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LICHEN ASCI AND SPORES 189
The development of the thickened wall of polarilocular spores has been
studied by Hue 1
, who
contends however that there is no true septation in
the colourless spores so long as the central canal remains open. According
to his observations the wall of the young spore is formed of a thin tegument,
everywhere equal in thickness, and consisting of concentric layers. This
tegument becomes continually thicker at the equator of the spore by the
addition of new layers from the interior, and the protoplasmic contents are
compressed into a gradually diminishing space. In the end the wall almost
touches at the centre, and the spore consists of two polar cell-cavities with
a narrow open passage between. A median line pierced by the canal is
frequently seen. In a few species there is a second constriction cleavage
and the spore becomes quadrilocular.
Hue insists that this spore should be regarded as only one-celled; for
though the walls may touch at the centre, he says they never coalesce. He
has unfortunately given no cytological observations as to whether the spore
is uni- or binucleate.
In Xanthoria parietina, one of the species with characteristic polaribilocular
spores, germination, it would seem, takes place mostly at one end
only of the spore, though a germinating tube issues at both ends frequently
enough to suggest that the spore is binucleate and two-celled. The absence
of germination from one or other of the cells only may probably be due to
the drain on their small resources. Hue has cited the rarity of such instances
of double germination in support of his view of the one-celled nature of the
spore. He instances that out of fifteen spores, Tulasne 2 has figured only
three that have germinated at each end; Bornet 3
figures one in seven with
the double germination and Bonnier 4 one in sixteen spores.
spores.
Further evidence is wanted as to the nuclear history of these hyaline
In the case of the brown spores, which show the same thickening
of the wall and restricted cell-cavity, though with a distinct median septum,
nuclear division was observed by Rend Maire 5 before septation in one such
species, Anaptychia ciliaris.
II. SECONDARY SPORES
A. REPRODUCTION BY OIDIA
In certain conditions of nutrition, fungal hyphae break up into separate
cells, each of which functions as a reproductive conidium or oidhim, which
on germination forms new hyphae. Neubner 6 has demonstrated a similar
process in the hyphae of the Caliciaceae and compares
formation described by Brefeld 7 in the Basidiomycetes.
1 2 Hue 1 . 191
5 Maire 1905.
2 Tulasne 1852.
6 Neubner 1893.
3 Bornet 1873.
it with the oidial
4 Bonnier i889 2 .
7 Brefeld 1889.