PHYTOLOGY, 



NOTES ON THE GENUS USNEA, WITH DESCRIPTIONS 



OF NEW SPECIES. 



By JAMES STIRTON, M.D., F L.S. 



SINCE the publication in this journal for July 1881 (vol. vi. 

 p. 99) of a paper on the genus Us/iea, I have received 

 numerous additions to my stock from various parts of the 

 world. Pursuing my investigations in the direction indicated 

 in the paper, I have been led to attach considerable importance 

 to the relative thickness and density of the central axis, as well 

 as to the corresponding characters in the medullary fibres. In 

 the latter the terms compact and arachnoid and loosely arach- 

 noid are employed, and sufficiently explain themselves. In the 

 group of which U. articulata may be reckoned the centre, the 

 medullary fibres reach their minimum density, and often betray 

 themselves as merely scattered, somewhat radiating, threads 

 connecting the dense axis with the subcortical compact layer. 

 Between such extremes there is considerable diversity, but 

 meanwhile I shall confine myself to the terms compact and 

 arachnoid. 



The colour of these medullary fibres varies from white through 

 yellow to red or reddish purple, while in a few instances the 

 surface of the axis assumes a rufescent tint. 



Hitherto I have seen no valid reasons for mistrusting the 

 indications afforded by chemical tests in the discrimination of 

 species. It is true that the external characters of lichens often 

 manifest wonderful similarities in instances where such tests 

 indicate differences of physical constitution, but the same is 

 equally true of gonidia and gonimia in Stictce and Stictince, &c, 

 as well as of spores in another direction. Nay, spermogonia 

 afford differences in the life-history of lichens of sufficient im- 

 portance to warrant, according to some authors, valid grounds 

 for generic distinction. Our knowledge of the vegetative and 

 reproductive processes of this class of plants is in much too 

 rudimentary a condition for us to assign to any means of diag- 



