194 



POPULAR HISTORY OF LICHENS. 



It often grows beside a following species, P. stettaris, to 

 some varieties of which it bears a close resemblance. It 

 would appear that these species sometimes graduate into 

 each other not only in the characters of the thallus, but in 

 those of the spores, which in both resemble the spores of 

 Physcia ciliaris, with the exception that they are usually 

 smaller. In P. pulverulenta the spores we have found to 

 differ in size in specimens from different localities ; they are 

 sometimes as large as those of Pkyscia ciliaris, having a 

 form like the figure of 8, being bilocular and dark-brown 

 when mature, apple-green when younger. The protoplasmic 

 amorphous contents of the young thecse contain frequently 

 a considerable quantity of oil-globules. Under a lens the 

 spermogones may be recognized about the centre of the 

 thallus in the form of pruinose cones, or of tubercles having 

 a cracked and stellate apical pore ; the former are isolated, 

 the latter aggregated spermogones. Their tissue is whitish, 

 hygrometric, and dense ; their interior divided into sinuous 

 cavities or compartments ; their spermatia linear, straight, 

 and so numerous that when mixed with a drop of water they 

 immediately render it turbid. 



7. Parmelia stellaris {stella, a star) differs from the 

 preceding chiefly in the thallus being naked, not pruinose, 



