vill INTRODUCTION 
In the tropics there are lichens in which the fungus belongs to 
the Hymenomycetes. 
Lichen-hyphe, as they issue from the germinating spore and 
lay hold on the alge, are thin-walled, and similar to those of 
fungi (fig. 2). The various tissues are formed by the branching 
2 and septation of these hyphe. In the 
growing regions, at the apex or edges 
of the thallus, or in the gonidial 
region, the cells remain comparatively 
thin-walled ; but in the other parts of 
the thallus, especially in the medulla, 
the walls become very thick, with the 
exception of the gelatinous lichens, in 
which the thickening is less marked. 
In the cortex of the foliose and other 
species, there is frequently a formation 
of pseudo-parenchyma (plectenchyma). 
FIG. 2.—a, Germinating spores; 1t arises from the vertical, multiseptate 
2 clasping laments. e.alzl tips of the hyphie which lie closely 
packed together and present the ap- 
pearance of cellular structure. In many cortices, more especially 
of crustaceous lichens, the walls are so swollen that the cell- 
lumen practically disappears and the tissue becomes an amorphous 
gelatinized mass. 
Strengthening elements are provided by the coherent parallel 
growth of thick-walled hyphze which form fibre-like bundles or 
chondroid strands that give support to the thallus. 
Lichen-hyphe retain many of the characteristics of those of 
the higher fungi. Pure cellulose has not*been found, the cell- 
walls being formed of various hemi-celluloses, and nearly all the 
cell-membranes contain more or less chitin. The cells enclose a 
nucleus, protoplasm, glucoses and sometimes oil-drops ; glycogen 
occurs in the cells of the reproductive system. Spheroid cells 
tilled with greenish oil are a constant feature of the lower 
rhizoidal layer of calcicolous lichens; oil cells also occur 
oceasionally in other lichens, 
Algal elements of the thallus.—The algal constituents of the 
composite thallus belong to the tavo classes—(1) Myxophyce:e, 
generally termed blue-green alge, and (2) Chlorophycex, which 
are referred to as bright-green. Most of them are aerial forms 
and, in a free condition, they inhabit moist shady situations. 
They multiply by division within the thallus; zoospores are 
