ae * ny ’ - ' oe 7 := sie 
N ' *y 
- 
; 
286 ~— Structural ana Physiological Botany... — | 
like that of Fungi, of rows of cells which form a fibrous — 
weft or pseudo-parenchyma. In addition to this there are — 
also round or oval cells of a green or blue-green colour, +. 
the gonidia or gemme. ‘Three types of structure of the 
thallus may generally be distinguished : the Stratified, the 
Pictorial, and the Gelatinous, furnishing the characteristics 
of these three descriptions of Lichens ; but there are some 
anomalous forms which cannot be included under either of 
these types. 
Sections through the thallus of the stratified or AHesero- 
merous Lichens (Fig. 409 11.) almost always show two distinct 
layers of tissue, a comparatively thin, usually transparent 
but close outer layer or cortex, and an inner looser hyphal 
layer enclosed by the former, the medulla. ‘The two layers 
are, however, formed of ramifications of the same filaments. 
At the line where they meet the gonidia almost always con- » 
stitute a zone of variable thickness, the gonzdzal layer (Fig. 
Aog 11. g). In the Arutecose Lichens, the cortex commonly 
constitutes a layer of uniform thickness all round; while in 
the Folzaceous forms it is usually different on the upper surface 
exposed to the light, and on the under surface. The thallus 
of the Crustaceous Lichens presents an affinity to the 
latter form in being fixed to the substratum by capillary or 
 pristle-like rAzzines, so that it cannot be removed without 
injury. , 
The Pictorial Lichens or Graphidez form thin patch- 
Fic. 410.—Graphes elegans, a Pictorial Fic. 411.—Chain of gonidia of a Pictorial os Py 
Lichen (natural size). ‘Lichen, Graphis scripta (x 390.) 
