116 PKACTICAL BOTANY. 



them in glycerine, and examine under a low power: 

 observe 



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 (com- 

 pare Collema, pages 117-118). 



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 constituent 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 



