404 DIVISION III-—MODE OF LIFE OF THE FUNGI. 
medullare). Both layers belong to the hyphal system, their constituents are ramifi- 
cations of the same hyphae. The cells of the Alga are in almost all cases inserted 
where the medulla and the rind meet. Together they form a green zone of varying size 
projecting into the medullary tissue to a different depth at different points, everywhere 
traversed by single hyphae of the medulla running to the rind, and in some places 
showing larger interruptions. This is the algal layer or algal zone which is commonly 
termed the third tissue-layer. But single algal cells or groups of cells are often found 
scattered through the medulla, as in Solorina and Placodium, or the whole mass of 
algal cells is distributed with tolerable uniformity through the medulla, as in Bryopogon. 
Sundry modifications of this type and small deviations from it occur in different 
species and groups of species. The following are the most important ones :— 
Some fruticose thalli, especially if cylindrical in form, are covered all round with 
a uniform rind, as in Usnea (Fig. 171), 
Bryopogon, Roccella, Sphaerophoron 
and others. In many fruticose forms 
with a flat thallus (species of Evernia 
and Cetraria) and in most foliaceous 
forms the rind of the upper surface 
which is towards the light is different 
from that of the lower surface (Figs. 
172, 173); in Anaptychia, Peltigera, 
Solorina and most of the foliaceous 
portions of the thallus in the Cladonieae 
the upper side only has the rind up to 
the margin, the lower surface has no 
rind. 
When there is this difference in the 
two surfaces, the algal zone apart from 
the scattered cells only appears on the 
side towards the light. Even in the 
atc, Lele nt, Kiron ie mat Tas 4 almost cylindrical thallus of Sphaero- 
vertical longitudinal section through the same. 7 black marginal phoron with a uniform rindall round it, it 
bands, g groups of Algae, 2p A slighty Circum- = 
ference of B magn. 45 times, but the whole completed from greater is often more strongly developed onthe 
re side towards the light than on the lower 
; one. The podetia of Cladonia, which 
according to Krabbe! are to be regarded as strongly developed parts (envelope-appara- 
tus) of apothecia but which always have the structure and growth of a fruticose thallus, 
are always without a rind when young, and in some species, as C. rangiferina, never 
have a rind, their surface being formed of a loose hyphal weft containing Algae. Most 
species are at a later period of their life partially covered with small wart-like rind-scales, 
and some are completely covered with a rind, as C. furcata. 
Some of the crustaceous forms, for example the genera Psora, Psoroma, Placodium, 
Endopyrenium &c. and Thalloidima candidum are only distinguished from the folia- 
ceous by having their under surface covered all over with rhizoid-hairs which penetrate 
into the substratum. These hairs either spring as branches from the elements of a 
distinct rind, as in species of Placodium and Endopyrenium, or where the rind 
is wanting on the under surface they are direct continuations of the medullary hyphae. 
Other crustaceous Lichens, those namely with areolate surfaces or which have a 
granular appearance, have essentially the same structure in the fully developed areolae 
as the species mentioned above which are without a rind on the under surface. Some of 



1 See on page 222. 
