xii INTRODUCTION 
a thin structureless mixture of hyphe and algz. Though mostly 
superficial the crustaceous thallus is, in certain genera or species, 
partly or wholly embedded in the bark or rock on which it 
grows. It is thus often difficult to recognize the different tissues. 
The lower hyphz in many of the superficial species form a thin 
spreading layer called the hypothallus; it is usually dark in 
colour and often appears as a black border to the thallus, either 
as a firm limiting line or as dendritic filaments. A patch of 
crustaceous lichen on tree or rock may belong to one species and 
yet be composed of many individuals which have started from 
different centres, each growing centrifugally. The dark lines. 
chiefly occur where the different individuals encounter each 
other. <A striking instance of such intersecting lines occurs in 
the thallus of the well-known Rhizocarpon geographicum. Strong 
boundary lines also frequently divide different species inhabiting 
the same substratum. 
2. Foliose, foliaceous or leafy lichens; as the name implies, 
these are spreading leaf-like expansions of one or many lobes, 
which adhere more or less firmly to the support on which they 
grow. Like the crustaceous forms, they are dorsiventral in 
structure; the upper surface is provided with a distinct 
hyphal cortex, beneath which lies the gonidial zone and the 
medulla. There is also a lower cortex which is frequently of a 
darker colour and which is mostly provided with hairs or with 
rhizoids formed of strands of hyphe that are chiefly organs of 
attachment. 
3. Fruticose or shrubby lichens: these are upright branching 
forms rising from a basal point, and they are either cylindrical or 
strap-shaped. The structure is radial, with a central pith of 
fungal hyphe, a surrounding band of alge and an outer cortex 
of fungal elements. There is considerable variety of form in 
this group from the short, stiff, strap-shaped lobes of some 
Ramaline to the long pendulous thread-like strands of Alectoria 
or Usnea. Intermediate forms connect these different groups. 
In the Cladoniacez there is a basal thallus, either crustaceous or 
of small lobes, and there is also an upright stalk called a 
“podetium,” which in many cases opens out into a cup-like 
structure called a “ scyphus.” The podetia may be simple or 
branched, and as the reproductive bodies in this family are borne 
on the tips of the branches or on the edges of the seyphi, the 
upright thallus has been frequently regarded as a modified fruit- 
stalk. 
