200 



As P. stellaris has no soredia, any lichen resembling it, but 

 having soredia, should be compared with P. caesia, also with 

 P. astroidea and P. sorediata. The substance of P. caesia is thinner 

 than P. sorediata, the pale gray color considerably brighter. The 

 sorediate masses rest on the center of the branch rather than on 

 its margin, and, most important, the pith, when the surface is 

 scratched away, is white, not yellow. The smaller P. astroidea 

 has soredia loosely scattered rather than compact. 



Physcia astroidea. Smaller Star Lichen 



Found occasionally on trees or rocks, forming much-branched 

 rosettes about 2 or 3 cm. across, with parts less than 1 mm. wide 

 and almost uniformly flat, lying close to the foothold. The color is 

 pale gray or whitish. Toward the center appear loosely clustered 

 granules and soredia. The under surface is white, with many pale, 

 root-like holdfasts, but in var. hypomela this surface and its hold- 

 fasts blacken. 



Fruits up to 1.5 mm. in diameter, black or frosted whitish, with 

 a wavy, narrow, gray rim. Spores 2-celled, brown, 17 to 25 by 

 7 to 11 microns. 



P. astroidea is a difficult subspecies intermediate between the 

 two variable species P. stellaris and P. tribacia, and is seldom 

 clearly determined, especially because the young rosettes of the two 

 other species abound on the bark where it grows. The presence of 

 soredia, together with its small size and thin texture, separate it 

 from P. stellaris, but only the blunt tips, which lie rather flat, 

 separate it from P. tribacia, whose tips rise, and divide into lacey 

 points, dusted white. In contrast to P. caesia, which besides is 

 almost confined to rocks, the white soredia of P. astroidea are not 

 massed densely in large clusters, but usually loose aggregations 

 of granules scattered over the vague central crust. It may some- 

 what resemble the larger Cetraria aleurites (Group 5), which how- 

 ever has the tips shiny instead of dull, while the central prongs and 

 granules are without soredia, therefore not dusty. 



Physcia tribacia. Lace-Tipped Lichen 



A tiny species covering large areas everywhere on trees and 

 rocks equally. So small are the parts in some specimens that they 



