PDF - CES (IISc)

LICHEN GONIDIA 27
F. COMPOSITE NATURE OF THALLUS
Schwendener 1 meanwhile was engaged on his study of lichen anatomy.
Though at first he adhered to the then accepted view of the genetic connection
between hyphae and gonidia, his continued examination of the
vegetative development led him to publish a short paper 2 in which he
announced his opinion that the various blue-green and green gonidia were
really algae and that the complete lichen in all cases represented a fungus
living parasitically on an alga: in Ephebe, for example, the alga was a form
of Stigonema, in the Collemaceae it was a species of Nostoc. In those lichens
enclosing bright green cells, the gonidia were identical with Cystococcus
humicola, while in Graphideae the brightly coloured filamentous cells were
those of Chroolepus (Trentepohlia). This statement he repeated in an
appendix to the larger work on lichens 3 and again in the following year 4
when he described more fully the different gonidial algae and the changes
produced in their structure and habit by the action of the parasite: "though
eventually the alga is destroyed," he writes, "it is at first excited to more
vigorous growth by contact with the fungus, and in the course of generations
may become changed beyond recognition both in size and form." In support
of his theory of the composite constitution of the thallus, Schwendener
pointed out the wide distribution and frequent occurrence in nature of the
algae that become transformed to lichen gonidia. He claimed as further
proof of the presence of two distinct organisms that, while the colourless
filaments react in the same way as fungi on the application of iodine, the
gonidia take the stain of algal membranes.
G. SYNTHETIC CULTURES
Schwendener's "dual hypothesis," as it was termed, excited great interest
and no little controversy, the reasons for and against being debated with
considerable heat. Rees 5 was the first who attempted to put the matte*. to
the proof by making synthetic cultures. For this purpose he took spores
from the apothecium of a Collema and sowed them on pure cultures of Nostoc,
and as a result obtained the formation of a lichen thallus, though he did not
succeed in producing any fructification. He observed further that the
hyphal filaments from the germinating spore died off when no Nostoc was
forthcoming.
Bornet 6 followed with his record of successful cultures. He selected for
experiment the spores of PJiyscia (Xanthoria) parietina and was able to
show that hyphae produced from the germinating spore adhered to the free-
1 Schwendener 1860, etc.
4 Schwendener 1869.
2 Schwendener 1867.
5 Rees 1871.
3 Schwendener 1868, p. 195.
6 Bornet 1872.