444 DR LAUDER LINDSAY ON THE 



acquired colour only from the parasite, with which they have come into acciden- 

 tal contact (fig. 42). 



The other form occurs as maculae, more or less raised and difform, sometimes 

 apparently destroying the superficial tissues of the apothecium or thallus — some- 

 times simply seated thereon (fig. 39 b). These maculae have much the external 

 characters of the Celidia of the Stictce. They contained no spores, and were in an 

 imperfect and unsatisfactory condition for determination. It remains for Local 

 Botanists to determine whether the parasitic spermogones, perithecia, and 

 maculae above described are referable to the same plant, or to different species, 

 genera, or orders. 



The nature or relations of these isolated forms of fructification cannot be 

 understood without comparison with a suite of European and foreign specimens of 

 the same or allied species of Usnea, on which I have found at various times simi- 

 lar parasites in more perfect conditions. Of these it will be most instructive first 

 to describe the highest forms of the most nearly allied parasites — those which 

 exhibit sporiferous apothecia. 



1. Sub. nom. Abrothallus Usnea?, Rabenhorst exs. (" Lichenes Europsei") 

 No. 537; sent me some years ago by Dr Rabenhorst himself from Germany; a 

 fine specimen, abounding in the parasitic apothecia in every stage of their growth ; 

 affecting var. florida, L., of Usnea barbata, Ft., in abundant fructification. The 

 parasite, if all its diverse forms are referable to one species, equally affects the 

 thallus from base to apex; the apothecia normal and degenerate — upper and 

 under surface ; and the cephalodia of the Usnea. 



The latter, as they occur in the genus Usnea, are so little known, and their 

 true nature so much misunderstood, that they demand some description before . 

 we proceed further. I have repeatedly met with them both in British and foreign 

 forms of U. barbata, and in foreign allied species, and I was long puzzled with 

 their true character. A careful examination of a series of specimens leads me to 

 concur with Nylander (Syn. p. 15), in regarding them as warts or excrescences 

 of the cortical layer or tissue of the thallus, having, however, a modified struc- 

 ture, consisting of a very dense, finely striated (filamentous) tissue, not unlike 

 that of many hymenia, where the paraphyses are very closely aggregated; a 

 circumstance, which, along with the frequently pale-bluish or violet coloration 

 with iodine, gave certain grounds for the apparently common belief that the 

 growths in question were really degenerate apothecia. This coloration with iodine 

 is, however, a fallacious indication of hymenial structure; for, in the cortical 

 tissue of the thallus of the Usnea on which the Abrothallus is parasitic — far re- 

 moved from all forms, both of apothecia and cephalodia — towards the base of 

 attachment of the plant, I sometimes met with a distinct, though pale, violet-blue 

 colour under iodine. I doubt* not that the thallus of other of the higher Lichens, 

 containing Lichenine in their tissues, will be found to yield a similar reaction. 



