GROWTH AND DURATION 255 



in the same direction, and the plant increased i'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 

 I 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'S 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. 



B. Season of Fruit Formation 



The presence of apothecia (or perithecia) in lichens does not always 

 imply the presence of spores. In many instances they are barren, the spores 

 having been scattered or not yet matured ; the disc in these cases is composed 

 of paraphyses only, with possible traces of asci. In any month of the year, 

 however, some lichens may be found in fruit. 



Baur^ found, for instance, that Parmelia acetabulum developed carpogonia 

 the whole year round, though somewhat more abundantly in spring and 

 autumn. Pertusaria communis similarly has a maximum period of fruit- 

 formation at these two seasons. This is probably true of tree-lichens 

 generally: in summer the shade of the foliage would inhibit the formation 

 of fruits, as would the extreme cold of winter ; but were these conditions 

 relaxed spore-bearing fruits might be expected at any season though perhaps 

 not continuously on the same specimen. 



An exception has been noted by Baur in Pyrenula nitida, a crustaceous 

 tree Pyrenolichen. He found carpogonia only in February and April, and 

 the perithecia matured in a few weeks, presumably at a date before the trees 

 were in full leaf; but even specimens of Pyrenula are not unusual in full 

 spore-bearing conditions in the autumn of the year. 



To arrive at any true knowledge as to the date and duration of spore 

 production, it would be necessary to keep under observation a series of one 

 species, examining them microscopicallyat intervals of a few weeks or months 



^ Baur 1901. 



