LICHENS. 117 



hyphse come into very close communication with 

 the gonidia. ' 



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,^ which is 

 to be found growing on moist soil, stones, &c. The foliaceous 

 thallus is somewhat thick and bulky, and of a gelatinous consist- 

 ency 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 95). 



2. Ohaplets 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 



