256 BIONOMICS 



and noting any conditions that might affect favourably or unfavourably the 

 reproductive organs. A comparison between corticolous and saxicolous 

 species would also be of great interest to determine the influence of the 

 substratum as well as of light and shade. But in any case it is profitable to 

 collect and examine lichens at all seasons of the year, as even when the 

 bulk of the spores is shed, there may remain belated apothecia with a few 

 asci still intact. 



C. DISPERSAL AND INCREASE 



The natural increase of lichen plants may primarily be sought for in the 

 dispersal of the spores produced in the fruiting-bodies. These are ejected, 

 as in fungi, by the pressure of the paraphyses on the mature ascus. The 

 spores are then carried away by wind, water, insects, etc. In a few lichens 

 gonidia are enclosed in the hymenium and are ejected along with the spores, 

 but, in most, the necessary encounter with the alga is as fortuitous, and 

 generally as certain, as the pollination of anemophilous flowers. A case of 

 dispersal in Sagedia microspora has been described by Miyoshi 1 in which 

 entire fruits, small round perithecia, were dislodged and carried away 

 by the wind. The addition of water caused them to swell enormously and 

 brought about the ejection of the spores. Areas covered by the thallus 

 are also being continually enlarged by the spreading growth of the hypo- 

 thallus. 



a. DISPERSAL OF CRUSTACEOUS LICHENS. These lichens are distributed 

 fairly equally on trees or wood (corticolous) and on rocks (saxicolous). Some 

 species inhabit both substrata. As regards corticolous lichens that live on 

 smooth bark such as hazel or mountain-ash, the vegetative body or thallus 

 is generally embedded beneath the epidermis of the host. Soredia are absent 

 and the thallus is protected from dispersal. In these lichens there is rather 

 an abundant and constant formation of apothecia or perithecia. 



Other species that affect rugged bark and are more superficial are less 

 dependent on spore produ'ction. The thallus is either loosely granular, or is 

 broken up into areolae. The areolae are each a centre of growth, and with 

 an accession of moisture they swell up and exert pressure on each other. 

 Parts of the thallus thus become loosened and are dislodged and carried 

 away. If anchored on a suitable substratum they grow again to a complete 

 lichen plant. Sorediate lichens are dependent almost wholly on these bud- 

 like portions for increase in number ; soredia are easily separated from 

 the parent plant, and easily scattered. Darbishire 2 noted frequently that 

 small Poduridae in moving over the surface of Perttisaria amara became 

 powdered with soredia and very evidently took a considerable part in the 

 dissemination of the species. 



1 Miyoshi 1901. 2 Darbishire 1897, p. 657. 



