THE BOTANY OF THE SOUTH ORKNEYS. 27 



another, this lichen appears to be crustaceous. The exposed parts of the plant are light 

 yellow or orange coloured, hut those more hidden are paler, and in part even white. 

 The lowest portions of the podetia can obtain a thickness of about 1'5 ram., the tips 

 being as much as 1 mm. across. The podetia measure up to 2 cm. in height, and are 

 generally cylindrical in section near the margin. Near the margin of the whole thallus 

 they generally assume a more typical Placodium structure. The marginal podetia show 

 a dorsi-ventral arrangement, the short assimilators springing from the upper side 

 only. But even here, near the margin, the dorsiventral and free podetia can lie 

 distinguished perfectly from the protothallus, which is firmly attached to the rocky 

 substratum. 



The protothallus consists of fine strands of fungal hyphse, which, white in colour, 

 radiate out in an irregular manner from the base of the podetia. At this latter point 

 the protothallus is often very thick. 



The gouidia are fairly evenly distributed in the podetia, where these are exposed 

 to light, but the gonidia are massed together at those points where a new branch or an 

 assimilator is about to sprout. 



The general structure of the apotheciuni is that typical of Placodium species. It is 

 up to 4 mm. in diameter, with orange epithecium and distinct thalline margin, which, 

 however, gradually sinks below the level of the epithecium. The light hyaline spores 

 are polar-bilocular ; parathecium and amphithecium are colourless, and green gonidia 

 are found under the hypothecium. 



This plant is very nearly related to Placodium coralloides, Tuck. (Synopsis of the 

 North American Lichens, i. p. 169), and P. dadodes, Tuck. (foe. cit.). It differs from 

 the latter by having colourless spores in each ascus instead of one brown one. It is 

 also stouter and bigger than both species of Tuckerman. I have only seen specimens of 

 P. coralloides. The big apothecia of P. regale also retain their amphithecium through- 

 out life. 



Placodium regale is an interesting plant which belongs to the subgenus Thamnoma 

 of Placodium, created by Tuckermau for his species coralloides and cladodes. The 

 thallus is throughout distinctly diploblastic, the protothallus being easily separated 

 from the podetia, even when the latter are prostrate, near the margin of the plants. 



Several species of Placodium have a tendency to become fruticulose. Thus in 

 H. Lojka Lich. Regni Hung, exsic. i. (1882), n. 26, Lccanora elegai/s Lk. v. compacta 

 (Arn.) Nyl. ( = Placodium) shows fruticulose podetia in the centre of the thallus. 



XANTHORIA LYCHNEA (Ach.), Th. Fr., North and South America, North Asia and 

 Europe. A number of small plants were found between some podetia of Placodium 

 regale. Rocks in Scotia Bay, South Orkneys. 



Some fragments of crustaceous lichen are amongst the material brought from the 

 South Orkneys, which, however, it is impossible to identify at present. 



But disregarding these, we have before us, brought back by the Scottish National 



