from its outer wall, the brown tint which it assumes,
and finally the formation of the sporidia in the ascus. The
antheridium remains for a long time recognizable without undergoing
any essential modification, but the dark colour of the perithecium
soon hides it from the observer's eye. De Bary thinks that he is
authorized in assuming the probability that the conceptacles and
organs of fructification of others of the _Ascomycetes_, including
the _Discomycetes_ and the _Tuberacei_, are the results of sexual
generation.

Certain phenomena which have been observed amongst the _Coniomycetes_
are cited as examples of sexual association. Amongst these may be
named the conjugation of the slender spores of the first generation,
produced on the germinating threads of _Tilletia_,[O] and similar
acts of conjugation, as observed in some species of _Ustilago_.
Whether this interpretation should be placed on those phenomena in the
present condition of our knowledge is perhaps an open question.

[Illustration: FIG. 101.--_Tilletia caries_ with conjugating cells.]

Finally, the spermogonia must be regarded as in some occult manner,
which as yet has baffled detection, influencing the perfection of
sporidia[P] In _Rhytisma_, found on the leaves of maple and willow,
black pitchy spots at first appear, which contain within them a golden
pulp, in which very slender corpuscles are mixed with an abundant
mucilage. These corpuscles are the spermatia, which in _Rhytisma
acerinum_ are linear and short, in _Rhytisma salicinum_ globose. When
the spermatia are expelled, the stroma thickens for the production of
asci and sporidia, which are afterwards developed during the autumn
and winter.

Several of the species of _Hysterium_ also possess spermogonia,
notably _H. Fraxini_, which may be distinguished from the ascigerous
perithecia with which they are associated by their smaller size and
flask-like shape. From these the spermatia are expelled long before
the maturity of the spores. In _Hypoderma virgultorum_, _H.
commune_, and _H. scirpinum_, the spermogonia are small depressed
black capsules, which contain an abundance of minute spermatia.
These were formerly regarded as distinct species, under the name of
_Leptostroma_. In _Stictis ocellata_ a great number of the tubercles
do not pass into the perfect state until after they have produced
either linear, very short spermatia, or stylospores, the latter
being reproductive bodies of an oblong s