474 PRACTICAL BOTANY 



1. The irregular outline of the section. 



2. The greater part of the thallus made up of 

 colourless transparent tissue (fungal tissue). 



3. The green gonidial layer, which is not very 

 regular or sharply limited, and is situated at some 

 distance below the upper surface of the thallus : 

 since the gonidia are confined to a certain region 

 of the thallus, and not distributed uniformly through 

 it, this may be distinguished as a heteromerous 

 Lichen. 



4. The rhizines, which may be seen as irregular 

 brushes of colourless tissue projecting from the lower 

 surface. 



Examine the thinnest section on the slide under a 

 high power, and passing successively from the upper to 

 the lower surface, observe 



1. The upper broad band of pseudo-parenchyma, 

 the outer limiting part of which is coloured yellow : 

 the cell-walls of this tissue are swollen, and the cell- 

 contents not voluminous or obvious : this band passes 

 without any clearly marked limit into 



2. The gonidial layer, where may be seen, em- 

 bedded in the rather more lax, colourless, fungal tissue, 

 round cells with definite cell-walls, and green-coloured 

 contents the gonidia, or Algal constituents of the 

 Lichen thallus. 



3. Passing downwards, the colourless tissue of the 

 thallus becomes more lax, and has a distinctly 

 hyphal character, betraying more clearly here 

 its Fungal characters. Note carefully how the 

 hyphse come into very close communication with 

 the gonidia. 



