EXTERNAL INFLUENCES 359 



observed the gradual colonization of the depressions, first by a growth of 

 hepatics and mosses and by such ground lichens as Peltigera canina, P. 

 aphthosa and Nephromium arcticum ; they cover the soil and in time the 

 hollow becomes filled with a mass of vegetation consisting of Cladonias, 

 mosses, etc. On reaching a certain more exposed level these begin to wither 

 and die off at the tips, killed by the high cold winds. Then arrives Lecanora 

 tartarea, one of the commonest Arctic lichens, and one which is readily 

 a saprophyte on decayed vegetation. It covers completely the mound of 

 weakened plants which are thus smothered and finally killed. The collapse 

 of the substratum entails in turn the breaking of the Lecanora crust, and 

 the next high wind sweeps away the whole crumbling mass. How long 

 recolonization takes, it was impossible to find out. 



Upright fruticose lichens are necessarily more liable to damage by wind, 

 but maritime Ramalinae and Roccellae do not seem to suffer in temperate 

 climates, though in regions of extreme cold fruticose forms are dwarfed and 

 stunted. The highest development of filamentous lichens is to be found in 

 more or less sheltered woods, but the effect of wind on these lichens is not 

 wholly unfavourable. Observations have been made by Peirce 1 on two 

 American pendulous lichens which are dependent on wind for dissemina- 

 tion. On the Californian coasts a very large and very frequent species, 

 Ramalina reticulata (Fig. 64), is seldom found undamaged by wind. In 

 Northern California the deciduous oaks Quercus alba and Q. Douglasii are 

 festooned with the lichen, while the evergreen " live oak," Q. chrysolepis, 

 with persistent foliage, only bears scraps that have been blown on to it. 

 Nearer the coast and southward the lichen grows on all kinds of trees and 

 shrubs. The fronds of this Ramalina form a delicate reticulation and when 

 moist are easily torn. In the winter season, when the leaves are off the 

 trees, wind- and rain-storms are frequent ; the lichen is then exposed to 

 the full force of the elements and fragments and shreds are blown to other 

 trees, becoming coiled and entangled round the naked branches and barky 

 excrescences, on which they continue to grow and fruit perfectly well. 

 A succeeding storm may loosen them and carry them still further. Peirce 

 noted that only plants developed from the spore formed hold-fasts and 

 they were always small, the largest formed measuring seven inches in length. 

 Both the hold-fast and the primary stalk were too slight to resist the tearing 

 action of the wind. 



Schrenk 2 made a series of observations and experiments with the lichens 

 Usnea plicata and U, dasypoga, long hanging forms common on short-leaved 

 conifers such as spruce and juniper. The branches of these trees are often 

 covered with tangled masses of the lichens not due to local growth, but to 

 wind-borne strands and to coiling and intertwining of the filaments owing 

 1 Peirce 1898. 2 Schrenk 1898. 



