may be in part
smaller or larger than the dimensions given in the key. The
dimensions given in the descriptions of species are either those
of the original collection, or the dimensions as they may have
been modified by the study of subsequent collections. How great
a departure from the original is necessary to warrant the segre-
gation of a new species cannot be determined by any simple rule.
Study and restudy of many collections will show that some species
are only narrowly variable, while others vary within wide limits.
These remarks are equally applicable to the species of other genera
of the Zygnemataceae.
In the descriptions of species the characteristics of the outer
spore wall are usually omitted since there are only a few species
in which they are not thin, smooth, colorless, and transparent.
The median spore wall may be variously colored and ornamented.
I have tried to distinguish among punctate, scrobiculate, and pit-
ted according to the size of the pits, and in many instances have
been able to give the diameters of the pits and the distances between
them in microns. Errors sometimes occur in descriptions
and figures where contraction ridges and irregular folds are mis-
taken for ornamentation. These may be due to contraction of the
spore contents either by drying or by plasmolysis, and may be
recognized readily by applying a dilute solution of potassium
hydroxide. Real structural patterns are enlarged and made more
distinct by this treatment, while contraction ridges disappear. In
studying dried specimens it is well first to wet them with water
on the slide and then to apply a drop of lactic acid, and after that
to heat the slide until the acid begins to boil. This treatment will
remove calcium carbonate and clarify both the walls and cell
contents of most species. Some of the older species were described
as having smooth median spore walls, although recent study of
the type specimens has shown them to be punctate. This circum-
stance is probably explained by the poor resolving power of the
microscope lenses of the last century as compared with those
available today.