TRANSACTIONS OF THE SECTIONS. 89 



great age. The object of the author's paper was to exliibit the result of certaiu 

 researches on the subject of Lichen-growth in rekition to the age of the structures 

 on which it occurs, in order to determine how far the current opinions in question 

 are founded on fact. His immediate object was to determine the rapidity of Lichen- 

 development under favourable conditions, or, in other words, How soon" mioht a 

 fresh surface of wood or stone become so Lichen-clad as to assume tlie " lioarj' " or 

 "time-stained" appearance that is popularly associated with the idea ofgi-eat a"e? 

 In endeavouring to solve this question he assumed, as standards of comparison, the 

 megaliths of Stennis in Orkney and Callernish in Lewis, botli of which groups of 

 prehistoric remains, undoubtedly of great though imdetermined age, he had visited 

 in May 1866, and of whose Lichen-Flora he had published an account in tlae 

 Transactions of the Botanical Societj' of Edinburgh (vol. ix. p. 154). On the other 

 hand, he noted the development of Lichen-gi-owth on a variety of recent structures 

 of known age, including 



I. Walls of buildings, gardens, and roads : bridges and other edifices of stone : as 



well as the mortar or cement used in their construction and in their repair 

 from time to time. 



II. Fences of sawn timber around fields and gardens : posts, gates, and other struc- 



tures of fabricated wood. 



III. Young trees and shrubs in nurseries, plantations, forests, gardens, shrubberies, 

 and cemeteries. 



The conclusion at which heariives is that within a quarter of a century, in periods 

 ranging from two to five years and upwards, as copious a clothing of Lichens as 

 that which covers the monoliths of Stennis or Callernish may be produced in favour- 

 able conditions of growth, and hence that Lichen-growth furnishes no criterion of 

 the antiquiti/ of prehistoric or other structures. 



Additional corroborative evidence was adduced from the history of those Lichens 

 which were or are still collected in Northern Europe, on account of their economi- 

 cal applications, as food, fodder, or dye-stvifis,e.g.Zecanora tartarea, Cctraria Islait- 

 dica, and Cladonia rauf/iferina. Their collectors were familiar with the fact that 

 they may look for replacement of the species they remove in a limited number of 

 years, varying generally witliin a period of from three to fi^e. 



The author draws a distinction between rapidity of primary development and 

 slowness of subsequent growth, showing that tlie two phenomena may occur conse- 

 cutively in the same individual — a circumstance which serves to reconcile on the 

 one hand the facts observed as to the rapidity with which a fresh surfoce, whether 

 of earth, stone, or wood, may become Lichen-coated ; and on the other the current 

 opinion among botanists that Lichen-growth is essentially slow, and its duration 

 practically unlimited. 



On Polymorphism in the FrticHjication of Lichens*. 

 Bijyif. Laudkr LiNDS.iT, M.n., F.R.S'.E., F.L.S. 



Ten years ago, while engaged in researches on the secondary or complementarv 

 reproductive organs of Lichens, tlie author met witli a number of instances of 

 polymorphism or plurality thereof — especially of the occurrence in the same species 

 of more than one form of spermogonium or pycnidium ; and since that date further 

 instances have frequently occurred to him. The forms of polymorphism specially 

 described or referred to in the present communication are the occurrence in the 

 same species of — 



I. More than one form of spermogonium. 



IT. More than one form of pycnidium. 



III. Pycnidia in addition to spermogonia, or spermogonia in addition to pycnidia. 



IV. Pycnidia instead of spermogonia. 



V. Spermatia and sporidia in the same conceptacle. 



YI. Dift'erent sizes and forms of spermatia and sterigmata, or of stylospores and 



basidia. 

 These multiple fonns of reproductive organs or bodies were met with cliicfly in 



* The subject may be found treated more fully in the ' Quarterly Journal of Microsco- 

 pical Science,' Januai-y 1868. 



