Fink : LICHENS OF THE LAKE SUPERIOR REGION. 225 



furnished more species than number three rather than a smaller 

 number. But it was only on number two that we found the un- 

 mistakable evidence of the effects of the arboreal vegetation 

 in exterminating the lichens. Here too the species existing are 

 all but two the same as those found on one or both of the other 

 islands, but the number is reduced to little more than one-third 

 as many as occur on either of them. 



The succession of species is as apparent upon trees as upon 

 rocks and is constantly in evidence in this largely undisturbed 

 region where trees of various ages grow side by side. Some 

 of the crustaceous lichens, of such genera as Pyrenula, Ar- 

 thonia and Graphis^ were usually found on young trees with 

 smooth bark. As the substratum becomes more rugged with 

 the increasing age of the tree, these are gradually replaced by 

 foliaceous and fruticulose species as Ramalinas , Usneas, Par- 

 mclias, etc. Finally as the trees die certain species of Calicium 

 Cladonia,Peltigera, Parmelia, etc., become the dominant types. 

 It is not possible, nor is it necessary here, to give a detailed ac- 

 count of relation between each epiphytic lichen and its host, 

 but a few of the most apparent relationships are in order. 

 Acer spicatum Lam. supports A rthonia dispersa (Schrad.) Nyl. 

 over the whole area. Populus tremuloides Michx. and P. bal- 

 sa ntif era L. bear Pyrenula leucoplaca (Wallr.) Kbr. commonly. 

 Some conifers, as Pinus resinosus Ait., P. strobus L., Thuja 

 occidentalis L., serve for substrata for those species of the genus 

 Calicium which grow on living trees. The most luxuriant 

 growths of Usnea were found on Picea mariana (Mill.) and 

 Abies balsamea (L.) Mill. Gr aphis scripta (L) Ach. var. recta 

 (Humb.) Nyl. was almost wholly confined to Betula lutea 

 Michx. and this same tree also supports Sagedia oxyspora 

 (Nyl.) Tuck and two or three Pyrenulas. 



A close analysis of the distribution of species within the area 

 studied reveals much of interest even though it is a rather a re- 

 stricted region. Of the 258 species and varieties listed, 96 were 

 found only in one place, 32 in two, 31 in three, and the remain- 

 ing 99 were collected along lake Superior and also inland, in 

 four or more localities and are known to be generally distributed 

 over the whole of the two counties. Also of those found in 

 in two or three localities, 34 species were collected at some 

 point along lake Superior and also beyond the ridge of high 

 land formed by the Mesabi range and the Misquah hills and 



