_Bacidia | | | fuscorubella_ | | | | | | _Sphaeromphale | | | KOH, then H₂SO₄, clopismoides_ | | | then HNO₃: blackish | | | _Segestria | bright yellow | | dilute H₂SO₄: lectissima_— | | | bright yellow perithecia | | | | | | _Segestria | | | Strong H₂SO₄: deep lectissima_— | | | violet, then grey entire tissue | | | | | | _Parmelia | blue, | | CaCl₂O₂: blue, then glomellifera_ | then violet, | | grey; finally | at last grey | | decolourized | | | Parmelia-brown | bright | | | red-brown | | ----------------+---------------+-------------+------------------------- 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 somewhat disconnected observations have however been made that bear directly on the question, and they are of considerable interest. Meyer[896] 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._ (_Parmelia_) _tiliaceus_, _L._ (_Rhizocarpon_) _geographicus_, _L._ (_Haematomma_) _ventosus_, and _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_, the specimen 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.