LICHENS. 105 
of marine alge, which would be the first vege- 
table inhabitants of our globe, and which, on the 
emergence of the land from the water, would 
adapt themselves, by undergoing various modifica- 
tions, to their new element and circumstances. 
Lichens, I have said, are exceedingly simple in 
their construction. They are composed of two 
parts, the nutritive and the reproductive system. 
The nutritive portion is called the thallus, which, 
in the typical plant, spreads equally on all sides 
from the original point of development, in the 
form of an increasing circle ; the circumference of 
which is often healthy and vigorous while the 
central parts are decayed or completely wanting. 
It is composed of two distinct tissues. The central 
or medullary portion is composed of spherical 
cells, filled with a green matter, which seem to be 
the active vegetating part of the lichen. These 
cells called gonidia frequently accumulate in 
masses, burst through the layer above them, 
and appear in the form of a green, tenacious 
powder on the surface of the plant ; while they 
are capable, if detached from the parent, of con- 
tinuing the powers of cell-development, multi- 
plying by sub-division and spreading out into 
filaments which form the nucleus of new lichens. 
They perform in the economy of lichens an office 
analogous to that of the gemme or buds of the 
