_- -.-) se. 6h— CU 
ii 
witness the strife which has gone on now for twenty years as to 
whether Lichens are a distinct class, or only slightly modified 
Fungi. 
It will not be without use, nor I hope without interest, therefore, 
if I delay you a short time longer to consider these plants under 
the aspect revealed by the microscope. 
When sufficiently thin sections of a Lichen thallus are placed on 
the stage of this instrument, and examined with an adequate 
power, we perceive that in every instance they are made up of 
cells of two kinds, one of which contains chlorophyll, or some 
modification of it, while the contents of the other are transparent. 
In this respect Lichens agree with all higher plants, a few species 
excepted, and with the Algz, but they are completely different 
from the plants known as Fungi. 
The function of chlorophyll, which gives the green appearance to 
vegetation, you are doubtless acquainted with. It is by its means 
that plants are enabled to assimilate the carbonic oxide of the air 
and so to obtain materials for building up their structure. Plants 
destitute of it are, like animals, unable to live on inorganic sub- 
stances, and are compelled to make use of organic matter elaborated 
previously by some chlorophyll-producing plant. Thus Fungi live 
on vegetable and animal matter already organized, and it is to this 
necessity that their destructive—and also in many cases their 
useful—action in the general economy of the world depends. 
Though some of them do us very considerable damage, as in the 
case of “dry rot,” which destroys our timber, and in the various 
blights which make a prey of our crops, yet there are others whose 
activity is beneficial, either as in the case of the yeast plant by 
altering and preparing for us useful products, or in other instances 
by removing or changing putrid matter which might become a 
source of danger. 
But to return to our Lichens. All of them possess cells con- 
taining chlorophyll or some modification of it, but the manner in 
which the green cells are arranged among the clear or non- 
chlorophyllous cells is not the same in all. In the larger number 
of Lichens the green cells occupy a definite position and appear 
2 
