LICHENOLOGY OF ICELAND 149 



that they die away at their base make them unfit for life on the 

 bark of trees and on stones, and consequently they do not occur 

 there. 



Some fuller data concerning the biology of the fruticose lichens 

 will be given here : 



The hypothallus is the purely mycelial lichen-tissue, free of 

 gonidia, which is formed by the germination of the spore (the so- 

 redium or perhaps the pycnoconidium). It has been observed in 

 all the species of the genus Cladonia, and in Stereocanlon conden- 

 satnm; but as to all the other species of the latter genus it has, in 

 some, never been observed, and in others, is very insignificant, and 

 is then, only for a time, of importance to the life of the species, 

 as it dies away early. It lives long in all hypothallus-wanderers, 

 and constitutes - as suggested by the name their only means 

 of wandering. On the other hand, it disappears very early in the 

 primary-scale wanderers, and in the majority of the podetium- 

 wanderers. 



It is always formed of very loosely woven hyphae, which grow 

 centrifugally from the germination -centre. Wherever green algae 

 suitable for the species, is encountered by it on the surface of the 

 ground, it weaves its hyphae round them, and forms thereby the 

 first beginning either of primary scales (Cladonia) or of direct po- 

 detia (Stereocanlon). This process has, as regards Cladonia, been 

 described by Krabbe (1891) and Wainio (1898). 



Some of the hypothallal hyphae are often formed as fairly thick, 

 dark hyphal bundles, almost devoid of intercellular spaces, especi- 

 ally where they are continued up into the base of the primary scale 

 (Cladonia cornnta, C. uerticillata). The hyphae easily come into con- 

 tact with mineral-grains, humus-particles, plant-remains with their 

 structure still intact, and earth algse. About this the following is to 

 be noted: 



Mineral-grains, especially sand-grains, adhere to the hyphae of 

 several species. I believe that this happens through the cell-walls 

 being covered with a slight (microscopically-invisible) covering of 

 mucus. The sand-grains themselves are always finely striated on 

 the surface, no doubt from weathering, for it cannot be proved that 

 the hyphae exercise any chemical influence upon them, and we 

 must be careful not to state definitely that the roughnesses are marks 

 of corrosion. 



The humus-particles are opaque under the microscope. Where 



