MORPHOLOGY AND PHYSIOLOGY. 39 



SECTION IV. 



THE MORPHOLOGY AND PHYSIOLOGY OF 

 LICHENS. 



We have already explained, in a general way, the 

 morphological structure and the functional activities 

 of lichens. We shall now enter into a somewhat more 

 special comparison of the different lichen-forms. This 

 is necessary to a better understanding of the relative 

 functional activities of lichens and their tendency in 

 the scale of evolution. We will find that the higher 

 lichens are more highly specialized as chlorophyll- 

 bearing organisms, and tluit the evolutionary ten- 

 dency is away from tlie forms resembling fungi. We 

 will find that there is a reduction in the spore-produc- 

 ing function, accompanied by an increase in the assim- 

 ilative function. 



I. THE THALLUS. 



The thallus of lichens is the purely vegetative portion 

 of the plant. It performs the function of assimilation, 

 and in it the phenomena of growth are actively mani- 

 fest. It is comparable to the vegetative portion of 

 other cryptogams, such as mosses and liverworts. In 

 its gross appearance and general texture, it is, how- 

 ever, very characteristic ; so that a very casual ac- 

 quaintance with lichens enables one to distinguish 

 them from all other thallophytes, as the lower flower- 

 less plants are technically known. 



