I 4 2 



shows some resemblance to the cortical tissue of Psora in that the 

 cell-walls are much gelatinized and cell-lumina reduced ; the upper- 

 most cell-layers are dark colored, due to a deposit of coloring sub- 

 stance in and upon the cell-walls ; the algal layer is quite deficient 

 for so large a thallus and consists of the alga Cystococcus humicola, 

 enclosed by haustoria ; the medullary layer, which is quite normal 

 in structure, is also very deficient ; the lower cortical layer is, how- 

 ever, enormously developed, which is in distinct contrast to the ma- 

 jority of foliose lichens. The outermost layer of this tissue as well 

 as of the mechanical plates is dark colored. The lower cortical 

 layer is, however, not of uniform thickness, and in places may be 

 quite thin. It is usually dotted with warty elevations. 



The umbilicus is a short stem-like structure of cortical tissue 

 supporting the thallus. From its base the long branching root-like 

 rhizoids extend deep into the substratum ; these are in all respects 

 closely analogous to roots in the higher plants. 



The apothecia are very peculiar in structure. Each apothecium, 

 or apothecial group, consists of numerous short vertical, twice or 

 thrice dichotomously branched apothecial stalks. The stalks them- 

 selves are miniature fruticose thalli, analogous to the podetia of 

 Cladonia, growing from the upper surface of the thallus proper. 

 Structurally they consist of a thin external gelatinous or semicortical, 

 dark-colored hyphal tissue, the interior being occupied by a medul- 

 lary tissue bearing a few algae. The apothecia proper are terminal 

 on the stalks, and are very simple in structure, as will be seen from 

 a study of the plate ; no trace of a thalloid exciple is present. 



The apothecial branches are so closely united that the apothecial 

 patch has the appearance of a single apothecium with a convex con- 

 volute disk, adnate upon the thallus. The entire apothecial structure, 

 inclusive of the stalks, appears black to the naked eye. 



The spores are simple, elliptical, colorless ; their plasmic con- 

 tents are sometimes more dense at the middle, producing the sem- 

 blance of a septum. 



The Gyrophoras are northern in their range but are also quite 

 common in the mountainous regions of the temperate and torrid 

 zones. They occur principally upon rock and soil. The genus is 

 usually united with Umbilicaria. 



PLATE 26. 



GVROPIIORA MUHLENBERGII (Tuck.) 



i. Portion of thallus natural size; a, apothecial groups; b, umbilicus. 



