806 HALV-HOtTRS IN THE GREEN LANES. 



find the leading genera figured and coloured. 

 Many species of lichens have long been used ir 

 the arts for dyeing purposes ; whilst others are 

 esteemed equally valuable for medicine. They are 

 the first possessors of the soil, and their decay 

 often lays the foundation on which plants of a 

 higher structure can better flourish. Of late years, 

 the theory has been started that lichens are double 

 organisms a kind of terrestrial algae, which have 

 imprisoned the spore-like fungi. 



Section of Physcia parietina. a. Cortical Stratum ; 6. Gonidic 

 Stratum ; c. Medullary Stratum. 



Whilst treating on objects whose structure can 

 only be seen by microscopical aid, let us not omit 

 another class whose occurrence is frequently only 

 too common. They are parasitic fungi, which occur 

 as black pustules on dead twigs or leaves. Fig. 248 

 is an excellent example of these lovely objects as 

 seen when magnified. It occurs on the twigs of the 

 beech, where the pustules form small orifices, with 

 a ragged mouth, within which may be seen a black 



