CHAPTER IV. 



THE GROWTH, MECHANICS AND CHEMISTRY OF 



LICHENS. 



INTRODUCTION. 



In order that the student may have a better understanding of the 

 special discussion of modes of growth and development as well 

 as of mechanical adaptations, it is well to present a few explanatory- 

 statements, which will obviate the necessity of continually referring 

 to the exceptions to the types of growth under consideration. 



In the majority of lichens the fungal symbiont initiates and di- 

 rects the mode of growth. The algae are entirely enclosed and take 

 a position dependent upon the development of the hyphal portion as 

 well as upon the requirements of assimilation. This position also 

 controls their rate of growth and development in any lichen or in any 

 area of the plant; in other words, there is a mutual adaptation of 

 the two symbionts, so that the prime structural changes occur in the 

 fungal portion and the prime functional activities are manifest in the 

 algal portion. It seems as though there were some sort of agree- 

 ment that one specialized portion should do the advance work in 

 growth and supply the mechanical protection and support, while the 

 other portion, by its highly specialized functional activity, should pre- 

 pare the principal food substances. 



In a few lichens, as Ephebc pubescens, the algal portion predomi- 

 nates and directs the mode of growth. It is, of course, evident that 

 such lichens resemble the alga from which the algal symbiont is de- 

 rived, and Ephebe -pubescens is quite frequently classified as Sirosi- 

 phon pulvinatus ; a careful microscopical examination only will de- 

 cide whether the plant be lichen or alga. 



There are also a few lichen-groups in which neither symbiont 

 seems to predominate (examples : Collema, Lichina) . In Collema 

 we find hyphae and algae (JVostoc) almost equally intermingled ; 

 neither seems to take the lead in development, although there is con- 

 siderable mutual adaptation. 



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