LICHENOLOGY OF ICELAND 145 



gradually to the substratum, as they come in contact with it. How 

 they attach themselves, and how far they are of any other import- 

 ance than to fix the plant in the substratum, is not known. The 

 thallus itself is always dorsiventral and in some species it dies away 

 in the middle, its single lobes thus becoming isolated. Zukal (1895) 

 has shown that several of the earth-lichens "wander" by a kind of 

 mycelium, which proceeds from their rhizines, and run horizontally 

 below the surface of the ground, forming new 7 thalli here and there, 

 as in Peltigera uenosa and Solorina saccata. This mode of propaga- 

 tion corresponds exactly with that by which crustaceous lichens 

 with a mycelium of radiating, centrifugal growth, form numerous 

 small balls of gonidia, which by their abundance fuse into a granu- 

 lose thallus; or with that by which Cladonia forms its centrifugally- 

 growing scales (primary thallus) or Stereocanlon its scales which 

 afterwards develop into podetia (Danske Likeners 0kologi, fig. 91); 

 it is no doubt the most natural explanation of the fact, that the 

 form of many foliaceous lichens is that the thallus consists of one 

 or more lobes, which have a base on the surface of the earth itself, 

 and grow 7 from thence unilaterally forward away from that base. 



The Erect foliaceous lichens are, although erect, very com- 

 monly dorsiventral. So far as my investigations go, they die away 

 below (the spot corresponding with the centre of the procumbent 

 lichens) and keep on growing at the apex. They escape being blow r n 

 away by being fastened by their "haptera" each to the other or to 

 other things (Sernander, 1901). These haptera, which have not been 

 investigated more closely, have been found by Sernander in Ce- 

 traria islandica, cucnllata, hiascens and nivalis, and are transformed 

 cilia, which, as is well-known, are extremely common in this genus. 



The means of propagation in the foliaceous earth lichens appear 

 to be ascospores, or perhaps pycnoconidia. On the other hand, 

 soredia and detached portions of thallus do not appear to play any 

 part in the dispersal of these species either. Otherwise, the whole 

 class has been as yet very little investigated. It is evident that pro- 

 cumbent foliaceous lichens are w r eak competitors, and are easily 

 covered by other plants. As regards abundance of individuals they 

 also play but a slight role in nature. They have far better chances 

 on stones and trees, and are very common in such stations. The 

 erect foliaceous lichens have far greater advantages in competition, 

 and are much richer in individuals than are the others. 



Iceland has the following foliaceous earth lichens : 



The Botany of Iceland. Vol II. 10 



