5o 



lures to resist the lateral tearing and lifting force of air currents 

 (see discussion of mechanical adaptations). 



Foliose thalli are, as a rule, typically dorsiventral in structure. 

 The upper portion, turned toward the sunlight, is primarily adapted 

 to perform the function of assimilation ; the lower portion is adapted 

 to conduct food substances. There are also remarkable relative 

 adaptations of mechanical support and physiological function. The 

 anatomy of the foliose thallus has been explained in the discussion of 

 the thallus. 



(c) The Fruticose Tyfe. 



As the type of the fruticose thallus I have'selected Usnea barbata 

 in which it attains its highest development. In structure it is per- 

 fectly radial. There are, however, all gradations between a typi- 

 cally foliose and typically fruticose thallus. For convenience of 

 classification I have included under fruticose all vertical ascending 

 or pendant, structurally centric, thalli having flattened or cylindrical 

 branches and which are attached at one point (umbilicus). They 

 may be sparingly branched, as in Pilofthoron, or much branched, as 

 in Stcreocaulon and Usnea. As already indicated, there may be 

 marked tendency toward jlorsiventrality, as in Cetraria and Rama- 

 Una; it is less marked in Evernia. Typically radial structures are 

 met with in Pilofhoron, Stereocaidon, Bryofiogon, Alectoria, podetia 

 of Cladonia, etc. The general histological characters of this thallus 

 are similar to those met with in the foliose type ; hence we shall con- 

 tent ourselves with a description of the arrangement of tissues and a 

 hasty comparison of homologous tissue elements. 



There is no tissue deserving the name of epidermis. The outer 

 cylindrical layer which corresponds to the cortical layer of the 

 foliose thallus is not typically cortical in structure. The hyphae 

 usually extend at right angles to the longitudinal axis ; sometimes 

 they extend longitudinally. The cells are longer, the cell-walls much 

 thicker and more gelatinized, the cell lumina more deficient than in 

 the foliose cortex. Branching of the hyphae does not frequently 

 occur. Gelatinization of the cell walls sometimes proceeds to such 

 a degree that scarcely any structure can be discerned ; this is espe- 

 cially true in those cases where the hyphae extend at right angles to 

 the longitudinal axis and are considerably branched. If the hyphae 

 extend longitudinally, as in Theloschistes and Ramalina, there is less 



