402 DIVISION III.—MODE OF LIFE OF THE FUNGI, 
therefore be separated from it without much injury; and the erus/aceous (thallus 
crustaceus, lepodes), a flat crust on or in the substratum and adhering firmly to it 
at least by its whole under surface, so that it cannot be separated from it without 
injury. The genera Cladonia and Stereocaulon are peculiar, having shrub-like 
formations (fodelia) rising from scaly or granular foliaceous bodies (the thallus or 
protothallus of the Lichenologists). These forms all agree in two points of structure ; 
they are composed, as has been said, of hyphae and of algal cells inserted between 
them, and from the first commencement of their formation hyphal branches from the 
base or surface turned towards the substratum enter the subtratum as rhzzoid-hyphae, 
which sometimes, especially in foliaceous species, are united into rhizoid-strands. 
These organs by which the plant attaches itself to the substratum and takes up food- 
material are termed by Lichenologists 
rhizines, and they often penetrate, 
especially in species living on rock or 
soil, as much as a centimetre deep into 
the ‘substratum and spread through it in 
dense ramifications. The rhizoids of 
thesp ecies which live in the rind of trees 
seem never to penetrate far into it, at 
all events they do not reach the living 
tissue of the rind. In many species, 
which, like the Graphideae examined by 
Frank and Lecanora pallida, form thin 
crusts in the rind of trees, we find in 
place of distinct rhizoid-branches the 
system of hyphae described above, which 
spreads in superficial periderm-layers 
without going further inwards. These 
hyphae may form a persistent thallus 
which grows all its life entirely in the 
FIG. 171. Usnea barbata. A optical longitudinal section of periderm along with the Algae which it 
the extremity of a thin branch of the thallus which has become a ni 1 
ae ren ta GollMlca of peice GaiFig: Sn. Beans eee has attacked, as in the above-mentioned 
section through a stronger branch with the point of origin of an mi - . 
adventitious branchsa; 7 cortical layer, # medullary layer, x stout and similar Graphideae and ın Pyr enula 
ae ae" # apex ofthe branch.  nitida, and it may be covered by one or 
more layers of periderm-cells, in which 
case it is termed by Lichenologists Aypophlocodic; or it makes its way out to the outer 
surface of the periderm in consequence of the subsequent growth in thickness of 
the parts which contain the Algae, and forms there an epiphlocodtc crust. 
In other points of structure differences appear, which have no simple relation 
to these growth-forms, but may be repeated in each of them. Lichenology since 
Wallroth’s time has accordingly distinguished between the Aeferomerous and the 
homotomerous thallus. The former is peculiar to the ‘true Lichens’ (Lichenes of 
Fries, Lichenaceae of Nylander, Gnesiolichenes of Massalongo), the latter to the Phyco- 
lichenes of Massalongo (Collemaceae of Nylander, Byssaceae of Fries). The hetero- 
merous thallus occurs in the large majority of species, and displays in fact a structure 
the main features of which can be clearly defined. The homoiomerous on the contrary 

