Fink : Composition of a Desert Lichen Flora 93 



apparently developing gradually from these centers with con- 

 tinued exposure and thus being more abundant in older portions 

 of the thalli. 



The isidioid branches were unusually well developed in Par- 

 melia conspersa from the desert; and while these branches, of 

 course younger than the horizontal thalli on which they developed, 

 showed only slight and occasional development of the dark lines, 

 they usually showed a brownish coloration at their exposed and 

 tender, growing tips, after the manner of coloration of the tips of 

 branches in many fruticose thalli, which is doubtless protective. 

 Placodium elegans showed more or less of the development of 

 darker areas in lighter thalli of the species ; but it was not noted 

 in any other thalli of general dark color, these thalli having suf- 

 ficient coloration of the general surface for protection against 

 intense rays of light in the desert. Nor was it noted that the 

 thalli of any of the twenty-five species were otherwise more deeply 

 colored than lichens of the same species from other regions, 

 except through the development of the black areas and spots, 

 which we must conclude are a protective device. 



A very instructive study would be the observation of the rela- 

 tion of development of the black areas to the position of the 

 particular plant on the rocks with reference to the sun's rays. 

 Another problem of interest would be the study of the relation of 

 development of coloration at the tips of the branchlets in Par- 

 melia conspersa to the orientation of these branchlets with refer- 

 ence to the sun's rays. 



Seriatim Consideration of Structure 



We may now consider seriatim the structure of the thalli in the 

 genera most frequently seen in the collections sent for study. 

 End o car pis cum is pseudoparenchymatous throughout, and the 

 algal cells within are thus well protected against too intense 

 light or too much transpiration of moisture, or both. Acaro- 

 spora is also cellular throughout, so that the algal cells are un- 

 usually well protected for such small thalli. Like most other 

 lichens whose thalli contain blue-green algal cells, Endocarpis- 

 cum seems to be confined to the most moist situations in the 



