5i 



difficulty in tracing the outlines of the cells, provided the sections are 

 sufficiently thin. Sometimes this cortical layer is not continuous; 

 that is, it occurs in patches corresponding to the patches of algae be- 

 neath. There is no doubt that the prime function of this layer is 

 protective and preventive of excessive evaporation of moisture. 



Immediately following the cortical layer is the algal layer which 

 is quite generally more deficient than the corresponding layer in the 

 foliose thallus. The association of the algae and hyphae is similar 

 to that described in the foliose type. As indicated above, this layer is 

 not always continuous. In the less highly developed fruticose thalli, 

 as Pilofhoron, the algal layer occurs in patches corresponding to 

 the superimposed cortical areas. The medullary laver is usually 

 deficient, except in Alectoria and Bryofogon, where it fills the en- 

 tire central cavity as a very sparingly branched network of hyphae. 

 Structurally it is not different from the medullary layer of foliose 

 thalli. 



In the majority of fruticose thalli the medullary layer is followed 

 by a mechanical and conducting tissue of longitudinal hyphae. This 

 tissue may form either a hollow cylinder, as in the podetia of Cladonia, 

 or a solid strand, as in Usnea. The cells are much elongated, very 

 sparingly branched, and extend parallel to the longitudinal axis of 

 growth. The cell-walls of this tissue are much less gelatinized than 

 those of the cortical cells. 



In fruticose thalli built on the plan of a hollow cylinder, we find 

 numerous transverse supporting tissues, consisting of bundles of 

 hyphae. Their special function is mechanical ; that is, they prevent 

 the collapse of the cylinder due to sudden changes in turgor as well 

 as to lateral pressure. 



The question whether the fruticose or foliose thallus is the more 

 highly differentiated is rather difficult to decide. From a purely 

 structural point of view it would seem that the fruticose thallus is the 

 higher. There are several well-marked differences between the 

 foliose and fruticose types. The most important is the predominance 

 of structural adaptations favoring the assimilative function in foliose 

 thalli, while the mechanical adaptations predominate in fruticose 

 thalli. There is little doubt that the fruticose thallus has been 

 phylogenetically derived from the higher crustaceous forms ; per- 

 haps, in some cases, from some of the lower foliose forms. The 

 differences here indicated will be more fully explained in subsequent 

 chapters. 



