242 



LITHOPHYTES sect, viii 



eventually visible on the outside.^ But in the main, lithophytes are 

 dependent for their mineral nutriment on atmospheric precipitations 

 and on dust conveyed to them by wind. 



The substratum would seem scarcely favourable to saprophytes, yet 

 some of these seem to occur as soon as a small supply of organic matter 

 is present. In the Bernese Oberland, on the Faulhorn, a nitric bacterium 

 is alleged to permeate rocks and cause them to crumble. 



Lithophytes must be able for a time to endure great desiccation, 

 whether this be due to heat and the burning sun's rays, or to cold, dry 

 winds. Vegetation on rocks exposed to the sun may be heated up to 

 temperatures (5o-6o C.) which approach the normal hmits of life (such, 

 for instance, is the case with plants growing on the limestone mountains 

 of Dalmatia) ^ ; while at night-time the temperature may sink very low, 

 lower than in the case of plants occupying other substrata. Lithophytes 

 are oecologically closely related to epiphytes and often identical with 

 them. Furthermore, during frosty periods in winter lithophytes are but 

 feebly sheltered from desiccation, since snow can remain lying only on 

 a few spots on rocks.^ 



A rock substratum is the warmest of all soils.* Consequently, there 

 are numbers of plants which on high mountains occur only in fissures of 

 rocks, but in the plains grow on loose soil.^ 



ASSOCIATIONS. 



Lowly organized though they be these plants display great differences 

 in their ecological demands ; and different formations and associations 

 of them can be established in accordance with the like conditions prevail- 

 ing on rock. The oecological factors concerned include the following : 



Nature of the rock (with great differences according as the rock is 

 eugeogenous or dysgeogenous) ; 



Exposure (involving differences in illumination, heating, wetting 

 by rain, exposure to wind) ; 



Steepness of the rock-surface ; 



Supply of flowing water ; 



Geographical situation and altitude. 



Kihlman's investigations, for instance, prove that different species 

 of lichens show different powers of resistance to the action of cold winds. 

 According to Zukal,' species of Parmelia require more moisture than do 

 crustaceous species of Lecanora ; the latter are less capable than the 

 former of absorbing water-vapour from the atmosphere. 



Even among lithophytes one sees competition, struggles for space, 

 and the suppression of one species by another. In North America useful 

 contributions to our knowledge on this matter have been made by Bruce 

 Fink,^ who recognized various associations of lichens, including ' Leca- 

 nora-associations of exposed boulders ', ' Lecanora calcarea contorta- 

 associations of exposed, horizontal limestone surfaces,' and others ; 

 and in Sweden the competition between species has been studied by 

 A. Nilsson.^ One sees crustaceous lichens settling at first in patches, 

 and the patches gradually extending until they meet, either preserving 



' Bachmann, 1904. * Kerner, 1868. " Ottli, 1903. * SeeHomen, 1897. 



' Flahault, 1893, 1901a; Vahl, 19046. * Kihlman, 1890; seep. 208. 



' Zukal, 1895. ' Fink, 1903. A. Nilsson, 18996. 







