GROWTH AND DURATION 253 



and suggests 20 years as a sufficient time for small plants to establish them- 

 selves 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^ found that a Pertusaria had increased about half a millimetre 

 from the ist February to the end of September. Vallot^ kept under obser- 

 vation 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^ 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 oi Aesculus in California had grown longitudinally v^ cm. and trans- 

 versely I 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 tem- 

 perate or northern countries, and growth must be more rapid. 



A series of measurements was also made by Heere'' on Ramalina reti- 

 culata (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' 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 speci- 

 mens in northern latitudes 18 inches long (50 cm.), which, on that computa- 

 tion, 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" 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' in the account 

 of his synthetic cultures which developed apothecia only after two to three 

 years. The culture experiments of Darbishire* and Tobler" with Cladonia 

 soredia are also instructive, the former with synthetic spore- and alga-cultures 



' Scott Elliot 1907. 2 Vallot 1896. ' Bitter 1901. ^ Heere 1904. ^ Krabbe 1891, p. 131. 

 ' Reinke 1894, p. 18. ' Bonnier, see p. 29. ' Darbishire, see p. 148. ' Tobler, see p. 148. 



