REPRODUCTIVE ORGANS 281 



lichen genus Lichina, considered then a kind of Fucus, and had observed 

 its similarity with true lichens. " The cavity," he writes, " at the top of the 

 fructification (in Lichina) is a proof how nearly this species of Fucus is 

 related to the scutellated lichens. Nature disdains to be limited to the 

 systematic rules of human invention. She never makes any sudden starts 

 from one class or genus to another, but is regularly progressive in all her 

 works, uniting the various links in the chain of beings by insensible con- 

 nexions." 



III. THE THALLUS 



A. GENERAL OUTLINE OF DEVELOPMENT 



a. PRELIMINARY CONSIDERATIONS. The evolution of lichens, as such, 

 has reference mainly to the thallus. Certain developments of the fructification 

 are evident, but the changes in the reproductive organs have not kept pace 

 with those of the vegetative structures: the highest type of fruit, for instance, 

 the apothecium with a thalline margin, occurs in genera and species with a 

 very primitive vegetative structure as well as in those that have attained 

 higher development. 



Lichens are polyphyletic as regards their algal, as well as their fungal, 

 ancestors, so that it is impossible to indicate a straight line of progression, 

 but there is a general process of thalline development which appears once 

 and again in the different phyla. That process, from simpler to more com- 

 plicated forms, follows on two lines : on the one there is the endeavour to 

 increase the assimilating surface, on the other the tendency to free the plant 

 from the substratum. In both, the aim has been the same, to secure more 

 favourable conditions for assimilation and aeration. Changes in structure 

 have been already described 1 , and it is only needful to indicate here the main 

 lines of evolution. 



b. COURSE OF EVOLUTION IN HYMENOLICHENS. There is but little 

 trace of development in these lichens. The fungus has retained more or less 

 the form of the ancestral Thelephora which has a wide-spreading superficial 

 basidiosporous hymenium. Three genera have been recognized, the differences 

 between them being due to the position within the thallus, and the form of 

 the Scytonema that constitutes the gonidium. The highest stage of develop- 

 ment and of outward form is reached in Cora, in which the gonidial zone 

 is central in the tissue and is bounded above and below by strata of hyphae. 



c. COURSE OF EVOLUTION IN ASCOLICHENS. It is in the association 

 with Ascomycetes that evolution and adaptation have had full scope. In 

 that subclass there are four constantly recurring and well-marked stages 

 of thalline development, (i) The earliest, most primitive stage, is the 



1 See Chap. III. 



