o_19nkc155213bd1lg5uoe1rn4pi0a.pdf

LICHENS UNDER THE MICROSCOPE. 667
The distinctive features of lichens Consist in their having
a thallus containing peculiar green cells, called gonidia, and
in their spores being contained in asci, or spore-cases. In
the latter particular the ascomycetous fuIngi resemble them,
but these are always destitute of g(onidia. A bluish reaction
of the gelatinous substance of the apothecia is also characteristic
of most lichens, though in some it is brown or red.
In the fungi the reaction with Fig. 139.
iodine is yellow, except in a
very few instances, where it is
blue.
In order to investigate more
closely the strLUCtUlre of the
lichens, let us take any foliaceous
lichen, Theloschistes pa-
2'ietinus (Fig. 139), for instance,
the common orangre-colored wall
lichen, which occurs everywhere
on stories and trunks;
and having inserted a portion
of the thallus in a slit made im Section of thallus of Theloschistes parieci;
el, cortical layer; g, goniija; ml,
a piece of soft cork, with a razor medullary layer; si, inferior layer.
slice off as thin a cross-section as possible, and put it on a
F-g. 140. slide, with a drop of water, beneath a
piece of thin glass, under the lens of our
microscope. We shall see that it is comnposed
entirely of cellular tissue, differing
iii this respect from those plants which
-have a vascular tissue. The upper surface,
ci, we shall perceive'to consist of a
liyer of cells composed of this tissue.
Colleina leptlietem; a, section Next beneath this is a stratum of round,
of thallus; b, monfiliform
gonidia.
greenish yellow bodies, g, called gonidia;
then a stratum of eloingated cells, or filaments, mel, crossing
each other in various directions, constituting the medullary
layer; and lastly another row of cells forming the lower sur-
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