DISK LICHENS | 215 
some species have more or less elongated branching 
stems. 
341. Lichens propagate by the escape of some of the 
algal cells, with attached fungal filaments by means of 
eruptive areas (‘“‘soredia’’) on the plant body. When 
one of these comes to rest upon a favorable substratum 
it grows directly into a lichen plant body like the original. 
Asexual spores appear to be wanting. 
342. The sexual organs as far as known remind one 
of those of the Red Algae. The oogone, which is a spiral 
coil of cells, sends up a slender trichogyne to the surface 
of the plant body. Fertilization takes place by means of 
minute non-ciliated sperms which are 
produced in countless numbers in nearby 
cavities (spermogones) in the plant body. 
The sperms come in contact with the 
projecting trichogyne (doubtless aided % 
by water) and fertilize the oogone, the ti6. 967 C3imual 
result of which is the rapid upward 
growth of filaments, the enlarged terminal cells of which 
become asci. Mingled with the asci are long sterile cells 
(paraphyses) for the protection of the asci and ascospores 
in the hymenial layer, which forms a more or less disk- 
shaped, or cup-shaped fruit. Such open fruits are known 
as ‘“‘apothecia,”’ in contrast with the closed fruits (‘‘ peri- 
thecia’’) of many of the fungi to be taken up later. 
343. The ascospores germinate by sending out one or 
more tubes which develop directly into the ordinary fila- 
ments of the lichen-body. Experiments have shown that 
these filaments will not grow for any great length of time 
unless they come into contact with green algae of the 
proper species, to which they become attached, growing 
rapidly and surrounding them. On the other hand, in 
the moist tissues thus formed the green algae find protec- 
