AN INTRODUCTION TO THE STUDY OF LICHENS, 89 
The horizontal thallus embraces the crustaceous and the foliaceous 
species. The crustaceous (fig. 12,) predominates in number over 
——— = — 
Fig. 12. 
all other forms. It closely adheres to the surface on which it 
grows. In growing, it sometimes assumes a determinate shape, 
and has a distinct margin when it is called determinate or uniform. 
When it spreads itself irregularly it is said to be effuse. The thick- 
ness of the thallus, and the condition of its surface, show many 
modifications ; which are all considered in the distinguishing of 
species. The foliaceous or frondose thallus is the most leaf-like of 
all lichen-growths, and is considered by some as the highest lichen 
development. It is a flat, light grey or green expansion. Its 
margin varies in different species. Sometimes they are irregularly 
torn into lobes or /acerated, sometimes the lobes are crenate or 
sinuate, and upturned. The margins or lobes are at other times 
cut into /acini@, these are narrow and linear, or sinuate segments. 
The foliaceous thallus is most frequently green above, and light or 
dark coloured beneath. On the under side it is sometimes villose, 
or else has a number of short fibres or bundles of fibres called 
rhizine. These have the appearance of small rootlets, but do not 
act as such; they simply fix the plant to the matrix or place of 
growth. Within these general distinctions, the Lichens show many 
features and variations, some of which are characteristic and per- 
manent, others dependent and changing. But these can only be 
understood by the initiated, yet they are gradually and soon 
learned by the careful and diligent student. 
The structure of the lichen thallus is much alike in all forms; 
but there is a difference between the gelatinous family, and the 
rest. In the large family of Lichenacei or non-gelatinous plants, 
the thallus is almost invariably stratified, and consists of three dis- 
tinct layers of cellular tissue. But in the family of Collemacei, 
while there are found the three forms of tissue just referred to, yet 
they are not stratified. The hyphoid and green cells being mingled 
together throughout the thallus. In one branch of this family— 
the Collemas, there is no distinct cortical layer. Another point of 
this difference between these two families, is observed in the 
