4 8 



nomic characters they are of importance only in the minor subdivisions. 

 The attempt to classify all lichens under the great divisions crus- 

 taceous, foliose and fruticose is wholly artificial and must be dis- 

 carded for a more scientific method. 



(a) The Crustaceous Type. 



As is to be expected from the origin 'and development of lichens,, 

 this thallus occurs only in the lower forms — those which have 

 not yet reached any high degree of development in the scale of 

 lichen evolution. The cortical tissue is typically wanting ; in its 

 simplest form, as met with in the lower Caliciaceae, and in all hypo- 

 phloeodal thalli, there is merely a hyphal network bearing groups of 

 algae which as a rule are quite uniformly distributed, so that there is 

 scarcely any distinction into hyphal and algal layers. It is a dorsi- 

 vental structure, although not well marked. The lower surf ace is dis- 

 tinguished from the upper by the presence of numerous hyphae 

 which serve the function of rhizoids. 



In the more highly developed crustaceous thalli three layers may 

 be recognized : i . The upper, usually a colorless hyphal network 

 devoid of living algae but containing in its meshes the cell walls of 

 dead algae, which intermingled with the hyphae aid in forming a 

 rudimentary protective covering for the underlying algal layer ; the 

 hyphae of this layer are more branched and the cell walls somewhat 

 more thickened and gelatinized than in the normal hyphae ; they 

 also have a tendency to extend in a vertical direction. 2. The layer 

 immediately beneath the foregoing consists of the algae enclosed by 

 the hyphae or united with them. On making a comparative study of 

 this layer and the algal layer of higher lichens we note the follow- 

 ing differences : The algae are in much less intimate association with 

 the fungal hyphae ; the haustoria are less numerous and less per- 

 fectly developed and rarely penetrate the algal cells ; the upper- 

 most algae continually die, while new ones are formed by direct di- 

 vision. 3. Below the algal layer is another layer of hyphal tis- 

 sue free from algae. It differs from the upper hyphal layer in 

 that the filaments are somewhat less branched, and they usu- 

 ally extend in a longitudinal direction ; the cell-walls are also thin- 

 ner and less gelatinized. It corresponds to the medullary layer of 

 the higher lichens. Below this layer are the hyphae, which perform 

 the function of rhizoids and which usually extend in a vertical direc- 



