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.
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 1 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[897] quotes a statement that a specimen of Lobaria pulmonaria had been observed to occupy the same area of a tree after the lapse of half a century. Berkeley[898] records that a plant of 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. Weddell[899], 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[900] on the other hand argues against the very great age of lichens, and suggests 20 years as a sufficient time for small plants to establish themselves on hard rocks and attain full development. He had observed a small vigorous plant of Xanthoria parietina that in the course of five years had extended outwards to double its original size. The centre then began to break up and the whole plant finally disappeared.
Exact measurements of growth have been made by several observers. Scott Elliot[901] found that a Pertusaria had increased about half a millimetre from the 1st February to the end of September. Vallot[902] kept under observation at first three then five different plants of Parmelia saxatilis during a period of eight years: the yearly increase of the thallus was half a centimetre, so that specimens of twenty centimetres in breadth must have been growing from forty to fifty years.
Bitter’s[903] observations on Parmelia physodes agree in the main with those of Vallot: the increase of the upper lobes during the year was 3-4 mm. In a more favourable climate Heere found that Parmelia caperata ([Fig. 49]) on a trunk of Aesculus in California had grown longitudinally 1·5 cm. and transversely 1 cm. The measurements extended over a period of seven winter months, five of them being wet and therefore the most favourable season of growth. In warm regions lichens attain a much greater size than in temperate or northern countries, and growth must be more rapid.
A series of measurements was also made by Heere[904] on Ramalina reticulata ([Fig. 64]), a rapid growing tree-lichen, and one of the largest American species. The shorter lobes were selected for observation, and were tested during a period of seven months from September to May, five of the months being in the wet season. There was great variation between the different lobes but the average increase during that period was 41 per cent.
Krabbe[905] took notes of the colonization of Cladonia rangiferina ([Fig. 127]) on burnt soil: in ten years the podetia had reached a height of 3 to 5 cm., giving an annual growth of about 3-5 mm. It is not unusual to find specimens in northern latitudes 18 inches long (50 cm.), which, on that computation, must have been 100 to 160 years old; but while increase goes on at the apex of the podetia, there is constant perishing at the base of at least as much as half the added length and these plants would therefore be 200 or 300 years old. Reinke[906] indeed has declared that apical growth in these Cladina species may go on for centuries, given the necessary conditions of good light and undisturbed habitat.
Other data as to rate of growth are furnished by Bonnier[907] in the account of his synthetic cultures which developed apothecia only after two to three years. The culture experiments of Darbishire[908] and Tobler[909] with Cladonia soredia are also instructive, the former with synthetic spore- and alga-cultures having obtained a growth of soredia in about seven months; the latter, starting with soredia, had a growth of well-formed squamules in nine months.
It has been frequently observed that abundance of moisture facilitates growth, and this is nowhere better exemplified than in crustaceous soil-lichens. Meyer found that on lime-clay soil which had been thrown up from a ditch in autumn, lichens such as Gyalecta geoica were fully developed the following summer. He gives an account also of another soil species, Verrucaria (Thrombium) epigaea, which attained maturity during the winter half of the year. Stahl[910] tells us that Thelidium minutulum, a pyrenocarpous soil-lichen, with a primitive and scanty thallus, was cultivated by him from spore to spore in the space of three months. Such lichens retain more of the characteristics of fungi than do those with a better developed thallus. Rapid colonization by a soil-lichen was also observed in Epping Forest by Paulson[911]. In autumn an extensive growth of Lecidea uliginosa covered as if with a dark stain patches of soil that had been worn bare during the previous spring. The lichen had reached full development and was well fruited.
These facts are quite in harmony with other observations on growth made on Epping Forest lichens. The writers[912] of the report record the finding of “fruiting lichens overspreading decaying leaves which can scarcely have lain on the ground more than two or three years; others growing on old boots or on dung and fruiting freely; others overspreading growing mosses.” They also cite a definite instance of a mass of concrete laid down in 1903 round a surface-water drain which in 1910—seven years later—was covered with Lecanora galactina in abundant fruit; and of another case of a Portland stone garden-ornament, new in 1904, and, in 1910, covered with patches of a fruiting Verrucaria (probably V. nigrescens). Both these species, they add, have a scanty thallus and generally fruit very freely.
A series of observations referring to growth and “ecesis” or the spreading of lichens have been made by Bruce Fink[913] over a period of eight years. His aim was mainly to determine the time required for a lichen to re-establish itself on areas from which it had been previously removed. Thus a quadrat of limestone was scraped bare of moss and of Leptogium lacerum, except for bits of the moss and particles of the lichen which adhered to the rock, especially in depressions of the surface. After four years, the moss was colonizing many small areas on which grew patches of the lichen 2 to 10 mm. across. Very little change occurred during the next four years.
Numerous results are also recorded as to the rate of growth, the average being 1 cm. per year or somewhat under. The greatest rate seems to have been recorded for a plant of Peltigera canina growing on “a mossy rock along a brook in a low moist wood, well-shaded.” A plant, measuring 10 by 14 cm., was deprived of several large apothecia. The lobes all pointed in the same direction, and the plant increased 1·75 cm. in one year. Two other plants, deprived of their lobes, regenerated and increased from 2 and 5 cm. respectively to 3·5 and 6 cm. No other measurements are quite so high as these, though a plant of Parmelia caperata (sterile), measuring from 1 to 2 cm. across, reached in eight years a dimension of 10 by 13 cm. Other plants of the same species gave much slower rates of increase. A section of railing was marked bearing minute scattered squamules of Cladonia pityrea. After two years the squamules had attained normal size and podetia were formed 2 to 4 mm. long.
Several areas of Verrucaria muralis were marked and after ten months were again measured; the largest plants, measuring 2·12 by 2·4 cm. across, had somewhat altered in dimensions and gave the measurements 2·2 by 3 cm. Some crustose species became established and produced thalli and apothecia in two to eight years. Foliose lichens increased in diameter from 0·3 to 3·5 cm. per year. So far as external appearance goes, apothecia are produced in one to eight years; it is concluded that they require four to eight years to attain maturity in their natural habitats.