THE STUDY OF A LICHEN FROM OBAN. 
275 
be free and to have but little connection with the other parts 
of the plant. Yet they perform a very important part in 
perpetuating the species by bursting through the cortical 
layer in various ways, and under favourable conditions 
developing into a new plant like the parent. So different 
does this gonidial layer seem from the other parts of the 
plant that Scliwendener and some other botanists have 
ventured to suggest that the gonidia may be a mass of 
separate plants making the Lichen their home, thus forming a 
“dual life,” but however fascinating this theory may be it 
requires a clearer elucidation of the facts of the life history 
of the lichen to support it. 
The medullary layer is marked in in Fig. 9, and is seen 
magnified in Fig. 6 ; it consists of a mass of threads which 
on examination will be found to be tubes divided by septa; 
most probably elongated cells joined end to end. By these 
moisture is rapidly absorbed and doubtless retained for the 
future use of the plant, yet they do not seem to exhibit any 
ordinary kind of circulation. 
Below this is the subcortical layer sc, similar to the 
upper layer but not so dense, and from it bundles of filaments 
proceed called rhizinse r, which serve to attach the plant 
firmly to the bark of the tree, but here their work ends, as 
they do not perform the part of true roots in supplying 
nutriment. 
The apothecium is one of those cup-shaped discs which 
lie scattered over the central portion of the thallus in Fig. 1, 
and a section of which is given at d in Fig. 9. The outer 
portion of the apothecium is formed of the thallus, and is 
really a continuation of its various layers, including the green 
gonidial layer. 
The central portion, the liymenium or disc, is formed of 
parapliyses and asci; the parapliyses (Fig. 4) are slender 
filaments enlarged and coloured at their ends, the mass of 
which packed closely together gives the red colour to the 
liymenium. 
The asci are transparent sacs which, when mature, are 
club shaped, and contain the spores—usually eight, placed in 
a spiral manner in the ascus—but when young are slender 
and filled with protoplasm (Fig. 5). 
The spores (Fig. 3) are long, and taper at the ends, with 
a division in the middle separating them each into two cells. 
When the spores are fully ripe they are expelled from the 
summit of the ascus by the pressure caused by the swelling of 
the parapliyses and asci by the absorption of moisture, and 
probably by a simultaneous contraction of the lower portion 
