Fink: Composition of a Desert Lichen Flora 91 



being equal. One may look through the whole list of twenty- 

 five lichens of the rocks without finding more than four species 

 with conspicuously lobed thalli. These four are Placodium ele- 

 gans, Placodium murorum, Lecanora muralis and Parmelia con- 

 spersa; and these plants, when compared with lichens of the same 

 species from more moist climates, show, as a whole, a perceptible 

 shortening of the lobes of the thalli. Many lichens having the 

 fruticose habit, as certain species of Evernia, can scarcely main- 

 tain themselves in open places, where subjected to strong gales, 

 but seek protected habitats, as in dense forests, where they will 

 not be torn from their substrata. Also, these fruticose species 

 are usually conspicuously branched and present much surface 

 and many tender, growing areas to the drying effects of winds 

 and dry atmosphere. It is, therefore, not quite certain after all, 

 until further investigation can be made, whether the restriction 

 of lichens about the Desert Laboratory to closely adnate and 

 poorly lobed or branched forms is wholly due to demand for 

 decrease of surface in contact with a drying environment, or 

 whether it is in part a mechanical response against destruction 

 by being torn from their substrata by desert gales. 



In general, the twenty-five lichens collected on the rocks of 

 Tumamoc Hill are protected above by some sort of mechanical 

 device, usually a definite pseudoparenchymatous cortex (and en- 

 closed, dead algal cells), which protects the living algal cells and 

 the fungal hyphae of the medullary layer against the drying 

 effects of high winds and the direct rays of sunlight. Zukal 

 has observed that the cortex is thicker in certain lichens growing 

 in places where exposed more than usual to intense light and dry 

 conditions than in the same species in less exposed positions.* 

 One of the most helpful studies in connection with the present 

 problem would be the comparison of some of the species with 

 lichens of the same species from regions having average con- 

 ditions of light, moisture, temperature, wind, etc., with respect 

 to development of cortex. This, with a more exhaustive study 

 of the functions of coloring matter in the cortex, would help 

 to determine whether the development of cortex in lichens is, as 

 Zukal thinks, mainly a light relation. f 



* Zukal, H. I. c. 



f Zukal, H. Op. c. 209. Mr 1896. 



