I 4 I 



or less immersed in the thallus ; sometimes almost entirely so {P. 

 Russellii). The entire apothecial structure is more or less rusty 

 colored ; there is no thalloid exciple. 



The spores are simple, elliptical, colorless, closely resembling 

 those of Biatora. 



The Psoras occur upon rock and soil and seem to be quite gen- 

 erally distributed ; by many authors they are included in Biatora 



PLATE 25. 

 Psora Russellii (Tuck.) 



1. Plants natural size upon a piece of rock; a, apothecia ; b, single 

 thallus. 



2. Thallus 'with apothecium, enlarged. 



3. Section of apothecium. 



4. Section of thallus. 



a, epidermal layer; b, dermal layer; c, algal layer; d, medullary 

 layer; e, lower limiting layer. 



5. Paraphyses and spore-sac. 



6. Spores. 



15. Gyrophora Ach. Lich. Univ. 36. 1810. 



The representatives of this genus form a remarkable group of 

 lichens, both as to size and structural characteristics. Their relation 

 to other genera is as yet not definitely determined; their life-history 

 has not been sufficiently studied to warrant any reasonable conject- 

 ures as to their phylogenetic relationships. 



The thallus, which is in all cases typically foliose, varies in size 

 from medium to very large. The smaller thalli may either be single, 

 or form a cluster, more or less imbricated. No matter what the size 

 of the thallus may be, it is always entire, fastened to the substratum 

 by the umbilicus. The margin is quite generally more or less torn 

 and frayed out or somewhat incised, rarely also bearing thalloid 

 branches or cilia. The prevailing colors of the upper surface are 

 grey to black. The lower surface is usually black and presents 

 various structural modifications, sometimes bearing black cilia (usu- 

 ally aerial), and there are generally special mechanical adaptations 

 consisting of vertical as well as horizontal plates of cortical tissue for 

 the purpose of supplying an adequate support to the large thallus. 

 (See discussion of Mechanical Adaptations, p. 81.) 



The following structures are revealed upon the examination of 

 vertical sections : The upper cortical layer is comparatively thin and 



