LICHENS UNDER THE MICROSCOPE. 661 
The distinctive features of lichens consist in their having 
a thallus containing peculiar green cells, called gonidia, au] 
in their spores being contained in asci, or spore-cases. In 
the latter particular the ascomycetous fungi resemble them, 
but these are always destitute of gonidia. A bluish reaction 
of the gelatinous substance of the apothecia is also character- 
istic of most liehens, 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 structure of the 
lichens, let us take any folia- 
ceous lichen, Theloschistes pa- 
rietinus ( Fig. 139), for instance, 
the eommon orange-colored wall 
lichen, which occurs every- 
where on stones and trunks; 
and having inserted a portion 
of the thallus in a slit made in Section of thallus of Theloschistes parie- 
el, rtical layer; g, A ml, 
à piece of soft cork, with a razor medullary layers sh Inferior lay 
slice off as thin a cross-section as possible, and put it on a 
slide, with a drop of water, beneath a 
piece of thin glass, v under the lens of our 
microscope. W e shall see that it is com- 
posed entirely of cellular tissue, differing 
in this respect from those plants which 
have a vascular tissue. The upper sur- 
face, cl, we shall perceive to consist of a 
layer of cells composed of this tissue. 
poena ailas; 6, tucnilitorm Next beneath this is a stratum of round, 
em greenish yellow bodies, g, called gonidia ; 
then a stratum of elongated cells or filaments, ml, crossing 
each other in various directions, constituting the medullary 
layer; and lastly another row of cells forming the lower sur- 
F'g. 140. 
