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[914] 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 microscopically at intervals of a few weeks or months 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.