PARHELIA 475 



4. Proceeding to the lower limit of the section, a 

 second band of pseudo-parenchyma is reached, which 

 is colourless, but otherwise similar to the upper. 



For comparison with the above type, other Lichens should also 

 be examined : for instance, Usnea barbata, which is frequently to 

 be found growing on the bark of trees in hilly districts : it is a 

 fruticose Lichen, with a cylindrical, much branched, grey 

 thallus, which bears near the tips of the branches the disk-shaped 

 apothecia. Cut transverse sections of the thallus and treat as 

 before : observe 



1. The irregular outline of the nearly circular section. 



2. The broad band of external pseudo-parenchyma. 



3. The gonidial layer as in Parmelia. 



4. A broad, and very lax hyphal band. 



5. A central dense strand of pseudo-parenchyma with thick 

 swollen walls. 



Thus Usnea is again an example of a heteromerous Lichen, but 

 of the fruticose type. 



Examine the gelatinous thallus of Collema pulposum, or other 

 species, which may be found growing on moist soil, stones, &c. 

 The foliaceous thallus is somewhat thick and bulky, and of a gel- 

 atinous consistency when moist ; when dry it is relatively thin, 

 and brittle. It is well to observe the changes which moistening 

 produces on dry specimens. 



Cut sections through the dry thallus, soak them in water, and 

 mount in water, or in weak glycerine : examine under a high 

 power : it will then be seen that the thallus consists of 



1. A gelatinous transparent matrix, similar to that of Nostoc 

 (see page 451). 



2. Chaplets of cells coloured greenish-blue (algal cells), occa- 

 sionally interrupted by larger cells, with thicker walls and no 

 green colour (heterocysts). 



3. Branched and colourless fungal hyphse. 



Note that the Algal and the Fungal constituents are distributed 

 uniformly throughout the thallus, the Alga not being restricted 

 to a definite zone : this is thus an example of the homobmerous 

 Lichens. 



