CHAPTER VI 



BIONOMICS 



A. Growth and Duration 



Lichens are perennial plants mostly of slow growth and of long continuance ; 

 there can therefore only be approximate calculations either as to their rate 

 of increase in dimensions or as to their duration in time. A series of some- 

 what disconnected observations have however been made that bear directly 

 on the question, and they are of considerable interest. 



Meyer' was among the first to be attracted by this aspect of lichen life, 

 and after long study he came to the conclusion that growth varied in 

 rapidity according to the prevailing conditions of the atmosphere and 

 the nature of the substratum ; but that nearly all species were very slow 

 growers. He enumerates several, — Lichen (Xanthoria) parietinus, L. {Par- 

 melia) tiliaceus,L. {Rhizocarpon) geographicus, L.{Haematommd) ventosus,s.nA 

 L. (Lecanora) saxicolus, — all species with a well-defined outline, which, after 

 having attained some considerable size, remained practically unchanged for 

 six and a half years, though, in some small specimens of foliose lichens, he 

 noted, during the same period, an increase of one- fourth to one-third of their 

 size in diameter. In one of the above crustaceous species, L. ventosus,t\\e speci- 

 men had not perceptibly enlarged in sixteen years, though during that time 

 the centre of the thallus had been broken up by weathering and had again 

 been regenerated. 



Meyer also records the results of culture experiments made in the open, 

 possibly with soredia or with thalline scraps: he obtained a growth of 

 Xanthoria parietina (on wrought iron kept well moistened), which fruited in 

 the second year, and in five years had attained a width of 5-6 lines (about 

 I cm.); Lecanora saxicola growing on a moist rock facing south grew 4-7 lines 

 in six and a half years, and bore very minute apothecia. 



Lindsay^ quotes a statement that a specimen oi Lobaria pulmonaria\i2A 

 been observed to occupy the same area of a tree after the lapse of half a 

 century. Berkeley^ records that a plant oi Rhizocarpon geographicum remained 

 in much the same condition of development during a period of twenty-five 

 years. The latter is a slow grower and, in ordinary circumstances, it does 

 not fruit till about fifteen years after the thallus has begun to form. WeddellS 

 also commenting on the long continuance of lichens, says there are crustaceous 

 species occupying on the rock a space that might be covered by a five-franc 

 piece, that have taken a century to attain that size. 



Phillips' on the other hand argues against the very great age of lichens, 

 1 Meyer 1825, p. 44. 2 Lindsay 1856. ^ Berkeley 1857. * Weddell 1869. " Phillips 1878. 



