The Scottish Naturalist. 101 



fluctuating, — so much so, indeed, that botanists have virtually 

 given up the attempt to discriminate species by this means. An 

 investigation of the structure of the different parts of the stem 

 and branches, taken in conjunction with the reactions by the 

 usual chemical reagents on the medullary fibres more especially, 

 has satisfied me that our knowledge of this perplexing genus may 

 be much extended ; and although I am not entirely reconciled 

 to all the results arrived at, I consider myself warranted in stat- 

 ing them broadly, leaving to future investigators the task of 

 modifying them. My conclusions have been deduced from a 

 pretty extended series of specimens (about ninety) from various 

 parts of the globe. 



In the first place, however, I should like to define, or rather 

 re-define, two species concerning which authors are by no means 

 agreed — viz., U. hirta and U. florida. 



Usnea hirta has thallus erect, rigid, either simple or loosely 

 divaricato-ramose, and everywhere covered with short, spreading, 

 usually simple branchlets or spines, analogous, in appearance at 

 least, to those of Rosa spinosissima. 



Usnea florida erect, densely and more or less intricately 

 branched at various angles, the branches more or less (in different 

 instances) divaricato-fibrillose, but not papillose. The fibrillar are 

 longer and less closely set than in U. hirta; at times nearly absent. 

 This definition is at variance with the views of Tuckerman, &c. 



My reason for amending (as I think) the distinction between 

 U. florida and hirta is, that the branchings (main and secondary) 

 are so very variable, that practically (whatever may be said to 

 the contrary) hirta is known by the abundance of soredia, more 

 especially on the branches, while in florida they are rarer, but 

 seldom absent. Another remarkable coincidence is the fact that 

 in both species these soredia are tinged a decided yellow by K. 

 In this country there are two forms of hirta ; in one, the more 

 common, the medullary fibres are tinged a decided yellow by 

 K ; in the other there is no evident reaction. The latter I have 

 also from the Himalayas. An authentic specimen of U. hirta 

 has the medullary fibres K — . 



It is evident that the possession (which I have not) of the 

 original types could not help me, inasmuch as the two forms 

 could not have been discriminated by Linnaeus. In these cir- 

 cumstances let that which has a negative reaction by K on the 

 medullary fibres still retain the name hirta, 1 while the other, 

 1 Hirta is here employed in the original acceptation. 



