GENERAL INTRODUCTION 357 



Ecological plant-geography has been well defined by Macmillan 1 as 

 "the science which treats of the reciprocal relation between physiographic 

 conditions and life requirements of organisms in so far as such relations 

 manifest themselves in ghoice of habitats and method of establishment 

 upon them... resulting in the origin and development of plant formations." 



B. EXTERNAL INFLUENCES 



The climatic factors most favourable to lichen development are direct 

 light (already discussed) 2 , a moderate or cold temperature, constant moisture 

 and a clear pure atmosphere. Wind also affects their growth. 



a. TEMPERATURE, Lichens, as we have seen, can endure the heat of 

 direct sunlight owing to the protection afforded by thickened cortices, colour 

 pigments, etc. Where such heat is so intense as to be injurious the gonidia 

 succumb first 3 . 



Lichens endure low temperatures better than other plants, their xerophytic 

 structure rendering them proof against extreme conditions: the hyphae 

 have thick walls with reduced cell lumen and extremely meagre contents. 

 Freezing for prolonged periods does them little injury ; they revive again 

 when conditions become more favourable. Efficient protection is also afforded 

 by the thickened cortex of such lichens as exist in Polar areas, or at high 

 altitudes. Thus various species of Cetrariae with a stout "decomposed" 

 amorphous cortex can withstand very low temperatures and grow freely on the 

 tundra, while Cladonia rangiferina, also a northern lichen, but without a con- 

 tinuous cortex, cannot exist in such cold conditions, unless in localities where 

 it is protected by a covering of snow during the most inclement seasons. 



b. HUMIDITY. A high degree of humidity is distinctly of advantage to 

 the growth of the lichen thallus, though when the moist conditions are ex- 

 cessive the plants become turgid and soredial states are developed. 



The great abundance of lichens in the western districts of the British 

 Isles, where the rainfall is heaviest, is proof enough of the advantage of 

 moisture, and on trees it is the side exposed to wind and rain that is most 

 plentifully covered. A series of observations on lichens and rainfall were 

 made by West 4 and have been published since his death. He has remarked 

 in more than one of his papers that a most favourable situation for lichen 

 growth is one that is subject to a drive of wind with much rain. In localities 

 with an average of 216 days of rain in the year, he found abundant and 

 luxuriant growths of the larger foliose species. In West Ireland there were 

 specimens of Ricasolia laetevirens measuring 1 65 by 60 cm. In West Scotland 

 with an "average of total days of rain; 225," he found plants of Ricasolia am- 

 plissima 150 x 90 cm. in size, of R. laetevirens 120x90 cm., while Pertusaria 



1 Macmillan 1894. - See p. 240 et seq. 3 See p. 238. 4 West 1915. 



