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264
BIONOMICS
several different hosts. In some instances the thallus is apparently quite
unaffected by the presence of Abrothallus, in others, as in Cetraria glauca,
there is considerable hypertrophy produced, the portion of the thallus on
which the parasites are situated showing abnormal growth in the form of
swellings or pustules which may be regarded as gall-formations. Crombie 1
points this out in a note on C. glauca var. ampullacea, figured first by
Dillenius, which is merely a swollen condition due to the presence of
Abrothallus.
The internal structure and behaviour of Abrothallus has more recently
been followed in detail by Kotte 2 . He recognized a number of different
species growing on various thalli of Parmelia and Cetraria, but Abrothallus
Cetrariae was the only one that produced gall-formation. The mycelium of
the parasite in this instance penetrates to the medulla of the host lichen as
a loose weft of hyphae which are divided into more or less elongate cells.
These send out side branches, which grow towards the algal cells, and by
their short-celled filaments clasp them exactly in the same way as do the
normal lichen hyphae. Thus in the neighbourhood of the parasite an algal
cell may be surrounded by the hyphae not only of the host, but also by
those of Abrothallus. The two different hyphae can generally be distinguished
by their reaction to iodine: in some cases Abrothallus hyphae take
the stain, in others the host hyphae. In addition to apothecia, spermogonia
or pycnidia are produced, but in one of the species examined by Kotte,
Abrothallus Peyritschii on Cetraria caperata, there was no spermogonial
wall formed. The hyphae also penetrate the host soredia or isidia, so that
on the dispersal of these vegetative bodies the perpetuation of both organisms
is secured in the new growth.
Abrothallus draws its organic food from the gonidia in the same way as
the host species, and possibly the parasitic hyphae obtain also water and
inorganic food along with the host hyphae. They have been traced down
to the rhizinae and may even reach the hypothallus, but no injury to the
host has been detected. It is a case of joint symbiosis and not of parasitism.
Microscopic research has therefore justified the inclusion of these and other
forms among lichens.
d. PARASYMBIOSIS' OF FUNGI. There occur on lichens, certain parasites
classed as fungi which at an early stage are more or less parasymbionts of
the host ;
as growth advances they may become parasitic and cause serious
damage, killing the tissues on which they have settled.
Zopf 3 found several instances of such parasymbiosis in his study of
fungal parasites, such as Rhymbocarpus punctiformis, a minute Discomycete
which inhabits the thallus of Rhizocarpon geographicum. By means of
staining reagents he was able to trace the course of the parasitic hyphae,
1 Crombie 1894.
2 Kotte 1910.
3
Zopf 1896.