372 ECOLOGY 



different thalli and fruits mingling together. Some forms such as Rhizo- 

 carpum geographicum which have a very close firm thallus do not break away. 

 In the course of time lichen communities come and go, and the plants of 

 one locality may be different from those of another for no apparent reason. 



The question of colonization^ was studied by Bruce Fink^ on a "riprap" 

 wall of quartz, 30 years old, built to protect and brace a railway in Iowa. 

 Nearby was a grass swamp which supplied moisture especially to the lower 

 end of the wall. A few boulders were present in the vicinity, but the nearest 

 lichen "society", was on trees about 150 metres away and these bore corti- 

 colous Parmelias, Physcias, Ramalinas,Placodiums, Lecanoras and Rinodines 

 which were only very sparingly represented on the riprap. Moisture-loving 

 species never gained a footing; the extreme xerophytic conditions were 

 evidenced by the character of the lichens, Biatora myriocarpoides {Lecidea 

 sylvicold) occupying the driest parts of the wall. Lower down where more 

 moisture prevailed Bacidia imindata and Stereocaulon paschale were the 

 dominant species. Some 30 species or forms were listed of which 1 1 were 

 Cladonias that grew mainly on debris from- the disintegration of the wall. 

 With the exception of two or three species the number of individuals was 

 very small. 



Some of these lichens had doubtless come from the boulders, others from 

 the trees ; the Cladonias were all known to occur within a few miles, but 

 most of the species had been wind-borne from some distance. The Stereo- 

 caulon present did not exist elsewhere in Iowa ; it had evidently been 

 brought by the railroad cars, possibly on telegraph poles. 



A similar wall on the south side of the railway, subject to even more 

 xerophytic conditions but with less disintegration of the surface, had a larger 

 number of individuals though fewer species. Only one Cladonia and one 

 Parmelia had gained a footing, the rest were crustaceous, Buellia myriocarpa 

 being one of the most frequent. 



There are two types of rock of extreme importance in lichen ecology: 

 those mainly composed of lime (calcareous), and those in which silica or 

 silicates preponderate (siliceous). They give foothold to two corresponding 

 groups of lichen communities, calcicolous and silicicolous. 



c. Calcicolous. The pioneer in this section of lichen ecology is 

 H. F. Link, who was a Professor of Natural Science and Botany at Rostock, 

 then at Breslau, and finally in Berlin. He" published in 1789, while still at 

 Rostock, an account of limestone plants in his neighbourhood, most of them 

 being lichens. In a later work he continues his Botanical Geography or 

 " Geology " and gives more precise details as to the plants, some of which 

 are essentially calcicolous though many of them he records also on siliceous 

 rocks. 



1 See also p. 254. 2 jri^i^ ,pQ^_ 3 Lj^k 1789. 



