INTRODUCTION 
THE LICHEN-PLANT. 
LicHEns are a class of thallophytes of lowly organization that 
inhabit soil, rock, wood, trees, etc. The vegetative thallus is of 
varying form and colour ; the reproductive organs are akin to 
those of fungi. They differ from all other members of the 
vegetable kingdom in their composite structure, being formed 
from the union in intimate symbiotic relationship of two separate 
plants, a fungus and an alga. This can easily be demon- 
strated in any part of the thallus or 
vegetative structure; a thin section, 
examined under the microscope, shows 
a ground structure of colourless cells 
or hyphe, the fungal elements ; and a 
series of green cells which are the 
alge. If the latter are confined to a 
narrow zone near the upper surface, 
the thallus is termed heteromerous (see 
figs. 5, 6), but if they are scattered 
more or less evenly through the thallus 
it is then described as homoiomerous  yyq. 1.—Section of homoio- 
(fig. 1). merous barre: Collema. 
/@ This old division of lichens into 
heteromerous and homoiomerous does not entirely correspond 
with the modern more exact system of classification based on 
the reproductive organs; but the type of structure thus indi- 
cated is generally well marked, and a valuable character in 
identification. 
Fungal elements of the thallus.—These have undergone con- 
siderable modification as lichen constituents, and cannot as a 
rule be traced back to any particular species or genus of fungi. 
The British lichens, however, are all associated with Ascomycetes, 
and approximate to certain groups within that class of fungi. 
Te b 
