* MORPHOLOGY AND PHYSIOLOGY. 49 



interior of which algae occur. Each body is in reality 

 a miniature thallus, and is capable of developing into 

 a new lichen should it find a suitable lodging-place. 

 It is the true propagative organ of lichens. The 

 majority of lichens depend almost wholly upon this 

 method of reproduction. 



It is well to bear in mind that lichens are also 

 propagated in a purely vegetative way ; that is, a portion 

 of a lichen-thallus, or even the entire thallus, may be 

 torn away and carried to a new locality, where a por- 

 tion of it will develop into a new lichen. Some lichens 

 also possess what may be designated as continuous re- 

 juvenescence ; that is, there is continuous apical growth, 

 accompanied by a continual dying away of the older 

 portion. Tliis phenomenon occurs typically in the 

 higher Cladonias as well as in Thamnolia, and is also 

 met with in the higher foliose lichens, as Parmelia. 

 The central, hence older, portion dies away, while the 

 margin continues to grow, producing an appearance 

 not unlike the "fairy rings" of certain mushrooms. 

 Nevertheless the soredia constitute the most important 

 means of propagating lichens. They are developed 

 from the algal layer of the thallus, finally pushing 

 their way through the upper cortical layer to the ex- 

 terior. Each body is known as a soredium ; a collec- 

 tion of them forming one of the patches above men- 

 tioned is designated as a soralium, plural soralia. 



2. The Cyphellce. 



Upon examining the lower surface of the thallus of 

 a Sticta or Stictina there will be found a number of 



