94 



some other locality, where a portion of it will develop into a new indi- 

 vidual ; this happens quite frequently among the higher crustaceous 

 lichens, especially the Lecanoras. Lecanora ventosa, for example, 

 is only loosely attached to the substratum ; during dry periods large 

 masses of this lichen are torn away and scattered by the wind, 

 and, falling upon other suitable localities, they again begin growth. 



Another and perhaps more frequent method of vegetative propa- 

 gation is by means of the soredial branches (including warts and 

 isidia) and thalli (secondary thalli). Typical soredial branches are 

 very numerous on Usnea barbata. These structures, sooner or later, 

 become detached and develop into new lichens very much in the 

 manner of soredia. 



Some lichens also possess what may be designated as continuous 

 rejuvenescence, that is, there is unceasing apical growth accom- 

 panied by a continuous dying away of the basal portion. This 

 phenomenon occurs typically in the higher Cladonias as well as in 

 Thamnolia, and is also met with in some foliose lichens, as Pannelia. 

 The central, hence older, portion dies away, while the margin con- 

 tinues to grow producing an appearance not unlike the " fairy rings" 

 of certain mushrooms. 



IV. LIFE PERIOD OF LICHENS. 



No reliable observations have as yet been made as to the life- 

 period of the individual lichen. It is supposed that some species 

 live on indefinitely, as for example the Cladonias, Thamnolia and 

 some of the foliose lichens ; it has been roughly estimated that 

 many Cladonias are hundreds of years old. Among the lower 

 crustaceous lichens it is difficult to make any estimates as to the 

 age of the individual plant, because we can not determine the limi- 

 tations of its growth. 



In general, it may be stated that lichens are perennial land-plants 

 of comparatively slow growth. Meyer, whom we have already 

 mentioned in the historical review, states that from actual measure- 

 ments he has found that Physcia parietina grew six lines in six 

 years when in a protected position and twelve to fourteen lines 

 when growing on the weather side of a tree. Lecidiella sabulc- 

 torum upon sandstone grew two lines in diameter in four years. 

 Aspicilia cincrea on the same substratum grew two lines during the 

 same period. Since the thalli of the above lichens attain very large 

 diameters, it is evident that they must become very old. 



