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i 3 8 MORPHOLOGY
the plant on account of the supposed resemblance of the dotted thallus to
the infantile ailment of " thrush." Babikoff l has published an account of
the formation and development of these Peltidea cephalodia. He determined
the algae contained in them to be Nostoc by isolating and growing them on
moist sterilized soil. He observed that the smaller, and presumably younger,
excrescences were near the edges of the lobes. The cortical cells in that
position grow out into fine septate hairs that are really the ends of growing
hyphae. Among the hairs were scattered minute colonies of Nostoc cells
lying loose or so closely adhering to the hairs as to be undetachable (Fig.
78 A). In older stages the hairs, evidently
stimulated by contact with the
Nostoc, had increased in size and sent
out branches, some of which penetrated
the gelatinous algal colony; others,
spreading over its surface, gradually
formed a cortex continuous with that
of the thallus. The alga also increased,
and the structure assumed a rounded or
lentiform shape. The thalline cortex
immediately below broke down, and
Fig. 78
the underlying gonidial zone almost
A. Hairs of Peltigera aphthosaVIi\\&.
associated with Nostoc colony much mag-
7 S S
wholly died off and became absorbed.
nified (after Babikoff). The hyphae of the cephalodium had
meanwhile penetrated downwards as root-like filaments, those of the thallus
growing upwards into the new overlying tissue (Fig. 78 B). The foreign
alga has been described as parasitic, as it draws from the lichen hyphae the
necessary inorganic food material; but it might equally well be considered
as a captive pressed into the service of the lichen to aid in the work of assi-
milation or as a willing associate giving and receiving mutual benefit.
Th. M. Fries 2 had previously described the development of the cephalodia
in Stereocaulon but failed to find the earliest stages. He concluded from his
observations that parasitic algae were common in the cortical layer of the
lichens, but only rarely formed the " monstrous growths " called cephalodia.
b. ENDOTROPHIC. Winter 3 examined the later stages of internal cephalodine
formation in a species of Sticta. The alga, probably a species of
Rivularia, which gives origin to the cephalodia, may be situated immediately
below the upper cortex, in the medullary layer close to the gonidial zone,
or between the pith and the under cortex. The protuberance caused by the
increasing tissue, which also contains the invading alga, arises accordingly
either on the upper or the lower surface. In some cases it was found that
the normal gonidial layer had been pushed up by the protruding cephalodium
1 Babikoff 1878.
2 Th. M. Fries 1866. 3 Winter 1877.