_Cephalodia._--In about 100 species of lichens peculiar growths are
developed in the interior of the thallus which cause a slight
projection of the upper or lower surface. These structures are known
as _cephalodia_ and they usually occupy a definite position in the
thallus. They are distinguished by possessing as gonidia algae foreign
to the ordinary part of the thallus. The foreign algae are always
members of the Cyanophyceae and on the same individual and even in the
same cephalodium more than one type of gonidium may be found. The
function of these peculiar structures is unknown. Zukal has suggested
that they may play the part of water-absorbing organs.
The exact relation of gonidia and hyphae has been investigated
especially by Bornet and also by Hedlund, and very considerable
differences have been shown to exist in different genera. In _Physma_,
_Arnoldia_, _Phylliscum_ and other genera the gonidia are killed sooner
or later by special hyphal branches, _haustoria_, which pierce the
membrane of the algal cell, penetrate the protoplasm and absorb the
contents (fig. 11, C). In other cases, e.g. _Synalissa_, _Micarea_, the
haustoria pierce the membrane, but do not penetrate the protoplasm (fig.
11, D). In many other cases, especially those algae possessing
_Pleurococcus_ as their gonidia, there are no penetrating hyphae, but
merely special short hyphal branches which are in close contact with the
membrane of the algal cell (fig. 3).
[Illustration: From Strasburger's _Lehrbuch der Botanik_, by permission
of Gustav Fischer.