390 ECOLOGY 



fruticose structure though he thinks the latter are prevented from developing 

 by the exposure to high winds and driving sand storms. Herre's 1 study of 

 the desert lichen flora at Reno, Nevada, is full of interest. The district is 

 situated at an altitude of 4500 feet east of the Sierra Nevada Mountains. 

 The annual rainfall averages 8*2 1 inches, and a large part falls as snow during 

 the winter months or as early spring rain. The summer is hot and dry and 

 the diurnal changes of temperature are very great. Strong drying winds 

 from the west or north are frequent. 



At 5000 feet and upwards lichens are, in general, exceedingly abundant 

 on all rock substrata and represent 57 species or subspecies, only three- of 

 these being arboreal : Buellia triphragmia occurs rarely, Xanthoria poly- 

 carpa is frequent on sage brush, while Candelariella cerinella though a rock- 

 lichen grows occasionally on the same substratum. Caloplaca (Placodiuni) 

 elegans is one of the most successful and abundant species and along with 

 Lecanora (nine forms), Acarospora (seven forms) and Lecidia (five forms) com- 

 prises three-fourths of the rock surface occupied by lichens. The addition 

 of Rinodina with two species and Gyrophora with four brings the computation 

 of individuals in these desert rock formations up to nine-tenths of the whole. 

 As the desert rocks pass to the Alpine, Gyrophora becomes easily the domi- 

 nant genus followed by Acarospora, Caloplaca and Lecidea. . 



"The colouring characteristic of the rock ledges of the desert and canon 

 walls is often entirely due to lichens, and in a general way they form the 

 only brilliant plant formations in a landscape notable for its subdued pale 

 monotonous tones. Most conspicuous are Acarospora chlorophana and 

 Caloplaca elegans, which form striking landmarks when covering great crags 

 and rock walls. The next most conspicuous lichens are Rinodina oreina and 

 Lecanora rubina and its allies, which often entirely cover immense boulders 

 and northerly sloping rock walls." Herre concludes that though desert con- 

 ditions are unfavourable to most species of lichens, yet some are perfectly 

 at home there and the rocks are just as thickly covered as in regions of 

 greater humidity and less sunshine. 



f. AQUATIC LICHENS. There is only one of the larger lichens that has 

 acquired a purely aquatic habit, Hydrothyria venosa, a North American 

 plant. It grows on rocks 2 in the beds of streams, covering them often with 

 a thick felt ; it is attached at the base and the rather narrow fronds float 

 freely in the current. The gonidium is Nostoc sp., and the thallus is of a 

 bluish-grey colour ; the fruits are small discoid reddish apothecia with an 

 evanescent margin. It is closely allied to Peltigerae, some of which are 

 moisture-loving though not truly aquatic. 



The nearest approach to aquatic habit among the foliose forms in our 



1 Herre iiii 2 . 2 See p. 97. 



